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Original research: Article 11. The hydrology of ocean current formation and the boundaries between different seawater layers.
Author: Liu Huan (1983-), Master of Science (First Class Honours, 2009), The University of Auckland.
1.Introduction
The ocean (Sea) is the widest water area on the Earth’s surface, which are separated by continents but organically connected to each other on the Earth. Usually the central part of the water area is called the ocean and the peripheral part is called the sea, which connect each other to form a unified water area [18].
The total area of the ocean on the Earth’s surface is approximately 360 million square kilometers with an average water depth of about 3795 meters, accounting for approximately 71% of the Earth's total surface area, which contains over 1.35 billion cubic kilometers of water, accounting for approximately 97% of the total water on Earth, whereas only 2% of ocean water is available for human consumption. The four main oceans on Earth include the Pacific Ocean, Atlantic Ocean, Indian Ocean, and Arctic Ocean, where most of area are bounded by land or seabed topography lines [18]. Marine hydrology study is closely related to marine transportation, coastal protection, marine resource development, marine pollution, national defense construction, maritime medicine and rehabilitation medicine [4].
In this paper, the main study objectives include: the materials’ energy transport, motion, mechanic dynamics of ocean ecosystem will be fully reviewed on the basis of the past hydrology theories, and the boundary layer mechanism discussed in my previous article is applied on the formation of turbulent flow in hydrology [1][3], with emphasis on the physical, chemical and biological exchange mechanism of ocean pollutants; Then the hydrology elements forming various types of ocean currents will be comprehensively summarized and the future research gaps is identified and filled; Finally the representative case studies that have been published in China are compared and contrasted, aiming to comprehensively analyze the methods and relevant theories necessary on the basis of published research data.
2.Marine hydrology
2.1.Marine hydrology elements and mapping
Marine hydrology elements that mainly include parameters of seawater temperature, salinity, density, ocean currents, tidal currents, waves, water transparency, water color, sea luminescence, sea ice, ocean-atmosphere interactions and etc, rely on hydrological observation at a certain geography point or section of rivers, lakes and oceans to analyze and sort out the observation data. Hydrological measurement provides necessary physical and chemical parameters of seawater such as underwater terrain measurement, water depth measurement and positioning, whose data can draw ocean hydrology maps of different ocean elements, representing the general patterns of both horizontal and vertical distribution of each element and displaying their general patterns and characteristics over time. For example, by collecting the measuring data of temperature, salinity or density of seawater, the speed of sound wave propagation in water can be calculated and estimated according to the empirical equations; Tidal observation can provide instantaneous vertical reference data for underwater terrain measurement, while wave correction can improve the accuracy of depth measurement and positioning. Consequently, these hydrology conditions can be estimated indirectly on the basis of observation parameters that are more convenient to collect [4].
Various hydrological maps add thematic elements to the geographic base map, which are indicated by using various methods. Among them, the contour line method is commonly used to indicate the horizontal distribution of hydrological elements such as temperature, salinity, density and sound velocity, while the cross-sectional diagram is used to represent their vertical distribution; Water color chart is generally adopted by using the base color method, and tides and ocean currents are represented by different colored arrows using the dynamic line method, whose length is also used to indicate the flow velocity at different depths [4].
The temperature of seawater is a physical parameter that reflects the thermal condition of seawater, whose variation generally ranges from -2 ℃ to 30 ℃ over the world's oceans, and half of the entire ocean area displays an annual average temperature exceeding 20 ℃. The temperature of seawater shows periodic and irregular changes such as daily, monthly, yearly, and multi-year variations, which are mainly depended on the oceanic heat balance and its temporal changes. Direct observation shows that the daily variation of seawater temperature is usually very small, and the temperature varies along water depth from 0 to 30 meters, but in comparison to daily variation, the annual temperature variation along water depth can be apparent from the water depth of 0 to 350 meters. At the water depth of about 350 meters, there is a constant temperature layer, below which the water temperature gradually becomes colder with the increasing water depth (about 1°-2℃ descending per 1000 meters of depth), and when the water depth reaches 3000-4000 meters, the temperature decreases to -1℃ ~ 2℃. Sea water temperature is one of the most important factors in marine hydrological conditions, which becomes a fundamental indicator commonly used for studying the properties of water mass and describing their movements[5].
There are several factors affecting seawater temperature in seawater hydrology, mainly covering (1) Latitude: the temperature varies depending on the solar radiation obtained at different latitudes, so the distribution pattern of global seawater temperature decreases from low latitude of sea areas to high latitude of sea areas; (2) Ocean currents: along the same latitude of sea area, there is higher temperature when warm currents flow through seawater, while lower temperature is discovered when cold currents flow through seawater; (3) Season: the seawater is also significantly influenced by seasons, and usually seawater temperature is high in summer but low in winter; (4) Depth: the surface seawater decreases significantly with the increase of water depth, and there is more noticeable change within the water depth of 1000 meters, while the smaller change is usually displayed between 1000 and 2000 meters, but finally the consistently low temperature is found at the water depth below 2000 meters [5].
2.2.Marine hydrology measurement methods
The temperature, salinity and density of seawater can be measured both physically or chemically by equipping with the temperature-salinity depth meter, which are the three main parameters to indirectly calculate the sound velocity in seawater. For direct measurement of sound velocity in seawater, a sound velocity profiler can be used to obtain the sound velocity of the corresponding depth layer by measuring the propagation time of sound waves at different depths and at a certain distance, thus calculating the sound velocity profile [4].
The flow velocity and direction of seawater are two dynamic factors in the development process of the seabed, which can be measured by ADCP (Acoustic Doppler Current Profile). ADCP applies the Doppler effect of ocean currents and several ultrasonic columns on calculating flow velocity and calculating flow direction based on the position of the columns. According to different working methods, ADCP can be divided into two types including stationary and mobile ones, which measure the flow velocity and direction data to draw a vector diagram of the water flow field distribution [4].
Tidal observation data value reveals the local tidal level and analyze tidal variation characteristics, providing the vertical reference surface for underwater terrain measurement, whose observation can usually include water gauge tide measurement, ultrasonic tide measurement, float tide measurement, pressure tide measurement, and GPS tide measurement[4].
After the formation of ocean currents, the continuity of seawater can lead to the formation of both upwelling and downwelling in areas where seawater diverges or gathers, which are commonly characterized and measured by the Euler method requiring simultaneous observation of ocean currents at certain stations in the ocean. Based on the measurement results, vectors representing the magnitude and direction of ocean currents are labeled, and streamline diagrams are drawn to describe the distribution of velocity in the flow field. If it is assumed that the flow field does not change over time in the long term, then the streamline represents the trajectory points of water mass in the ocean current flow diagram [7].
2.3.Marine hydrology and ocean current classification
According to the hydrology factors discussed above, the prevailing wind is the main driving force of ocean currents, so the larger currents in the ocean are mostly blown by strong and stable winds. ‘Wind-driven ocean current’ is directly generated by the wind, which is also named as ‘drifting’; The flow of seawater that is caused by uneven density distribution is called ‘density flow,’ also known as ‘gradient flow’ or ‘geostrophic flow’; In the process of studying ocean currents, scientists often classify them into warm currents and cold currents based on their temperature characteristics; There is also horizontal flow phenomenon of seawater caused by the tidal force generated by the moon and sun, which is called as the tidal current generated together with the tide [7].
3.Ocean current
Ocean currents are the water masses with different densities formed by multiple factors including thermal radiation, evaporation, precipitation, cold shrinkage, etc. In combination with wind stress, geostrophic forces, tidal forces and other effects, relatively stable flow of seawater can be formed in large scale, as one of the common forms of seawater movement. There are many streams of ocean currents in the ocean, each of which flows along a relatively fixed route throughout the year. It is like the blood circulation of the human body, connecting the entire world's oceans and enabling them to maintain relatively long-term stability on the basis of various hydrological and chemical elements. For examples, the most famous ocean currents in the world wide are the Kuroshio Current and the Gulf Stream [7].
For the convenience of discussion, ocean currents can also be classified and named on the basis of different perspectives including the stress conditions and causes of seawater. For example, ocean currents caused by wind are called wind-induced currents or drift, while those caused by temperature and salinity variation in spaces are called thermohaline circulation; According to the force situation, ocean currents can be classified into geostrophic flow and inertial flow; Considering the different regions where ocean currents occur, there are also different ocean currents classified into sea currents, shelf currents, equatorial currents, east-west boundary currents, etc [7].
3.1.Ocean current measurement
There are two methods to describe the movement of seawater, including the Lagrangian method and the Euler method. The Lagrangian method is to track water mass points to describe their spatio-temporal changes, which is difficult to implement, but in modern time by using drift bottles and neutral floats, it is to track flow traces approximately characterizing the changes in flow patterns [7].
The ocean current velocity in SI units is measured by meter per second, denoted as m/s, while flow direction is represented by geographical azimuth, indicating the direction of seawater flows. For example, if seawater flows northward at a speed of 0.10m/s, the flow direction is denoted as 0°, while it is indicated by 90° for eastward flow, 180° for southward flow, and 270° for westward flow respectively. It is worthwhile noting that the definition of ocean current flow direction is exactly opposite to that of wind direction which indicates the direction in which the wind blows. Arrow symbols are commonly used when drawing ocean current maps, in which the length of the arrow represents the magnitude of the flow velocity and the direction of the arrow reflects the flow direction, respectively [7].
The content below tries to summarize the common types of ocean currents focusing on the hydrology elements forming and driving each type of ocean current:
3.2.Ocean tide current
3.2.1.Ocean tide formation
Under the tidal force of celestial bodies such as the sun or moon, seawater exhibits a periodic fluctuation phenomenon that the seawater rises rapidly and reaches a climax at a certain time but after some time the rising seawater is receded on its own, leaving behind a sandy beach and experiencing a low tide. This ocean seawater cycle repeats itself endlessly in the long term fluctuation, whose movement is called as tides that are mainly caused by tidal forces imposed by celestial bodies. It manifests as tidal level rising and falling in the vertical direction, while tidal current comes and leaves in the horizontal direction [6].
There are two gravitational forces imposed on the objects of earth planet per unit mass: one is the gravitational forces on the surface objects of earth planet exerted by the moon, sun, or other celestial bodies while the other gravitational force is exerted at the center of the Earth, and the difference in both gravitational forces causes tidal force. Consequently, the magnitude of the tidal force exerted by any celestial body at a certain point on Earth surface is directly proportional to the mass of that celestial body, but inversely proportional to the square of the distance between the center of the Earth and the center of the celestial body, which is also related with the zenith angle from the celestial body to that point, and the tidal force is gradually reduced when the zenith angle is closer to 90°, whose magnitude and direction of tidal forces on Earth vary both temporally and spatially. Although the mass of the Sun is much larger than that of the Moon, its tidal force is only 46% of that of the Moon due to its distance from the Earth. Additionally, the tidal force of other celestial bodies on Earth is very small compared to the Moon or Sun, which can be ignored. The tide caused by the tidal force of the moon is called the lunar tide and the tidal force caused by the sun is called solar tide correspondingly, both of which belong to astronomical tides. Tidal forces do not only generate ocean tides, but also cause solid earth tides (earth tides) and atmospheric tides (air tides). For the ocean seawater, the solid earth tide is below the ocean tide, but the air tide is above the tide, both of which result in the impact on the ocean tide [6].
3.2.2.Ocean tide process
The movement modes of the moon and the sun relative to the earth display periodicity, so tides also show periodicity correspondingly. It is to describe the perspectives of periodic tidal processes below: when the tide level ascends to its peak level, it is defined as the high tide or full tide; If the tide level does not rise or fall during the period before or after this peak level, it is called a flat tide; Then the tide level begins to descend and reaches its bottom level, this critical tide level is called a low tide or dry tide; if the tide level does not rise or fall during a period of time before or after this critical level, it is called a tidal pause; Finally after the tide stops, the tide level begins to rise again for the second tide cycle. The duration of normal and stagnant tides varies, which is depended on the geographic location. For the flat tide, the middle time of the flat tide is defined as the high tide, and the height of the flat tide at that time is the height of high tide; For the tidal pause, the middle moment of the tidal pause is the low tide, and the tide height level at that time is the height of low tide correspondingly; The height difference between adjacent high and low tides is called tidal range. The periodic process from low tide to high tide is called as egre or rising tide, and the periodic process from high tide to low tide is called ebb tide. The tidal range during the rising tide stage is called the rising tide range, and the time interval between low tide and high tide is defined as the time of rising tide; The tidal range during the ebb tide stage is called ebb tide range, and the time interval is defined as the time of ebb tide [6].
3.2.3.Ocean tide features
The process of tides varies from day to day due to the constantly changing positions relatively to the moon, sun, and earth, causing that not only their distances vary, but also the three celestial bodies are not on the same plane. Consequently, the tidal forces of the moon and the sun on the earth sometimes strengthen each other but sometimes weaken each other, resulting in changes in tidal height and duration, among which there are four main phenomena: unequal half moon, unequal month, unequal declination and unequal day [6].
3.2.3.1.Half month unequal phenomenon
On the first day of the lunar calendar month and on the fifteenth or sixteenth day of the lunar month, the positions of the moon, sun and earth are roughly in a straight line, as the tidal forces of the moon and sun are in the same direction, so the tide forces themselves to cause mutual reinforcement to each other, resulting in a maximum tidal range that occurs in every half synodic month (14.7653 days), and the corresponding tide is called a spring tide or synodic tide. After the spring tide, the tidal range gradually decreases until the first quarter (on the eighth or ninth day of the lunar calendar) or until the second quarter (on the twenty second or twenty third day of the lunar calendar) of each lunar month, when the direction of the tidal force of the moon and the sun is close to orthogonal. At this time the mutual weakening is the most significant instead of mutual strengthening, resulting in a minimum tidal range, which also occurs in every half month and the corresponding tide is called as a neap tide or square tide. The successive and alternate replacement of large and small tides is called the phenomenon of half moon inequality. In fact, the high tide in the Chinese sea area usually occurs about two days after the lunar calendar days of ‘Shuo’ and ‘Wang’, while the low tide usually occurs about two days after the upper and lower chords [6].
3.2.3.2.Unequal monthly phenomenon
Due to the elliptical orbit of the Moon around the Earth, it takes 27.5546 days for the Moon to go through departure from perigee, to pass through apogee, and finally return to perigee. As a result, the tidal force exerted by the Moon on the Earth also undergoes the corresponding periodic changes, and the tidal force variation in tidal range caused by this reason is called the monthly unequal phenomenon of tides [6].
3.2.3.3.Declination inequality phenomenon
Due to the oblique intersection between the lunar orbit and the Earth's equatorial plane, the declination of the moon constantly changes: during each lunar month, half of the month is north of the equatorial plane whereas the other half of the month is south of the equatorial plane. Because the tidal force effect is the same between the first and second half of the month, the periodic duration is half a regression month (13.6608 days), and the corresponding tidal changes are called declination inequality phenomenon [6].
3.2.3.4.Daily unequal phenomenon
The phenomenon of daily variation in the height difference between adjacent high tides and low tides on the tidal curve, which goes through the circle with a period of 27.3216 days, is called as tidal diurnal variation. According to the overall effect of tidal inequality, the tidal forces can be analyzed on the basis of interaction between moon and sun motion orbits: for the moon, it passes through the equator approximately once in every two weeks, during which the inequality between adjacent high and low tides is minimal, and the corresponding tides are called equatorial tides; in comparison to the equatorial tides, when the moon is near its maximum declination in the north or south, there is significantly tidal inequality, and the corresponding tide is called the return tide. For the sun, its declination is the smallest around the vernal and autumnal equinoxes in each year. At this time if the moon just appears near the equator, then the tidal inequality phenomenon becomes the smallest due to the mutual minimization of inequality between the sun and moon, and the corresponding tide is called the equinox tide; During the summer and winter solstice, the sun's declination is at its maximum, and if the moon's declination is higher at this time, the tidal inequality phenomenon is the greatest due to the mutual maximization of inequality between the moon and sun, whose corresponding tide is called the solstice tide [6].
3.2.4.Main types of tidal
It is considered that tidal phenomena can be composed of many tidal components with different periods and amplitudes. Among these tidal components, the most important factors include the main lunar semi diurnal component M2, the main solar semi diurnal component S2, the lunar-solar declination daily tide K1 and the main lunar daily tide O1, with amplitudes of HM2, HS2, HK1 and HO1 respectively. Therefore, the ratio of the amplitudes of these four basic tidal components is usually used as to interpret and classify the characteristic value of tides [6]:
The first type of eigenvalue: (HK1 + HO1)/(HM2 + HS2)
The second type of eigenvalue: (HK1 + HO1)/ HM2
According to the two eigenvalue, tides are divided into four types, including semidiurnal tides, mixed irregular semidiurnal tides, mixed irregular diurnal tides and diurnal tides, which is described below [6]:
3.2.4.1.Half day tide
The eigenvalue range of half day tide is that the first type of eigenvalue is less than 0.25 or the second type of eigenvalue is less than 0.5, which has two high tides and two low tides in a cloudy day with the duration of 24 hours and 50 minutes in total, showing roughly equal heights between adjacent high tides or between adjacent low tides, such as the tides in Xiamen and Tanggu, China [6].
3.2.4.2.Mixed irregular half day tide
The eigenvalue range of mixed irregular half day tide is that the first type of eigenvalue ranges from 0.25 to 1.5, or the second type of eigenvalue ranges from 0.5 to 2. There are two high tides and two low tides for mixed irregular half day tide in a lunar day, but the tide heights between the two high tides or between the two low tides are different, and the tide rising and falling are also different correspondingly. The examples of this mixed irregular half day tide are the tides in Magong and Anping, Taiwan Province, China [6].
3.2.4.3.Mixed irregular diurnal tide
The eigenvalue range of mixed irregular diurnal tide is that the first type of eigenvalue ranges from 1.5 to 30, or the second type of eigenvalue ranges from 2 to 4, which becomes the mostly irregular half day tide within half a month, with a few days experiencing a full day tide phenomenon that only displays one high tide and one low tide in a cloudy day. The representative examples of mixed irregular diurnal tide include the tides in Yulin, Jieshi Bay, and Lingshui Bay in Guangdong Province, China [6].
3.2.4.4.Diurnal tide
The eigenvalue range of diurnal tide is that the first type of eigenvalue is greater than 30, or the second type of eigenvalue is greater than 4. For the diurnal tide, there are a high tide and a low tide in a single overcast day, whose duration can be more than 7 consecutive days in a lunar month. In a few days there is only a small tidal range, which exhibits the characteristics of semi diurnal tide phenomenon. The examples of diurnal tides include the tides in Beihai, Beili and Weizhou Island in China [6].
3.2.5.Ocean tide energy utilization
In summary, ocean tides are forced vibrations caused by tidal forces from the moon, sun and other sources, forming their own rotating tidal wave systems together with the influence of Earth's rotation and topography. Tidal energy is the ocean energy that appears in the form of potential energy, generated by the potential energy of water rising and falling of seawater tides, with theoretical reserve of tidal energy of approximately 3 × 109 kw worldwide. One of the important functions of tidal energy is to generate electricity, so many countries around the world have selected a considerable number of suitable sites for the development of tidal energy. According to the latest estimate, the tidal energy with potential for development is about 200 TW·h per year. For example, in 1912 the world's earliest tidal power station was built in Busum, Germany [6].
3.3.Windy ocean currents (Drift)
Wind current is the current caused by the continuous shear stress of prevailing winds on the sea surface, and its velocity is influenced by multiple factors, including the tangential force of the wind, the vertical turbulence coefficient, and the geographical latitude of the location. Because the wind directions in the trade-wind zone, westerly wind zone, and polar easterly wind zone are relatively stable, these directional winds rub against the surface water in the ocean and the wind can transfer some of its energy to the surface seawater through friction. In addition to forming waves, it also drives the surface seawater to move, thus forming wind currents [11].
In the Northern Hemisphere, the seawater circulation that flows around the center of the subtropical high is clockwise, while the circulation that flows around the sub-polar low (mid latitude low) is counterclockwise. However, in the southern hemisphere, the circulation corresponding to the subtropical high pressure zone is counterclockwise, but both the sub-polar low pressure and polar high pressure are basically strip-shaped, so the ocean currents in those zones are parallel to the latitude circle, which means that the clockwise cyclonic circulation corresponding to the northern hemisphere does not exist in the southern hemisphere [16].
The force driven by wind firstly imposes on the sea surface, forming wind-driven ocean currents at surface layer. However, this windy ocean forces is reduced by the consumption of momentum due to the viscosity in seawater movement, so that this flow weakens with the increasing water depth until it becomes negligible at the deep water, which causes that the windy ocean current depth involved is usually only a few hundred meters, becoming a thin layer compared with the total ocean depth of several thousand meters [7].
The surface layer with a depth of less than 200-300 meters in the ocean is defined as the wind-induced drift layer. The combined forces of both wind stress and horizontal turbulent stress exerted by the planetary wind system on the sea surface, which are balanced with the deflection force of earth rotation (also named as Coriolis force), together generate wind-induced drift [7]. Coriolis parameter characterizes the displacement of a mass particle undergoing linear motion in a rotating system due to its inertia relatively to the linear motion of the rotating system, which comes from the inertia of an object's motion [8]. The magnitude and direction of wind force in planetary wind systems varies with latitude, leading to both convergence and divergence of seawater on ocean surface, whose effects can be divided into two aspects: firstly, it redistributes the density of seawater and creates the horizontal pressure gradient on the surface of wind-induced drift layer. As the wind-induced force balances with the deflection force of earth rotation, the horizontal geostrophic flow can be formed in the relatively thick horizontal layer; Secondly, at the bottom of the wind-induced drift layer in the equatorial region the seawater flows upward into the sub-surface water, or flows downward from the subsurface water, forming the ascending and descending currents in the equatorial region [7].
3.4.Compensating current
Due to the continuity and incompressibility of seawater, when seawater flows away from one location, seawater from the adjacent sea area also flows into this location as to supplement it, resulting in compensatory flow that displays at both horizontal and vertical directions. On the continental shelf or the ocean areas with shallow water depth, due to the significantly increased friction on the seabed or on the coast, coupled with particularly strong currents, there are complex shallow sea currents formed, such as continental shelf circulation, shallow inland sea circulation and strait currents [7]. For these types of compensating currents, current direction can be horizontal or vertical, and vertical compensation flow is further divided into two types: upward flow and downward flow [15].
For example, compared with the seawater at the middle of the sea basin, the depth of the seawater is relatively shallow at the edge of the sea basin, receiving more heat from solar radiation, so the water temperature is higher, which causes the seawater to expand and rise, resulting in lower density and higher sea level. In the middle of the sea basin, the depth of the seawater is deeper, receiving less heat from solar radiation, so the water temperature is lower, which causes the seawater to shrink and sink, leading to higher density and lower sea level. As the consequences, surface seawater flows from the periphery of the basin to the middle of the basin, while the lost seawater around the basin is replenished by the rising seawater from the seabed, forming the seawater circulation, so upwelling compensating current is commonly distributed at the edge of sea basins [15].
3.5.Density current
The different temperature and salinity concentration of seawater in various sea areas can cause differences in seawater density, leading to variation in seawater levels, which results in a tilt of the sea surface between two sea areas of different seawater densities and causes the flow of seawater. The ocean currents resulted in by this density driving force are called density currents. The intrusion of high-density fluids into low-density fluids is mainly caused by static pressure due to gravity and density differences. For example, the Mediterranean evaporates vigorously, resulting in higher salinity and lower water surface, while the adjacent Atlantic ocean has higher water surface, so the surface seawater of the Atlantic flows into the Mediterranean through the Strait of Gibraltar, but the Mediterranean seawater flows into the Atlantic from the bottom of the strait as the circulation [10]. Consequently, density currents usually occur together with compensating currents: when the seawater in a certain sea area flows out due to the density variation, the seawater in the adjacent sea areas comes to supplement, and the resulting ocean currents should be classified as compensating currents, which can flow horizontally or vertically. For the vertical compensating currents, they can be divided into ascending currents and descending currents. For example, the Peruvian cold current is classified as the ascending compensating current [7].
The hydrodynamics of density current is discussed in more detail as below: although the density of seawater is determined by multiple factors including its temperature, salinity and pressure, with variation in its horizontal distribution from place to place, usually the salinity range of seawater is not large, whereas the temperature difference of seawater is large. Therefore, the density of seawater is mainly depended on the temperature of seawater. For example, in the sea area where the temperature increases due to receiving more heat from the sun, the volume of water body expands, resulting in the decrease of the density, and the sea surface (measured as isobaric surface) slightly rises; In another sea area where the density relatively increases because of less amount of solar heat received, the water temperature decreases, and the volume of water body shrinks, resulting in relatively lower sea surface (measured as isobaric surface). The equipotential surfaces of the sea surface between the two sea areas are inclined to some extent, meaning that the pressure on any horizontal plane (measured as equipotential surface) inside the seawater is not the same. Under the action of horizontal pressure gradient force, seawater flows from areas with high pressure to areas with low pressure. Once the seawater begins to flow, the Coriolis force immediately affects its motion, pulling the seawater that should have flowed in the direction of the horizontal pressure gradient to alter into the right direction in Northern Hemisphere, until the Coriolis force is equal in magnitude to the horizontal pressure gradient force but is opposite in direction. The ocean current then flows along the intersection of the isobaric and equipotential surfaces, moving at a constant speed. At this intersection line, the ocean current is exactly defined as density flow. Obviously, if the observer faces the flow direction in Northern Hemisphere, the left isobaric surface is low and the right isobaric surface is high; The density is higher on the left and lower on the right, while the water temperature on the left side is low and the water temperature on the right side is high. In the southern hemisphere, the above direction is the opposite one as counter hydrodynamics. The hydrodynamics of most powerful ocean currents in the world, such as the Gulf Stream, Kuroshio Current and Benguela Current, are all related to the distribution variation in seawater density [10].
Other factors forming density currents include the thermohaline effects of ice formation, melting, precipitation and evaporation in the ocean, resulting in uneven distribution of seawater density over a large area of the sea surface, which can cause high-density seawater to be generated in the surface layer of certain polar and high latitude sea areas, sinking into the deep and bottom layers. Under the effect of horizontal pressure gradient force, the high-density seawater flows horizontally in the deep layer, which subsequently flows upwards through the middle layer from the water bottom layer to the surface layer, forming the oceanic thermohaline circulation [7].
The wind drift generated on the surface of the ocean constitutes the wind-driven circulation on the surface of the ocean. For example, the North Equatorial Current at low latitude and the South Equatorial Current located at mid latitudes are blocked by the coast at the western boundary of the ocean, causing their main currents to turn north and south respectively. Due to the variation of Coriolis parameters with the change of latitude (β - effect) and the effect of horizontal turbulent friction, these wind drift forms the strengthened ocean westward flow with narrowed flow spokes and increased flow velocity [8]. Half of the total heat energy transferred from the equatorial region to the high latitude regions of the Earth in each year is carried by these westward strengthening flows along the western boundary of the ocean. The velocity of thermohaline circulation entering the upper layer of the ocean increases in the northern hemisphere due to its direction that is the same as the westward strengthening flow of the ocean. In comparison, due to the opposite direction and slower flow velocity, the westward strengthening influence on ocean circulation in the southern hemisphere is not very significant [7].
For another example, the wind-driven circulation in the mid latitude and high latitude regions of the southern hemisphere on the ocean surface forms an Antarctic Circumpolar Current that continuously flows around the Antarctic continent due to the absence of continental coastal barriers [7].
In the eastern and nearshore waters of the ocean, the wind blows uniformly, which is almost parallel to the coast for a long time, so that the wind drift is generated and horizontal convergence and divergence of seawater occurs, resulting in both upwelling and descending currents; Concurrently, due to the accumulation and loss of seawater near the coast, the sea surface tilts and the force of horizontal pressure gradient occurs, resulting in coastal currents and forming both upwelling and downwelling streams along the coast [7].
3.6.Cold current and warm current
Ocean currents generated by the temperature variation can be divided into two types: cold currents and warm currents, which is depended on whether their water temperature is lower or higher than the water temperature of the sea flowing through. The cold current comes from areas with lower water temperatures, while the warm current comes from areas with higher water temperatures. The horizontal flow velocity of ocean currents ranges from a few centimeters per second to 300 centimeters per second at the surface layers, while the horizontal flow velocity at deeper water depths is less than 10 centimeters per second. The vertical current velocity is very slow compared with horizontal flow, ranging from a few centimeters per day to several tens of centimeters per hour [7].
3.7.Geostrophic flow
Geostrophic flow is the stable ocean current when the forces are balanced between the horizontal pressure gradient and horizontal geostrophic force in the ocean area where the turbulent frictional force can be ignored [7]. Geostrophic current can be also defined as the the ocean current occurring in the deep ideal ocean where the horizontal pressure gradient force generated by the uneven distribution of seawater density is balanced with the horizontal geostrophic deflection force, ignoring the effect of turbulent frictional forces. These two different directional forces continuously change the direction of seawater flow, which leans to the right at the northern hemisphere and leans to the left at the southern hemisphere, until both the horizontal pressure gradient force and the Coriolis force reach equilibrium, and then the flow reaches stability [12].
There are mainly four characteristics with regards to the geostrophic current: the magnitude of the geostrophic flow velocity is directly proportional to the tangent of the angle between the isobaric and equipotential surfaces, but is inversely proportional to the Coriolis parameter; Flowing along the intersection line of two sides (one side is isobaric surface and the other side is equipotential surface), the flow direction in the northern hemisphere is 90 degrees to the right of the horizontal component of the pressure gradient force; In the Northern Hemisphere, when it is to face the direction of flow, the isobaric surface is higher on the right but lower on the left; The inclination of the isobaric surface caused by the internal pressure field is mainly reflected in the upper layer of the ocean and decreases with the increasing depth, whereas the inclination of the isobaric surface caused by the external pressure field directly reaches the seabed from the ocean surface [12].
For example, one of the representative theory is the Ekman Drift forming at the surface ocean layer, driven by three kinds of natural force: wind force, horizontal geostrophic deflection force (Coriolis force) and friction force from deeper water layer. The flow direction of surface seawater is 45 degrees away from the wind direction in theory, with the rightward deviation in the northern hemisphere and the leftward deviation in the southern hemisphere. When surface seawater begins to flow under the effect of wind shear stress, the Coriolis force immediately imposes on it, causing the direction of wind current flow to be inconsistent with the wind direction. Due to the fact that the direction of the Coriolis force in the northern hemisphere is biased to the right of the direction of object motion, and in the southern hemisphere it is biased to the left of the direction of object motion, the flow of wind currents is biased to the right of the wind direction in the northern hemisphere and to the left of the wind direction in the southern hemisphere, respectively. In addition to the shear stress and Coriolis force of the wind, the surface seawater flow is also affected by the internal friction force between surface water and the deeper seawater. Therefore, the factor determining the magnitude of the wind current and wind direction deviation angle is the balance of these three forces [13]. As the depth increases, the angle of deviation from the wind direction becomes larger, but the flow velocity decreases accordingly. When it reaches a certain depth, the direction of drifting is exactly opposite to the direction of surface drifting currents, with a flow velocity of only about one twenty third of the surface flow velocity, and then this critical depth is called the frictional depth in oceanography. Therefore, it is usually considered that Ekman Drift is only formed at the surface layer of seawater flow within the range of friction depth (usually about 500m of water depth), whereas the current velocity below this friction depth is slow, which can be ignored [16].
Although Ekman Drift is a kind of idealized ocean current, it can approximately reflect some of the motion laws of seawater in theory. For example, the ocean currents in the thicker lower layers of the ocean are similar to geostrophic currents only, whereas in the thinner and upper layer of the ocean, there are both geostrophic currents and Ekman drift, both of which become the fundamental flows of the ocean [12].
3.8.River discharge
River discharge current is the flow of seawater caused by river runoff into the sea near the estuary [7].
3.9.Rip current
The rip current propagates from the open sea to the coast and is generated when the wave breaks in the wave breaking zone, moving from the shore to the deep water direction [7]. Consequently, rip current is the narrow and strong flow moving from the coast into the sea, which flows outwards in the direction perpendicular to the coast. The narrow width of rip flow generally does not exceed 10 meters and its length is usually between 30-50 meters, sometimes reaching up to 700-800 meters. Although this stream of rip flow is not long, its speed can be very fast, with a flow rate of over 2 meters per second. The duration of each stream is usually two to three minutes, and sometimes can be even longer, so it can quickly drag people into the water. Rip current often occurs at the interruption of white waves, beach gaps or low-lying areas, rocky areas, which are perpendicular to the trenches of coast, pocket shaped coasts, etc [14].
The formation of rip flow can be explained as below: waves spread to the shore, or winds blow towards the shore, which may form the buildup of seawater on the shore firstly, subsequently leading to the backflow of seawater offshore. The returning seawater gathers in areas with small waves near the coast and then turns towards the sea along a narrow band, resulting in considerable flow velocity. For example, the most common case is when water breaks through the obstruction of sandbars. The sandbar is the narrow sand dune that accumulates along the outer side of the coast. Seawater usually clusters on the lower surface inside the sandbar, but in some cases, the receding water flow can generate sufficient backward pressure to form a breakthrough in certain parts of the sandbar, immediately forming the rip current with narrow width and strong velocity [14].
3.10.Longshore current
Compared with rip current, longshore current direction is the current parallel to the coast, which is generated when waves propagate from the open sea to the coast and break up in a fractured zone [7].
3.11.Summary of ocean current types
In summary, the actual ocean currents that occur are always the result of a combination of multiple factors, among which the main cause of ocean currents is the variation in wind speed and seawater density. In addition to tidal movements caused by tidal forces, seawater flows on a large scale along relatively fixed paths in the ocean, whose driving forces that cause ocean currents to move commonly include wind or the uneven distribution of seawater density caused by the thermohaline effect. For the thermohaline driving force, the seawater flows horizontally in a regular direction, which are classified into warm currents and cold currents according to the water temperature. Generally, ocean currents flowing from low latitudes to high latitudes are likely to become warm currents, while ocean currents flowing from high latitudes to low latitudes are likely to become cold currents; Ocean currents can also be classified into wind-driven currents, density currents, and compensating currents based on their causes. The prevailing wind blows over the sea surface, pushing the seawater to drift with the wind and causing the upper seawater to drive the lower seawater to flow, which forms a large-scale ocean current called wind-induced ocean current, and the ocean systems on the surface of earth planet are mostly composed of wind-driven currents [7].
4.Meso-scale vortex and circulation
The westward strengthening current of the ocean flows northward in the northern hemisphere (or southward in the southern hemisphere correspondingly), then turning eastward. When it reaches the specific area, the flow becomes unstable, and the axis of the current mapping bends in wave shape near its average position, resulting in the bending (or meandering) of ocean currents. Finally, it forms the circular flow and separates from its parental streams, generating a cold current ring with cold water from the continental shelf and a warm current ring with warm water from the ocean interior, which becomes a type of meso-scale vortex with medium scale. In addition, in other parts of the ocean, due to the instability of ocean currents, other types of meso-scale eddies can also be formed. These meso-scale eddies concentrate significant amount of energy on the ocean, forming various weather like eddies that stack over the ocean's climate like average circulation field, making ocean circulation more complex[7].
In the continental shelf or shallow waters of the ocean, due to the significant friction between the coast (or the seabed) and strong currents, complex shallow sea currents such as continental shelf circulation, shallow inland sea circulation and strait currents are formed [7].
4.1.Overall pattern of ocean surface circulation in Earth
There are various current circulations on the surface of ocean in our planet, mainly including the anticyclonic oceanic circulation centered around the subtropical high in mid to low latitude sea areas, the cyclonic oceanic circulation centered around a low-pressure area in the mid to high latitudes of the Northern Hemisphere, the westerly drift in the mid latitude sea area of the southern hemisphere, the circumpolar circulation formed around the Antarctic continent, and the monsoon circulation formed in the northern Indian Ocean [8].
4.2.Anticyclonic oceanic circulation
Under the influence of the trade-wind belt, the trade-wind drift, including both South and North Equatorial Warm Current, flows westward. When the drift encounters the continent, a portion of the seawater turns back due to the uneven north-south velocity components of the trade-wind drift that yields shearing stress and compensation effect, forming equatorial counter current/back flow and equatorial subsurface current from west to east as anticyclonic oceanic circulation, while another portion of the trade-wind drift moves towards the north and south of high latitudes, forming the Kuroshio Current in the North Pacific, the East Australian Current in the South Pacific, the Brazilian Current in the South Atlantic, the North Atlantic Gulf Stream in the North Atlantic, and the Mozambique Current in the South Indian Ocean [8].
Under the influence of the westerly wind belt, westerly winds drift eastward and diverge into high latitude and low latitude currents on both sides upon encountering continents, consequently forming compensating currents as anticyclonic oceanic circulation. The ocean currents flowing towards low latitudes include the California Current in the North Pacific, the Peru Current in the South Pacific, the Canary Current in the North Atlantic, the Benguela Current in the South Atlantic, and the Western Australian Current in the South Indian Ocean [8].
In summary, there are the compensating currents commonly including trade-wind drift, trade-wind drift turning towards high latitudes after encountering continents, westerly drift, and westerly drift turning towards low latitudes after encountering continents, which are classified into the anticyclonic ocean circulation in the subtropical sea areas of various oceans. These oceans cover the following sea areas of the ocean: North Pacific, South Pacific, North Atlantic, South Atlantic, and South Indian Ocean [8].
4.3.Cyclonic ocean circulation
The cyclonic ocean circulation from the mid to high latitude of sea areas in the Northern Hemisphere is composed of compensating currents formed by the westerly drift that encounters land and splits northward, as well as the ocean currents formed by polar easterly wind belts on the west coast from the mid to high latitude ocean [8].
The ocean current circulation in the North Pacific includes the North Pacific Warm Current, Alaska Current, and Kuril Current, while in the North Atlantic, there are the North Atlantic Warm Current, Norwegian Warm Current, and East Greenland Cold Current [8].
The North Indian Ocean monsoon drift is influenced by two types of wind driving forces: the first wind force is the South Asian monsoon, with the northeast monsoon prevailing in the half year of winter, forming the northeast monsoon drift, and the other wind driving forces is the southwest monsoon prevailing in the other half year of summer, forming the southwest monsoon drift [8].
4.4.Antarctic Circumpolar Circulation
Under the blowing of the polar easterly wind belt, the Antarctic circumpolar circulation is formed around the Antarctic continent, and then a westerly drift is formed around the Antarctic continent in the lower latitude direction. Due to the consistent natural characteristics of this sea area, the outer sea area of Antarctica is sometimes named as the ‘Southern Ocean’[8].
5.Function of ocean current in earth ecosystem
Ocean currents lead to the impact and constraint on various physical, chemical, biological and geological processes in the ocean, which also affect the formation and changes of climate and weather over the ocean [7]. The overall importance of ocean currents, which play the role in Earth ecosystem, is firstly to transfer thermal energy from multiple ocean regions to other different ocean areas, balancing thermal energy, then to transfer nutrients from multiple oceanic regions to other different oceanic areas, and finally to distribute seawater with different oxygen contents from multiple ocean regions to other ocean areas due to the motion of ocean currents [8]. My another paper has discussed that the terrestrial vegetation is the main source of providing oxygen in our earth planet [17], and the atmospheric oxygen should become the soluble oxygen in ocean water by the mixture effects of wind, sea wave and precipitation, supplying the compulsory survival condition for ocean creatures. This oxygen process among terrestrial vegetation, atmospheric oxygen, and soluble oxygen of ocean is discussed in detail in Figure 2 below.
5.1.The effects of ocean current on climate
For example, warm currents result in the warming and humidifying effect on coastal climate, whereas cold currents cause the cooling and dehumidifying effect on coastal climate; In the sea area where cold and warm currents intersect, the disturbance of seawater can bring nutrients from the lower layer to the surface layer, which is conducive to the large-scale reproduction of fish and provides bait for them; Both warm and cold currents can also form a kind of 'water barrier', hindering fish activity and concentrating fish populations, which is beneficial to form large-scale fishing grounds such as the Newfoundland fishing ground and the Hokkaido fishing ground in Japan [7].
The effect of ocean currents on precipitation and fog differs between warm and cold currents: heat and water vapor are transported upwards over warm currents, resulting in unstable stratification and increasing air humidity, which leads precipitation more likely to occur, whereas the cold current generates temperature inversion, so the stratification is stable and the water vapor is not easily transported upwards with weaker evaporation. Although the relative humidity in the lower layer is sometimes high over the cold current, it can only form fog without sufficient meteorological conditions forming rain. The surface of the cold current is often covered with advection fog, classified into sea land wind fog and sea fog. When sea land wind fog meteorological condition occurs, land wind flows to the surface of the cold current during the day and forms advection fog. In comparison, sea fog meteorological conditions is different from sea land wind fog, under which wind blows from the surface of the warm current to the surface of the cold current, forming advection fog at the confluence of both warm and cold currents [8].
Overall, the temperature and humidity is increased in the atmosphere of warm currents, while temperature and humidity is decreased in the atmosphere of cold currents. The influence of ocean currents on atmosphere temperature causes the transfer of heat from low latitudes to high latitudes, especially converted by the warm currents. However, for the same latitude, the effect of ocean currents on temperatures at both sides of continents is that warm currents pass through the coastal areas of continents with higher temperatures, while cold currents pass through the coastal areas of continents with lower temperatures [8].
5.2.Effects of ocean currents on marine organisms
For example, some sea areas are affected by offshore winds, and the upwelling of deep seawater brings a large amount of nutrients to the surface, forming fishing grounds such as the Peruvian fishing grounds [7]. This nutrition transport particularly occurs in the areas where the warm and cold currents intersecting causes disturbance to the seawater. The four famous fishing grounds in the world and their ocean currents include Hokkaido Fishing Ground located near Hokkaido Island, Japan, where the Japanese Warm Current and the Kuril Cold Current intersect; North Sea Fishing Ground located in the European North Sea, where the North Atlantic Warm Current and the Arctic East Wind Belt bring cold water southward from the Arctic Ocean; Peruvian fishing grounds where Southeast trade winds prevail along the coast, which are offshore winds that cause upwelling compensating currents; Newfoundland Fishing Ground located near Newfoundland Island, Canada, where the North Atlantic Warm Current and Labrador Cold Current intersect. Especially for another example, penguins species habitat in the Equatorial Region, which is distributed in the Colon Islands (also known as the Galapagos Islands) in the eastern Pacific Ocean, is also established naturally due to the Peruvian cold wave [8].
5.3.Ocean currents and sailing
Ocean currents influence the sailing and shipping transportation in the ocean, and usually sailing along ocean currents can save fuel and accelerate speed, but when warm and cold currents meet, sea fog is often formed, which is unfavorable for maritime navigation. In addition, the Arctic Ocean carries icebergs southward from the Arctic region, posing a significant threat to maritime navigation [7].
5.4.Ocean currents and environmental pollution
Ocean currents can also diffuse pollutants from nearby waters to other areas, which is beneficial for the diluting of pollution and accelerates the purification process, but as a result other sea areas may be contaminated as well, expanding the scope of pollution [7].
5.5.Ocean currents and electricity
In ocean motion, ocean currents do not only play an important role in the climate and ecological adjustment to achieve eco-balance of the Earth, but also can drive turbines to generate electricity and deliver green energy to people. The ocean currents follow the certain route and move repeatedly, with the scale that is thousands of times larger than the giant rivers and streams on Earth land, so the potential energy being capable of utilization is considerable. For example, among all the ocean currents there is a very large scale ocean current that can be called as ‘giant’ in ocean currents, which is the famous ‘Gulf Stream’ with width of 60-80 kilometers and thickness of 700 meters, and its total rate of flow is from 74 to 93 million cubic meters per second. This total flow is nearly twice as large as the Kuroshio Current in the North Pacific that is the world's second largest ocean current, and is even more than 80 times the total flow amount of all rivers on Earth land [7].
Consequently, understanding the motion orbits of ocean currents, the mutual interactions between large-scale sea air and long-term climate change are of great significance for fisheries, shipping, pollution control and military affairs [7]. In summary, ocean currents play an important and positive role in balancing and driving the Earth's biosphere and abiotic (both chemical and physical) environment, providing positive assistance not only for the survival of most marine organisms, but also for those on land [8].
6.Representative current
6.1.Kuroshio Current
The Kuroshio Current is the strongest ocean current in the Pacific region, named due to its deep blue color and black appearance. Compared with the sea area which it flows through, it displays the characteristics of high temperature and high salinity, which is hence called the Kuroshio Warm Current as well. There are several streams divided when the Kuroshio Current enters the East China Sea. According to its traditional definition, the Tsushima Warm Current runs north along the west coast of Kyushu, west of Amami Oshima Island. When the Kuroshio Current goes through the south of the Goto Islands, it splits into two streams: the main stream flows northeast through the Korea Strait into the Sea of Japan, while the western stream enters the South Yellow Sea through the south of Jeju Island, forming the Yellow Sea Warm Current. The Kuroshio Current is divided into two streams in the southeastern water of Taiwan. The mainstream heads northwards, and the other stream flows northwest into the Bashi and Balintang Strait (the western branch of the Kuroshio Current), which then splits into two streams in south of Taiwan: the larger stream flows southwest into the South China Sea, forming part of the winter circulation in the South China Sea, while another small stream enters the Taiwan Strait and heads north along the eastern side of the strait [7].
The Kuroshio Current is known due to its strong flow velocity, narrow flow amplitude and large thickness. The flow rate is generally 1-3 knots, reaching 3.5 knots at east of Suao. After entering the East China Sea, the flow velocity decreases slightly to approximately 1-2 knots, but the flow velocity increases again, reaching 2.5 -3 knots at the location around 26° N & 126° E and even up to 3.4 knots at southwest of Yakushima Island. On the cross-section of flow velocity, the Kuroshio Current is often consisted of two flow cores northward or northeastward, separated by a reverse flow core. The stronger flow core is located near the coast, indicating the strengthening of the oceanic circulation towards the west coast. However, in the vicinity of Okinawa and Amami, the Kuroshio Current often generates backflow on the right side of its main stem, due to the friction with the terrain, but this counter current velocity is not large, about 0.3-0.5 knots, and the thickness is relatively shallow, with an average flow rate of only approximately 1/5 of the mainstream [7].
The Kuroshio Current shows the relatively narrow range, with the averaging range of less than 100 nautical miles, and the strong current zone with more than 2 knots is only 25 nautical miles. The thickness of the Kuroshio Current is about 800 meters, which can be divided into four water layers from top to bottom: surface water, subsurface water, middle water and deep water. Although the Kuroshio is a stable and powerful ocean current, there is significant change in flow velocity, flow rate and amplitude. Moreover, the flow axis of Kuroshio Current also swings and bends along the current orbit. With variation in time, there are various cycles including long, medium and short cycles, and from the spatial perspective there is medium to small-scale change in spaces as well. To the east of Taiwan, the northward flow and velocity of the Kuroshio Current exhibits a six-month cycle, with the maximum velocity in spring and autumn (at the velocity of 120 cm/s) and the minimum velocity in winter and summer (at the velocity of 50 cm/s), respectively. In addition, there are several cold and warm eddies appearing on both sides of the Kuroshio Current [7].
6.2. The hydrology environment of Philippines Sea
The map of Temperature-Salinity point clustering in the sea area of Kyushu Palau Ridge displays as an inverse ‘S’ shape on the graph, which is roughly divided into four types according to the temperature and salinity structure: surface high temperature and low salinity water (HTLS), subsurface high temperature and high salinity water (HTHS), mid-level low temperature and low salinity water (LTLS), and deep low-temperature and low salinity water (LTHS). The Temperature-Salinity points in the surface, subsurface, and mid-level are discrete, indicating strong mixing of seawater in these depth levels. The surface high temperature and low salinity water are located at depths of 0-50m with the highest water temperature (28 ℃ -30 ℃) that is attributed to solar radiation on surface, but the lowest salinity (33.5 ‰ -34.5 ‰) is found at this depth, which is attributed to the dilution effects by surface precipitation; The secondary layer of high-temperature and high salinity water is located at the depth of 100-200m (24 equipotential density surface), where water temperature (15 ℃ -28 ℃) is lower than it at the surface layer, but the highest salinity (34.7 ‰ -35.3 ‰) is reported at this depth; Middle level low-temperature and low salinity water is located at the depth of 300-1000m, with water temperature (4 ℃ -16 ℃) higher than deep layers, but there is seawater with lower salinity (34.1 ‰ -34.5 ‰); Deep low-temperature and low salinity water is located at depths of over 1000m, and the lowest water temperature is less than 5 ℃, with the salinity ranging from 34.5 ‰ to 34.7 ‰ [19].
The Northeast trade winds prevail long-termly in low latitude regions of the Northern Hemisphere, coupled with the influence of geostrophic force tending to the right direction, which forms the Pacific North Equatorial Current flowing from east to west in the 8° N-20° N of sea area. This equatorial current crosses the Pacific Ocean, mainly flowing through the southern part of the Philippines, where the horizontal flow amplitude is about 2000km and the vertical flow amplitude is about 250m, with the average horizontal flow velocity of about 0.4m/s and constant vertical flow velocity at 0.1m/s, respectively. In the flow area of this equatorial current, the flow velocity near the equator is relatively high, reaching 0.55m/s in the southern Philippines, which provides the dynamic conditions for the consistency in both temperature and salinity structure of Philippine Sea, especially in the 10° N-20° N of sea area [19].
From the equator to 3° N, the Pacific South Equatorial Current is formed, featured by an average flow velocity of about 0.3m/s and a water depth of about 200m (0.1m/s isokinetic line). On the equatorial windless zone located at the south of the North Equatorial Current, Pacific Ocean, there is a west to east ocean current known as Equatorial Counter Current (NECC) passing across the Philippine Sea between the South and North Equatorial Currents. Analysis shows that the equatorial cross-section of southern boundary of this counter current (NECC) is located at about 3° N, and the northern boundary is about 5° N. The water depth of this equatorial counter current is below 500m, with the maximum flow velocity reaching 0.6m/s, but the subtropical counter current is weak or even no longer apparent. It should be noted that the North Equatorial Current and Kuroshio Current may lead to significant impact on the east-west distribution of both temperature and salinity structures in the sea areas, showing significant variation between south of 10° N and north of 20° N [19].
Overall, the ocean currents in the Philippine Sea are mainly distributed as a band-pattern in shallow water depth below 500m, and especially in shallow water below 200m, where the flow velocity is significantly affected by ocean current dynamics. The current velocity at a depth of 500m significantly slows down, especially with the disappearance of the North Equatorial Current. The sea area at north of 20° N is mainly consisted of the afterflow of Kuroshio Current and weaker subtropical counter currents, while both equatorial currents and western boundary currents are present with significantly increased flow intensity in the South China Sea below 20° N. Both the vertical and horizontal distribution of these currents may be the main driving factors causing significant regional differences in temperature and salinity structures at the upper layer of shallow water depths below 500m [19].
In summary, Philippine Sea exhibits significant differentiation in hydrological environment with dynamic configuration characteristics. The marine hydrology is characterized in horizontal and vertical distribution separately: the horizontal distribution of both temperature and salinity structures shows a differentiation pattern of five zones, in which the north-south boundary is located at approximately 20° N and 10° N, and the east-west boundary is located at approximately 137° E, showing significantly regional differences; The vertical distribution follows a pattern of ‘three lines and one circle’: at the water depth of 200m as the boundary, the upper layer exhibits significant seasonal variations and distinct regional characteristics, while the lower layer shows less seasonal variation, but the temperature and salinity structure at water depths of 200-500m shows an inverse distribution pattern compared with the upper layer. At deeper layers, the temperature and salinity structure at depths below 500m changes little without significant difference, and there is the sub-layer with low temperature and low salinity at water depths of 500-1000m in the sea area north of 20° N [19].
7.Discussion and research gaps in ocean hydrology
The process of the boundary layer formation at each substance state (gaseous, liquid, solid) can be modeled by 3D modeling technology discussed in my another article [2], and the preconditions of boundary layer formation should include two conditions: one is that the spatial variation in substance’s density and composition should reach the threshold, which is reflected by the variation along the spatial scale of modeling; the other is that the relative speed of substance motion is low enough to form boundary layers, which is measured by the variation along the temporal scale of modeling. The higher variation per spatial scale and lower variation per temporal scale in this 3D modeling would tend to form boundary layer more efficiently. The higher spatial variation in substance density and composition will facilitate magnetization between different substance layers, initiating boundary layer formation as discussed in my article [3], while the less temporal variation in substance density and composition reflects the lower relative speed of fluid substance’s motion, increasing the sink effects of fluid substances on the boundary layer, which consequently enhances the stability of substance boundary layers. Consequently, this boundary layer formation sub-model would fill in the research gaps in 3D modeling in atmospheric, geological and hydrology environment, which will be firstly discussed in this hydrology paper.
Various hydrology factors that form and drive ocean currents as described above have been fully summarized in Table 1. However, both formation and breaking out of water boundary layers, which plays the key role in the causes of turbulence flow, must be added to the Table 1 as the supplement to marine hydrology study, becoming the research gap in future ocean investigation. There is the Figure 1 below to illustrate the water boundary layer theory in my paper.
Table 1. Summary of hydrology forces forming and driving each type of ocean current.
Hydrology elements | The ocean current types applied |
| Warm current; density current |
| |
| |
Cold shrinkage; Convergence and divergence of seawater | |
| |
| |
Gravity forces (Earth, Moon and Sun) | |
Uneven density distribution of water | |
Temperature and salinity variation | |
| |
The vertical turbulence coefficient | |
The geographical latitude of the location | |
Friction forces on the seabed or coast | |
Friction forces between different water layers | |
Horizontal pressure gradient caused by various factors | Wind drift;density current;compensating current; |
The formation and breaking out of seawater boundary layer | Seawater circulation between surface layer and deeper layer; |
| |
The interruption of white waves, beach gaps or low-lying areas, rocky areas on coast | |
Breaking up in a fractured zone | |
My article above has discussed the hydrodynamics of ocean current according to the representative case study of Kuroshio Current. Here it is to further discuss the formation mechanism of the back flow/counter current in detail on the basis of Figure 1: firstly, along the coast area the current water is imposed by the friction force (F1) generated from the flushing movement between the coast and water, and the water pressure (P1) caused by the friction forces can be efficiently conducted in the same water layer; secondly, the breaking out of water boundary layer near the coast area results in the upwards water flow from bottom layer to surface layer, which leads to the resistance force and pressure (P2) in the surface layer against the flow water of Kuroshio Current. Consequently, both water pressure caused by the friction force and the resistance pressure imposed by the upwards water flow lead to the back flow/counter current, which becomes the reverse flow core in the middle separating two flow cores of Kuroshio Current described above. Additionally, the upward water flow from deeper layer to surface layer leads to higher water surface level and horizontal pressure gradient, which also causes the force driving back flow. This hydrodynamics is also applicable on the explanation of back flow near the coast of other locations in ocean.
发表于 2023-11-22 10:52:17
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