Choose a region of the world ocean and provide a concise review of its circulati

Question

Choose a region of the world ocean and provide a concise review of its circulation and the influence of these flows on an aspect of marine science of particular interest.

Include:

The forcing of these flows. Ex. Upwelling and/or downwelling; density fluxes tied to atmospheric forcing or riverine inflows…

Description of the dyna mics involved. Ex. River plume; boundary current; fronts….

Larger scale contribution and linkages to global ocean circulation and climate. Ex. Local thermohaline features; ENSO forcing, …

Discussion of role(s) circulation plays in aspects of biogeochemical processes’ coral or other benthic ecosystems; fisheries; and/or climate.

Explanation / Answer

Among all the world’s oceans, Pacific ocean is not only the largest and deepest, it is the seat of the global climate cycle known as "El Nino", which is also the one of the major controller of the global ocean circulation and global climate. Here I have talked about some of the important aspects of oceanography in terms of the tropical Pacific. The scope of discussion is huge, but I have confined myself to only those which are of prime significance.

The tropical Pacific is usually considered to lie between the astronomically-defined tropics - the Tropic of Cancer (23°N) and the Tropic of Capricorn (23°S).. There is a great number of islands around Tropical Pacific, and the warm surface water of tropics sustain the coral reefs, one of the most attracting nature built features seen. Compared with areas at higher latitudes, the frequency of storms and the average strength of the winds are low here.

Temperature : The tropics are a region of excess heating from the sun and towering cloud convection and rainfall in narrow bands across the Pacific . This leads to a sea surface temperature difference from equator to pole of about 30°C. In the tropical Pacific, the maximum sea surface temperature is around 28°C and can rise to at most 30°C. This is considerably cooler than the maximum temperatures regularly found over land, of about 50°C. current researches suggest that along with the greater sea surface temperature, cloud formation also increases significantly. As cloud are good reflector of the sunlight (increasing albedo), they hinder the solar radiation reaching the sea surface, thus keeping the surface temperature comparatively cooler.

The sea surface temperature is not uniformly distributed in the tropical Pacific. In normal situation (also “La Nada”), a large warm water pool is found in the central and western Pacific, while the Easter Pacific is much cooler. This is because of the vertical thermal structure of the upper ocean. In the western Pacific, the surface layer, which is fairly well mixed, is approximately 100 meters thick and warmer than about 28°C. Just below this surface layer, the temperature changes rapidly downward (for instance at 100-150 meters depth in the west in) this is called the "thermocline". In the central and eastern Pacific, the surface layer is shallower, and so colder water and the thermocline are found closer to the surface.

Salinity : Salinity of seawater is the factor of density (more density, more salinity), which in turn depend on temperature (warm water is less dense), amount of material dissolved, input and output to the sea, atmospheric circulation. Along the equator surface salinity is lowest in the western Pacific, where normally there is much more rainfall than in the central and eastern equatorial Pacific.

Surface circulation, Boundary Currents :

The Pacific sea surface circulation consists of two large "subtropical gyres" centered at 30°N and 30°S, which rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, a "subpolar gyre" centered at about 50°N and rotating counterclockwise, a major eastward flow which circles Antarctica called the "Antarctic Circumpolar Current", and complicated but predominantly zonal (east-west) currents in the tropics between about 15°N and 15°S. At the sea surface, flow is westward from 30°S up across the equator to about 5°N. This westward flow is all called the "South Equatorial Current". Between 5°N and 10°N lies a strong eastward flow, termed the North Equatorial Countercurrent. The westward flow between 10N and 30N is called the North Equatorial Current (NEC). The northern half of the NEC is actually part of the subtropical gyre and the southern half is part of the ITCZ's elongated counterclockwise flow. Sometimes a weak ITCZ (South Pacific Convergence Zone) is also present in the southern hemisphere, creating an occasional appearance of a South Equatorial Countercurrent analogous to the North Equatorial Countercurrent.

In the western tropical Pacific, the circulation is dominated by strong currents which abut the western boundary . Western boundary currents are a central feature of all circulation patterns worldwide. In the tropical Pacific, the western boundary currents are complicated by the many islands and deep ridges. In the North Pacific, the westward-flowing North Equatorial Current reaches the western boundary at Mindanao in the Philippines. It splits into a northward flow, called the Kuroshio, and a southward flow, called the Mindanao Current. The Kuroshio flows into the East China Sea and then northward to the southern end of Japan (Kyushu) where it splits into a major flow eastward along the eastern coast of Japan, and a weaker flow, called the Tsushima Current, into the Japan East Sea. The Kuroshio is one of the strongest currents in the world, similar to the Gulf Stream and the Antarctic Circumpolar Current in strength. It affects climate in Japan through its warmth and fisheries off Japan through both its warmth and relative lack of nutrients. The Mindanao Current flows southward along Mindanao and separates to flow eastward into the North Equatorial Countercurrent at about 5°N. A portion turns westward at the southern end of Mindanao and enters the Celebes Sea.

In the South Pacific, the very broad, westward-flowing South Equatorial Current reaches the western boundary through a complex of islands. The northern portion forms a northward-flowing western boundary current along New Guinea, called the New Guinea Coastal Current .This flows northward to the equator. A portion of it turns eastward along the equator and apparently forms part of the eastward-flowing subsurface Equatorial Undercurrent. A portion may continue slightly northward, joined by the westward flow just north of the equator, and then turns eastward, joining the separated Mindanao Current, into the North Equatorial Countercurrent.

The upper ocean circulation in the tropical Pacific is driven mostly by the stress from the wind. The prevailing winds in the tropical Pacific are the trades or easterlies, which blow from east to west. Together with the westerlies of higher latitudes, these force the large subtropical gyres. The dominant influence of these gyres on the tropics is the broad-scale westward flow, called the North Equatorial Current (north of 5°N) and the South Equatorial Current (from the equator southward).

Deep circulation : With increasing depth, the surface circulation weakens and shifts latitude. In the tropics, the surface circulation signatures disappear by about 500 to 1000 meters depth. Flow beneath this is predominantly zonal (east-west) with very slight north-south movement.

Upwelling : Upwelling in western pacific is weaker than the eastern Pacific. Upwelling is common along the west coast of South America, off Equador and Peru, and along the west coast of Central and North America. As a result of both the upwelling and the eastern boundary currents which flow towards the equator in these regions, sea surface temperatures are relatively low along these coasts. The winds which create upwelling are strongest in the area just west of Costa Rica.

Upwelling also occurs in the equatorial oceans due to the movement of the ITCZ, which lead to the broad line of high phytoplankton concentration in equatorial Pacific.

Biological productivity : Biological productivity in the ocean relies on nutrients in the sunlit surface layer (euphotic zone - about 100 meters depth). The principal nutrients are nitrate, phosphate and dissolved silica. They are consumed by plants and animals in the ocean's surface layer. They are "regenerated" at depth as the decaying plants and animals and fecal pellets fall through the water column, with some portion, especially of the silica-bearing hard parts, reaching the ocean bottom. Thus nutrients are severely depleted in the surface layer where they are used almost as quickly as they appear there. Nutrients are found in abundance below the surface layer, especially where waters have been separated from the sea surface for a long time. Nutrients reach the euphotic zone through upwelling, and so upwelling regions have slightly higher nutrient content and much higher biological productivity than downwelling regions. The most productive regions occur where upwelling is vigorous and where the nutrient-rich thermocline is near the sea surface. Near-surface nutrients in the Pacific are high in the equatorial and eastern tropical Pacific where upwelling is high, and low in the subtropical downwelling regions poleward of about 20°. Surface nutrients are higher in the eastern equatorial Pacific than in the western, reflecting the upwelling of the thermocline waters towards the east.

Climate variability : El Nino is the manifestation of true ocean – atmosphere coupling. It encompassed the entitre tropical Pacific ocean and atmosphere, Its effects reach far beyond the tropical Pacific through connections in the far-ranging atmospheric circulation. It occurs over the course of three to seven years.

In normal situations, known as La Nina, warm water pools in the west and cold water prevails in the east. This results to strong rainfall in the western Pacific, little rainfall in the eastern Pacific, and major fisheries production in the eastern boundary regions. But during El Nino, the easterly trade winds weaken in the tropical Pacific, warm water builds up across the equatorial Pacific, resulting in reduced westward flow at the equator which leads to a draining of the western warm pool towards the east. As the western warm pool cools slightly and the central and eastern equatorial Pacific warm, this further reduces the strength of the tradewinds, in other words, providing a positive feedback. The resulting disruption creates drought in some regions - the western tropical Pacific and northern Australia and large rainfall in other regions - the eastern tropical Pacific and the west coasts of North and South America. Upwelling also reduced significantly in equatorial ocean as well as eastern Pacific. This leads to great decline in production in important fisheries area, like the coast of Peru.

Tsunami : Tsunamis are gravity waves, just like those generated by the wind, but their period is much longer, on the order of 10 to 60 minutes. While earthquakes are the most common cause of tsunamis, the waves can also be generated by any phenomenon which rapidly changes the shape of the sea surface over a large area: volcanic eruption, landslide, even meteorite impact. Since the largest shallow focus earthquakes occur in the subduction zones which ring the Pacific, and since these same subduction zones are dotted with volcanoes, the tsunami hazard throughout the tropical Pacific is high.

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