Lynne Talley, 2000
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Reading and study questions
The classical explanation of these currents is that equatorward winds force Ekman flow offshore, which drives a shallow upwelling (order 200 meters deep) in a very narrow region adjacent to the coast (order 10 km). Winds along the eastern boundaries of all oceans are favorable to upwelling, probably as a result of topographic steering of the westerly winds as they reach land. The upwelling results in uplift of cooler, nutrient rich waters from just below the surface layer and hence higher productivity and cooler waters. (Both are clear in satellite images - AVHRR to look at infrared which is a measure of temperature, and Coastal Zone Color Scanner which indicates pigment concentration.)
The upwelling is accompanied by a rise in isopycnals towards the coast. This has an associated geostrophic flow which is equatorward. This equatorward flow is the intensified eastern boundary current, which we identify separately from the general equatorward flow of the eastern part of the subtropical gyre. The eastern boundary currents are shallow, meandering currents. The actual eastern boundary currents such as the California Current are narrow (< 100 km width), meandering and have speeds of 40 to 80 cm/sec. They are located at the upwelling front created by the offshore Ekman transport. They have strong seasonality, described below.
The equatorward surface flow creates in some sense a piling of water towards the equator and hence a pressure gradient force which is northward. This drives a poleward current at the coast and usually just beneath the equatorward eastern boundary current (at ~200 meters). When upwelling favorable winds weaken or disappear, the equatorward flow also disappears and the poleward undercurrent is found to the surface.
Upwelling occurs over a broader region than just the very narrow coastal strip. This may be because the wind stress curl associated with the topographically steered winds is positive, creating a broader upwelling zone than would a strictly uniform wind with an equatorward component.
Offshore Ekman transport does not occur as a simple uniform offshore flux all along the eastern boundary. Rather it occurs in jets. Along California there are semi-permanent locations for the jets, apparently associated with the coastal geography - jets occurring at capes such as Point Arena.
Seasonality of the California Current has been fairly well described. In winter the California Current is weak or absent. As upwelling- favorable winds begin to blow, the current forms near the coast but quickly moves offshore. It is most strongly developed at the height of the upwelling season, in July-August. Surface dynamic height on the inshore side of the current varies seasonally by about 30 to 40 cm.
Productivity in the eastern boundary current regions is enhanced by both the local upwelling and by the advection of higher latitude waters, from broad upwelling regions (like the subarctic Pacific), towards the equator. Climate fluctuations can change the relative amount of higher latitude waters reaching the eastern boundary region.
The only ocean without an equatorward eastern boundary current is the Indian Ocean. The Leeuwin Current along the west coast of Australia flows poleward, even though the winds are upwelling favorable and would drive a normal eastern boundary current there in the absence of other forces. However, there is a much larger poleward pressure gradient force along this boundary than along the others, due to the flow of water westward through the Indonesian archipelago from the Pacific to the Indian Ocean.
The equatorward eastern boundary currents:
The poleward eastern boundary currents:
Talley, L. D., G. Fryer, and R. Lumpkin, 1997. Physical oceanography of the tropical Pacific. In Geography of the Pacific Islands, ed. M. Rapaport. Bess Press, Honolulu, HI. In press.Some of the text here is taken from that article.
The trade winds are relatively steady easterlies. They are driven by warm waters in the western region and cooler waters in the east, which creates rising air in the west and sinking air in the east, and a thermally direct flow from east to west to feed this (Walker cell). (The tropical region in the atmosphere is roughly from 20S to 20N.)
In the ocean the true equatorial region is much narrower - about 2 degrees wide. Easterly trade winds at the equator drive (1) poleward Ekman transport and (2) westward surface flow, as follows:
The easterly trade winds cause northward Ekman transport just to the north of the equator and southward Ekman transport just to the south of the equator. This causes upwelling at the equator. As a result, the pycnocline shoals towards the equator. This drives a westward geostrophic flow at the sea surface, much like the eastern boundary current flow.
Directly on the equator, the effect of rotation on the circulation vanishes, and so the concepts of geostrophic and Ekman flow do not apply. At the equator, the easterly trade winds push the surface water directly (frictionally) from east to west. This water piles up gently in the western Pacific (0.5 meters higher there than in the eastern Pacific).
Figure. Sea surface heights from NASA/JPL. Look for one that is labelled either as "normal" or "La Nina" to get a sense of the mean sea surface height distribution at the equator.The pycnocline is deeper in the west also as a result, and much warmer water is found there ("warm pool").
Upwelling in the east draws cool water to the surface because of the shallow pycnocline there, but intense eastward-flowing upwelling in the west cannot create cold water at the surface there because of the thickness of the warm pool.
Because the sea surface is higher in the west than in the east, there is a pressure difference that causes the flow just beneath the surface layer to be eastward. This strong eastward flow is the Equatorial Undercurrent. It is centered at about 150 to 200 meters depth. EUC speeds are in excess of 100 cm/sec. The current is exceptionally thin vertically (about 150 meters thick).
The Equatorial Undercurrent shoals towards the east, as does the pycnocline. The shoaling is associated with upwelling of cool water in the central/eastern Pacific, giving rise to the "cold tongue" in non-El Nino years.
Figure. Sea surface temperature in the tropical Pacific during normal, El Nino and La Nina years, from the El Nino Theme Page produced by NOAA/PMEL.Below the Equatorial Undercurrent, the equatorial currents are complex. The narrow equatorial region is a waveguide for waves with a lot of vertical structure. These waves decay away quickly away from the equator. The quasi-permanent current structure reflects this complexity. A series of "stacked jets" is found on the equator down to about 1000 m.
Steady trade winds, which cause equatorial upwelling, are more prevalent in the east than in the west. There is seasonality in the winds, and equatorial upwelling is weaker in the northern winter and spring, giving rise to mini-El Nino conditions (topic 8) each year in the eastern equatorial Pacific.
When the trade winds weaken or even reverse, the flow of water westward at the equator weakens or reverses and upwelling weakens or stops. Surface waters in the eastern Pacific warm significantly since upwelling is no longer bringing the cool waters to the surface. The deep warm pool in the western Pacific thins as its water sloshes eastward along the equator in the absence of the trade winds which maintain it.
Off the equator, flow is geostrophic. Just north and south of the equator are found the North and South Subsurface Countercurrents ("Tsuchiya jets"), which flow eastward and sometimes appear to be slightly deeper poleward extensions of the EUC.
North of the equator (5N to 10N) in the Pacific and Atlantic is found the intense North Equatorial Countercurrent. This is driven by cyclonic wind stress curl associated with the Intertropical Convergence Zone. This current probably reaches very deep into the ocean. It is the southern side of a very long and narrow cyclonic circulation.
The north side of the circulation is part of the "North Equatorial Current", which is also the westward flow of the subtropical gyre. The North Equatorial Current in the Pacific reaches the western boundary and splits into the Kuroshio (northward flow for the subtropical gyre) and into the Mindanao Current (southward flow for the tropical cyclonic gyre).
In the southern hemisphere there isn't usually a strong counterpart to the North Equatorial Countercurrent. Most of the time the westward flow of the northern part of the subtropical gyre appears to merge smoothly with the westward surface in the tropics and at the equator. This is called the "South Equatorial Current".
In the Indian Ocean, the winds have very strong seasonality (monsoon). The equatorial current system is sensitive to the seasonality and complete reversals of currents occur. (See topic 8.)