OCEANOGRAPHY: EASTERN BOUNDARY

Eastern boundary currents in subtropical regions are considerd separately from the basin-wide subtropical gyres because they have unique and separate dynamics from the gyre-scale circulation. Each of the five oceans has an eastern boundary current, which is narrow and shallow, and generally extremely eddy-rich.

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: