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:
- North Pacific: California Current
- South Pacific: Peru/Chile Current
- North Atlantic: Canary Current
- South Atlantic: Benguela Current
- Indian: Leeuwin Current
source:
http://oceansjsu.com/105d/exped_buoys/13.html
See also:
Rahal Marsha Bala'zam 230210130006
Averous Mutahari, 230210130025
See also:
Rahal Marsha Bala'zam 230210130006
Averous Mutahari, 230210130025