A main objective for World Ocean Circulation Experiment (WOCE) was to quantify the size of western boundary currents. Current meter arrays were deployed for the first time to measure thermocline western boundary currents at 30ºS in the Pacific, Indian and Atlantic Oceans and at 25ºN in the Pacific Ocean while the time series Gulf Stream transport through Florida Straits at 25ºN continued to be monitored. Furthermore, array measurements were made in the Atlantic at 38ºN and in the Pacific at 32ºN to examine the fully developed Gulf Stream and Kuroshio. From these measurements, the nature and size of temporal variability in the western boundary currents can be reliably quantified for the first time. A notable feature of the direct current measurements at 30ºS is that all southern hemisphere western boundary currents exhibit an equatorward flowing undercurrent beneath the poleward thermocline flow in each basin. Comparison of measured western boundary current transports versus Sverdrup transports exhibits no clear relationship. Recirculations of arbitrary size appear to be necessary to explain the transports of the observed thermocline western boundary currents.
Deep western boundary currents were also measured in each ocean basin. In the Atlantic, the southward flowing North Atlantic Deep Water (NADW) transports at 26ºN and 18ºS are about 40 Sv, more than a factor of 2 larger than expected for the net export of NADW. Again, sizeable recirculations are called upon to explain the magnitude of the boundary currents. Consistent northward flows of Antarctic Bottom Water/Lower Circumpolar Deep Water were measured in the South Pacific and South Atlantic suggesting bottom water transports of 16 Sv in the Pacific and 6 Sv in the Atlantic, each decreasing in transport toward the equator. The direct measurements of northward bottom water transport in the Indian Ocean across 20ZS yield surprisingly small values with the transport in the anticipated major boundary current off Madagascar being effectively zero.
The principal issue for basin-scale circulation and western boundary currents is to understand what sets the size of the circulation or, alternately, what sets the size of the recirculations that can dominate both the thermocline and deep western boundary currents. |
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