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The dynamics of the eastern subtropical gyre of the North Pacific Ocean are distinct from those of the western Pacific, which are dominated by fluctuations in the Kuroshio Extension. Fluctuations in the eastern Pacific may be more related to subtropical/subpolar gyre interactions or to influences from the equatorial region. Measurements from WOCE have shown that geostrophic transport between Hawaii and the California coast deviates substantially from Sverdrup balance. this discrepancy could be due to errors in the wind field used to estimate the Sverdrup transport, but it may also indicated the importance of other mechanisms, such as seasonal heating, propagating Rossby waves which are aliased in the observations, mixing along the front between the interior of the gyre and the California Current, or the formation of mode water.
Data from the TOPEX/POSEIDON altimeter can show both the spatial and temporal evolution of fluctuations in the subtropical gyres, their relationships to fluctuations in the subpolar gyre and the equatorial region, an estimate of the annual signal in SSH, and wave propagation information. The relative contributions of thermal forcing and Rossby waves to the SSH variations can be estimated by assimilating SSH anomalies form the altimeter into a simple reduced gravity model of the wind and thermally forced circulation. However, seasonal SSH variations cannot be used to estimate seasonal variations in geostrophic transport; to obtain transport, it is necessary to know the depth of the seasonal thermocline.
Several of the other candidates for the deviation from Sverdrup balance require a very accurate specification of the wind field and a numerical circulation model. An estimate of the Sverdrup transport can be computed from scatterometer wind fields. Greatly improved spatial wind resolution will be available with the launch of the NASA scatterometer in August 1996. The structure of the actual wind field relative to the location of the outcropping layer, both of which may vary seasonally, may be important. Recent analyses of the California Current show that the location of the front is related to seasonal variations in the wind stress curl field. The effect of spatial variations in the wind field, in conjunctions with a seasonally varying front, has been the subject of modeling efforts by L. Thompson. These analyses are being done in collaboration with Susan Hautala and LuAnne Thompson at the University of Washington.
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