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Equatorial Undercurrent, thermocline, boreal winter

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Zonal volume transport is examined over the upper 150 m of the water column at 28-degrees-W, using the Seasonal Response of the Equatorial Atlantic experiment moored current meter data at 0.75-degrees-S, 0.0-degrees, and 0.75-degrees-N and a time domain empirical orthogonal function (EOF) analysis to improve spatial and temporal resolution. Gaussian distributions are fit to the mean volume transports per unit width and the fluctuations about these means that are found to be represented by a single EOF mode. The mean volume transport is distributed symmetrically both about the equator and about 75 m depth, with a meridional scale width of 101 km and a magnitude of 20.3 Sv. The standard deviation for the fluctuations is 4.7 Sv, and in contrast to the mean the fluctuations are confined to above 100 m depth, with a larger meridional scale width. The scale width for the mean is consistent with an inertial scale associated with conservation of potential vorticity, while the scale width for the fluctuations is consistent with an equatorial Rossby radius of deformation associated with equatorially trapped waves. These findings support a hypothesis that different physical processes are controlling the volume transport, as integrated from the surface across the Equatorial Undercurrent, over different time scales. On the time scale of the record length average, inertial dynamics appear to be controlling, while on the time scales of the seasonal fluctuations, linear equatorial long-wave dynamics appear to be controlling. However, given the limited record length, there are insufficient degrees of freedom to test this hypothesis statistically.

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Journal of Geophysical Research - Oceans, v. 98, no. C6, p. 10145-10153.