Meridional sections constructed from contemporaneous CTD and ADCP data taken across the Pacific, primarily during the 1990s, have been used to describe the mean zonal evolution of upper-ocean tropical zonal velocity, temperature, and salinity, as well as the seasonal cycle and the ENSO cycle. The data are unusual in that they allow direct estimates of properties of the near-equatorial currents, including the SEC(N), EUC, and EIC. The large number of sections taken, and the quality of the zonal velocity data, enable a four-dimensional exploration of the upper ocean zonal velocity field. This analysis should be useful for model comparisons. In addition it comprises a consistent climatology of simultaneously sampled zonal velocity, temperature, and salinity for the decade, to compare with past and future decades.
Some of the more novel results of this analysis involve the near-equatorial and subthermocline currents. For instance, the westward intensification of the EIC stands out in the mean velocity sections. A seasonal cycle of the EIC is also to be seen in the west. Another intriguing result in the eastern Pacific is the hint of a seasonal cycle of the SCCs at 110°W. Basin-wide estimates of the SEC(N) and its properties are unusual. The SEC(N) has a central Pacific transport maximum, a seasonal cycle nearly in phase with the NECC, and the suggestion of a large reduction during El Niño. The inclusion of so many realizations of the NGCUC in the analysis is also rare. The use of direct velocity data also allows inclusion of ageostrophic effects both around the equator (Joyce et al., 1988) and away from it. For instance, the subsurface velocity maximum in the NECC is some combination of the wind-driven Ekman layer and geostropic shear, a feature that geostrophic calculations alone would not resolve.
These data are also the backbone for an inverse model of the tropical Pacific circulation. They are being combined with meridional velocities estimated through geostrophy and Ekman dynamics, and the entire current system is adjusted to conserve mass, heat, and freshwater, including the contributions of surface fluxes. The model is initially being run using the mean fields, but will soon include a seasonal cycle. The results are allowing diagnosis of the effects of diapycnal mixing processes on the maintenance of the cold tongue. In addition, the role of the subtropical cell in cycling water from the South Pacific to the North Pacific to feed the Indonesian throughflow is being explored.
This work was partially funded by the NOAA Office of Oceanic and Atmospheric Research, the NOAA Office of Global Programs, and the NASA Physical Oceanography Program. This work was performed while BMS held a National Research Council Research Associateship Award at NOAA’s Pacific Marine Environmental Laboratory (PMEL). The analysis would not have been possible without the careful and sustained work of the TAO project, especially the officers, crew, and scientific parties of the NOAA Ships Discoverer, Ka’imimoana and Ronald H. Brown, especially Dennis Sweeny. Eric Johnson and Patricia Plimpton processed the earlier NOAA ADCP data. June Firing and Jules Hummon processed the later NOAA ADCP data as well as much of the ADCP data from other sources. Data from R/V Kaiyo were also essential, with contributions from the officers, crew, and all TOCS participants both at sea and ashore. Discussions with Mike McPhaden were useful. PMEL Contribution Number 2337.
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