Anatomy and decadal evolution of the Pacific Subtropical-Tropical Cells (STCs)

The output from an ocean general circulation model driven by observed surface forcing (1958-97) is used to examine the evolution and relative timing of the different branches of the Pacific Subtropical-Tropical Cells (STCs) at both interannual and decadal time scales, with emphasis on the 1976-77 climate shift. The STCs consist of equatorward pycnocline transports in the ocean interior and in the western boundary current, equatorial upwelling, and poleward flow in the surface Ekman layer. The interior pycnocline transports exhibit a decreasing trend after the mid-1970s, in agreement with observational transport estimates, and are largely anticorrelated with both the Ekman transports and the boundary current transports at the same latitudes. The boundary current changes tend to compensate for the interior changes at both interannual and decadal time scales. The meridional transport convergence across 9 degrees S and 9 degrees N as well as the equatorial upwelling are strongly correlated with the changes in sea surface temperature (SST) in the central and eastern equatorial Pacific. However, meridional transport variations do not occur simultaneously at each longitude, so that to understand the phase relationship between transport and SST variations it is important to consider the baroclinic ocean adjustment through westward-propagating Rossby waves. The anticorrelation between boundary current changes and interior transport changes can also be understood in terms of the baroclinic adjustment process. In this simulation, the pycnocline transport variations appear to be primarily confined within the Tropics, with maxima around 10 degrees S and 13 degrees N, and related to the local wind forcing; a somewhat different perspective from previous studies that have emphasized the role of wind variations in the subtropics.