Dynamics of the equatorial undercurrent and its determination

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1988

This study focuses on the zonal weakening, eastern termination andseasonal variations of the Atlantic equatorial undercurrent (EUC). Themain and most original contribution of the dissertation is a detailedanalysis of the Atlantic EUC simulated by Philander and Pacanowski's(1986)general circulation model (GCM), which provides a novel descriptionof the dynamical regimes governing various regions of a nonlinear stratifiedundercurrent.Only in a narrow deep western region of the simulation does onefind an approximately inertial regime corresponding to zonal acceleration.Elsewhere frictional processes cannot be ignored. The bulk ofthe mid-basin model EUC terminates in the overlying westward surfaceflow while only a small fraction (the deeper more inertial layers)terminates at the eastern coast. In agreement with observations, arobust feature of the GCM not present in simpler models is the apparentmigration of the EUC core from above the thermocline in the west to belowit in the east. In the GCM, this happens because the eastward flow iseroded more efficiently by vertical friction above the base of the thermoclinethan by lateral friction at greater depths. This mechanism is aplausible one for the observed EUC. A scale analysis using a depth scalewhich decreases with distance eastwards predicts the model zonal transitionbetween western inertial and eastern inertio-frictional regimes.Historical and recent observations and simple models of theequatorial and coastal eastern undercurrents are reviewed, and a newanalysis of current measurements in the eastern equatorial Atlantic ispresented. Although the measurements are inadequate for definitive conclusions,they suggest that Lukas' (1981) claim of a spring surge of thePacific EUC to the eastern coast and a seasonal branching of the EUCinto a coastal southeastward undercurrent may also hold for the AtlanticOcean. To improve the agreement between observed and modelled strengthof the eastern undercurrent, it is suggested that the eddy coefficientof horizontal mixing should be reduced in future GCM simulations.

This work was supported by NSF grantsOCE82-14771, OCE82-08744 and OCE85-14885.