Abstract:
This study focuses on the zonal weakening, eastern termination and
seasonal variations of the Atlantic equatorial undercurrent (EUC). The
main and most original contribution of the dissertation is a detailed
analysis of the Atlantic EUC simulated by Philander and Pacanowski's
(1986) general circulation model (GCM), which provides a novel description
of the dynamical regimes governing various regions of a nonlinear stratified
undercurrent.
Only in a narrow deep western region of the simulation does one
find an approximately inertial regime corresponding to zonal acceleration.
Elsewhere frictional processes cannot be ignored. The bulk of
the mid-basin model EUC terminates in the overlying westward surface
flow while only a small fraction (the deeper more inertial layers)
terminates at the eastern coast. In agreement with observations, a
robust feature of the GCM not present in simpler models is the apparent
migration of the EUC core from above the thermocline in the west to below
it in the east. In the GCM, this happens because the eastward flow is
eroded more efficiently by vertical friction above the base of the thermocline
than by lateral friction at greater depths. This mechanism is a
plausible one for the observed EUC. A scale analysis using a depth scale
which decreases with distance eastwards predicts the model zonal transition
between western inertial and eastern inertio-frictional regimes.
Historical and recent observations and simple models of the
equatorial and coastal eastern undercurrents are reviewed, and a new
analysis of current measurements in the eastern equatorial Atlantic is
presented. Although the measurements are inadequate for definitive conclusions,
they suggest that Lukas' (1981) claim of a spring surge of the
Pacific EUC to the eastern coast and a seasonal branching of the EUC
into a coastal southeastward undercurrent may also hold for the Atlantic
Ocean. To improve the agreement between observed and modelled strength
of the eastern undercurrent, it is suggested that the eddy coefficient
of horizontal mixing should be reduced in future GCM simulations.
