Thermocline processes in the tropical Pacific, and their role in decadal climate variability

Abstract

The thermocline is the layer of the ocean that is characterized by large vertical temperature gradients. Thermocline variability can arise through subduction processes as well as baroclinic Rossby wave propagation, processes that have been invoked in several recent theories of decadal climate variability.In the Pacific, the thermocline exhibits two tropical centers of variability, at ~10S and 13N. A large fraction of the variance in these areas is characterized by long timescales, in the decadal range.

In this talk we investigate the origin of the centers of variability at 10S and 13N using the output from a numerical simulation performed with the NCAR ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat and freshwater over the period 1958-1997. Both centers of variability are associated with first mode baroclinic Rossby waves forced by anomalous Ekman pumping. The waves propagate westward to the western boundary, and continue equatorward along the boundary.After reaching the equator, they propagate eastward along the equator, where they appear to produce a low-frequency modulation of the thermocline depth.

A simple Rossby wave model is used to examine which aspects of the forcing (amplitude, spatial and/or temporal coherency) are responsible for creating the large thermocline signals at 10S and 13N. At those latitudes, the thermocline slopes poleward in both hemispheres, so that meridional excursions of the thermocline can also give rise to large local changes in thermocline depth and temperature. The contribution of this process to the thermocline variability at 10S and 13N is also examined.