The Tropical Ocean Response to a Change in Solar Forcing

Abstract

Changes in the tropical oceans caused by a shift of 6 months in the date of perihelion are examined using a coupled atmosphere?ocean general circulation model (GCM). The changes in the annual cycle of sea surface temperature (SST) near the equator are described, and the mechanism for the changes is diagnosed. The GCM results are diagnosed using the ocean component model forced by the time mean fluxes from the coupled integration. This diagnosis shows that the changes in the annual cycle of near-equatorial SST are caused by different mechanisms in different regions. An extensive analysis of the changes in the eastern Pacific is given because of the importance of this region in modulating the global climate through teleconnections associated with ENSO. In the eastern Pacific, the change in the annual cycle of SST is found to be primarily due to zonal wind stress differences. The zonal wind stress differences are caused for the most part by changes in the precipitation distribution. The changes in the precipitation distribution are consistent with being caused by changes in both the low-level convergence forced by the surface temperature gradients and changes in the local evaporation. The physical process responsible for the change in low-level zonal wind and convergence are diagnosed using a steady-state linearized version of the atmospheric model that is forced by time mean fields from the coupled model. Interannual variability of eastern Pacific (Niño-3) SST under the modified solar forcing is found to have an amplitude and period similar to those observed in modern times. The only major difference in the interannual variability of tropical Pacific SST is found to be the timing of SST anomalies. This occurs because the interannual SST variability in the tropical eastern Pacific is phase locked to the annual cycle of SST, whose phase is itself dependent on the solar forcing.