Tuesday, 11 January 2005
The impact of Ekman transport on ENSO-induced SST anomalies
Atmospheric teleconnections associated with El Niņo/Southern Oscillation (ENSO) alter the near-surface temperature, humidity and wind, and the distribution of clouds, which in turn, can influence the ocean far from the equatorial Pacific. Thus, the atmosphere can act as a "bridge" between sea surface temperature (SST) anomalies in the equatorial Pacific and ocean conditions in the North Pacific and other ocean basins. Previous studies of the bridge have examined the impact of the surface heat fluxes and vertical mixing on the ocean in regions beyond the equatorial Pacific. However, recent studies suggest that Ekman transport may also contribute to extratropical SST anomalies on seasonal and longer time scales. Here, we examine how the inclusion of Ekman transport, the wind driven change in the surface ocean currents, influences the evolution of bridge-induced SST anomalies.
The atmospheric bridge is examined using observations and two sets of 16 atmospheric GCM simulations. In both, observed SST anomalies are specified in the tropical Pacific for the period 1950-1999, while the remainder of the global oceans are represented by a grid of column ocean models. In both experiments, the column models predict the mixed layer temperature salinity and depth by simulating vertical processes including, surface heat fluxes, entrainment and penetrating solar radiation. In the second experiment Ekman transport also influences the evolution of SST anomalies via its direct impact on the mixed layer temperature tendency (Ekman pumping and the ocean currents it induces are not included). By comparing the two sets of experiments we will show how changes in the Ekman transport impact ENSO-driven SST anomalies.