Numerical Simulations of the Atmospheric Response to a Sea Surface Temperature Anomaly over the Equatorial Eastern Pacific Ocean

Abstract

The atmospheric response to a sea surface temperature anomaly over the equatorial eastern Pacific Ocean (SSTA) is investigated using the UCLA General Circulation Model. The SSTA used is an idealization of that compiled by Rasmusson and Carpenter for the mature phase of El Niño. Two simulations over seasons, one without and the other with the SSTA, are performed and their results are compared for the Northern Hemisphere winter season. In the tropics the SSTA enhances precipitation over the central and eastern equatorial Pacific, while it decreases precipitation over the adjacent regions. The anomalous precipitation is predominantly balanced by the anomalous moisture flux convergence, which has comparable magnitude in the planetary boundary layer (PBL), and in the free atmosphere with quite different geographical distribution. This suggests that the anomalous precipitation, and hence the anomalous tropical cumulus heating, cannot be related exclusively to either flow anomalies in the PBL or in the free atmosphere. In the midlatitudes, it is found that the SSTA results in a more zonal flow over the Pacific with an intensification of the upper-tropospheric westerlies. Associated with this intensification, synoptic-scale transient baroclinic waves become more active. This is consistent with interannual differences in observed spectral distributions of transients for five winters, two of which were El Niño winters. Geographically, the increase in baroclinic wave activity occurs in a zonal bell extending from the northeastern Pacific to the northern Atlantic.