Simulation of ENSO-Related Surface Winds in the Tropical Pacific by an Atmospheric General Circulation Model Forced by Observed Sea Surface Temperatures

Abstract

The authors present the simulation of the tropical Pacific surfacewind variability by a low-resolution (R15 horizontal resolution and18 vertical levels) version of the Center for Ocean-Land-AtmosphereInteractions, Maryland, general circulation model (GCM) when forcedby observed global sea surface temperature. The authors have examinedthe monthly mean surface winds acid precipitation simulated by themodel that was integrated from January 1979 to March 1992. Analysesof the climatological annual cycle and interannual variability overthe Pacific are presented. The annual means of the simulated zonaland meridional winds agree well with observations. The only appreciabledifference is in the region of strong trade winds where the simulatedzonal winds are about 15%-20% weaker than observed, The amplitudeof the annual harmonics are weaker than observed over the intertropicalconvergence zone and the South Pacific convergence zone regions.The amplitudes of the interannual variation of the simulated zonaland meridional winds are close to those of the observed variation.The first few dominant empirical orthogonal functions (EOF) of thesimulated, as well as the observed, monthly mean winds are foundto contain a targe amount of high-frequency intraseasonal variations,While the statistical properties of the high-frequency modes, suchas their amplitude and geographical locations, agree with observations,their detailed time evolution does not. When the data are subjectedto a 5-month running-mean filter, the first two dominant EOFs ofthe simulated winds representing the low-frequency EI Nino-SouthernOscillation fluctuations compare quite well with observations. However,the location of the center of the westerly anomalies associated withthe warm episodes is simulated about 15 degrees west of the observedlocations. The model simulates well the progress of the westerlyanomalies toward the eastern Pacific during the evolution of a warmevent. The simulated equatorial wind anomalies are comparable inmagnitude to the observed anomalies. An intercomparison of the simulationof the interannual variability by a few other GCMs with comparableresolution is also presented. The success in simulation of the large-scalelow-frequency part of the tropical surface winds by the atmosphericGCM seems to be related to the model's ability to simulate the large-scalelow-frequency part of the precipitation. Good correspondence betweenthe simulated precipitation and the highly reflective cloud anomaliesis seen in the first two EOFs of the 5-month running means. Moreover,the strong correlation found between the simulated precipitationand the simulated winds in the first two principal components indicatesthe primary role of model precipitation in driving the surface winds.The surface winds simulated by a linear model forced by the GCM-simulatedprecipitation show good resemblance to the GCM-simulated winds inthe equatorial region. This result supports the recent findings thatthe large-scale part of the tropical surface winds is primarily linear.