SIMULATION OF ENSO-RELATED SURFACE WINDS IN THE TROPICAL PACIFIC BY AN ATMOSPHERIC GENERAL-CIRCULATION MODEL FORCED BY OBSERVED SEA-SURFACETEMPERATURES

The authors present the simulation of the tropical Pacific surface win d variability by a low-resolution (R15 horizontal resolution and 18 ve rtical levels) version of the Center for Ocean-Land-Atmosphere Interac tions, Maryland, general circulation model (GCM) when forced by observ ed global sea surface temperature. The authors have examined the month ly mean surface winds acid precipitation simulated by the model that w as integrated from January 1979 to March 1992. Analyses of the climato logical annual cycle and interannual variability over the Pacific are presented. The annual means of the simulated zonal and meridional wind s agree well with observations. The only appreciable difference is in the region of strong trade winds where the simulated zonal winds are a bout 15%-20% weaker than observed, The amplitude of the annual harmoni cs are weaker than observed over the intertropical convergence zone an d the South Pacific convergence zone regions. The amplitudes of the in terannual variation of the simulated zonal and meridional winds are cl ose to those of the observed variation. The first few dominant empiric al orthogonal functions (EOF) of the simulated, as well as the observe d, monthly mean winds are found to contain a targe amount of high-freq uency intraseasonal variations, While the statistical properties of th e high-frequency modes, such as their amplitude and geographical locat ions, agree with observations, their detailed time evolution does not. When the data are subjected to a 5-month running-mean filter, the fir st two dominant EOFs of the simulated winds representing the low-frequ ency EI Nino-Southern Oscillation fluctuations compare quite well with observations. However, the location of the center of the westerly ano malies associated with the warm episodes is simulated about 15 degrees west of the observed locations. The model simulates well the progress of the westerly anomalies toward the eastern Pacific during the evolu tion of a warm event. The simulated equatorial wind anomalies are comp arable in magnitude to the observed anomalies. An intercomparison of t he simulation of the interannual variability by a few other GCMs with comparable resolution is also presented. The success in simulation of the large-scale low-frequency part of the tropical surface winds by th e atmospheric GCM seems to be related to the model's ability to simula te the large-scale low-frequency part of the precipitation. Good corre spondence between the simulated precipitation and the highly reflectiv e cloud anomalies is seen in the first two EOFs of the 5-month running means. Moreover, the strong correlation found between the simulated p recipitation and the simulated winds in the first two principal compon ents indicates the primary role of model precipitation in driving the surface winds. The surface winds simulated by a linear model forced by the GCM-simulated precipitation show good resemblance to the GCM-simu lated winds in the equatorial region. This result supports the recent findings that the large-scale part of the tropical surface winds is pr imarily linear.