Modeling the seasonal dependence of the atmospheric response to observed El Ninos in 1962-76.

Abstract

Two 15-year atmospheric GCM integrations are conducted with the lower boundary over the tropical Pacific being forced by observed month-to-month sea surface temperature (SST) changes during the period 1962-76. A descriptive account is given on selected aspects of the 30-year model climatology, as well as the anomalous model behaviour through the life cycles of El Nino-Southern Oscillation (ENSO) episodes centered in the years 1965, 1969 and 1972. These model results are compared with available observations reported in the published literature. Particular attention is devoted to the timing of various simulated meteorological phenomena with respect to the spatially and temporally evolving SST forcing and to the climatological seasonal cycle. An assessment is made of the capability of the model to simulate the seasonal dependence of various climatological features relevant to ENSO. The phenomena examined include the flow field and rainfall in different monsoon regions, the planetary scale waves in the extratropics, and the low-level convergence zones in the tropical Pacific Basin. The evolutionary response of the model atmosphere in a typical ENSO event is examined using time series of selected circulation indices, composite charts and Hovmoller diagrams. -from Author