Modeling the Seasonal Dependence of the Atmospheric Response to Observed El Niños 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 behavior through the life cycles of El Niño?Southern Osicillation (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 Hoymöller diagrams. As the warm SST anomaly appears in the eastern equatorial Pacific during the boreal spring and subsequently spreads across the ocean basin, a well-defined sequence of meteorological events is evident in the model atmosphere. The most notable atmospheric response over the tropical Pacific Basin includes weakening of the east-west surface pressure gradient and easterly trades eastward displacement of the South Pacific Convergence Zone, southward displacement of the Intertropical Convergence Zone, above normal precipitation at and east of the date line, and below normal precipitation over the Indonesian Archipelago. The strongest anomalies am simulated in the northern winter following a warming off the Peruvian coast. The model response in this mature stage is characterized by tropospheric warming throughout the entire tropical zone, and by the appearance in the tropical upper troposphere of a pair of Pacific anticyclones straddling the equator. These anticyclonic centers appear as the starting points of well-organized wave trains spanning the midlatitude zones of both hemispheres. The Northern Hemisphere wave pattern in the Pacific-North American sector bears a strong resemblance to that reported in recent observational studies. The warm Pacific SST anomaly tends to be replaced a year later by a cold anomaly. The polarities of meteorological anomalies simulated during the cold phase of the ENSO cycle are mostly opposite to those occurring during the warm phase. Time series analysis of different circulation indices. as well as comparison between simulated amplitudes of atmospheric variability in this experiment and in a ?control? experiment without any prescription of interannual SST variations, indicate that the impact of equatorial Pacific SST anomalies on the tropical circulation is much greater than that on the flow patterns in middle latitudes. In particular, the temporal variance of 200 mb height in this perturbed SST experiment is larger than the corresponding quantity in the control experiment by a factor of 2?6 over the tropics; whereas the same SST fluctuations are much less effective in enhancing the variability in middle and higher latitudes. Moreover, perturbations in the equatorial Pacific SST are more strongly correlated with circulation changes in the tropical atmosphere than with changes in the extratropics.