INTERANNUAL FLUCTUATIONS IN ATMOSPHERIC ANGULAR-MOMENTUM SIMULATED BYTHE NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION MEDIUM-RANGE FORECAST MODEL

An earlier study by Dickey et al. [1992] established the existence of globally coherent interannual fluctuations in atmospheric angular mome ntum (AAM), associated with the El Nino-Southern Oscillation (ENSO) cy cle. In this paper, we pursue the origin and the structure of these fl uctuations using an ensemble of experiments generated by the National Centers for Environmental Prediction, medium range forecast model vers ion 9. In the control experiments, where the observed sea surface temp eratures (SSTs) were used as the lower boundary conditions, the model captures the characteristic V-like structure in time-latitude plots of zonally averaged AAM found by Dickey et al., while experiments with c limatological SSTs and those with either perpetual warm or cold ENSO c onditions superimposed on the climatological SSTs failed to reproduce this structure. The numerical results indicate that these AAM structur es are related to SST variations associated with transitions between d ifferent phases of the ENSO cycle and have both propagating and standi ng components. The largest zonal wind contribution from the levels stu died (850, 500, and 200 hPa) is at 200 hPa, where the tropical convect ive outflow is the strongest. Composites of zonal wind and geopotentia l height show a clear relationship between the stages of the global AA M oscillation and the ENSO cycle. The strong similarity between the si mulated and observed AAM series attests to the model's ability to real istically simulate the interannual response of the atmosphere to ENSO SST anomalies.