SEASONAL AND INTERANNUAL VARIABILITY IN A HYBRID COUPLED GCM

A hybrid coupled model for the tropical Pacific ocean-atmosphere syste m is used to simulate El Nino-Southern Oscillation (ENSO) interannual variability and to investigate the role of coupling in the seasonal cy cle. An ocean GCM (OGCM) is coupled to an empirical atmospheric model that specifies a wind stress field from a given sea surface temperatur e (SST) field. The stress is estimated by singular value decomposition of the covariance between observed surface wind stress and SST fluctu ations. Two versions of the atmospheric model are employed: one includ es only spatial patterns of the atmospheric feedbacks associated with interannual variability, whereas the other also includes spatial patte rns associated with the annual cycle. In the latter version, wind stre ss coupling in the seasonal cycle is modeled on the same basis as in t he interannual variability. The seasonal cycle enters through prescrib ed heat flux and is modified by momentum-flux feedbacks. In the OGCM, two vertical mixing schemes-Philander-Pacanowski (PP)and a modified sc heme-are used. Simulated ENSO anomalies have a reasonable spatial stru cture compared to observations, and the form is not strongly sensitive to the atmospheric model or mixing scheme. SST anomalies evolve large ly as a standing oscillation, though with some westward propagation; h eat content evolution is characteristic of subsurface memory, consiste nt with a mixed SST-ocean dynamics mode regime. In the absence of the seasonal cycle, the ENSO period is affected by vertical-mixing: about 2.3 years for the modified scheme and slightly less than 2 years for t he PP scheme. Indications of irregular or multifrequency behavior are also found. Interaction with the seasonal cycle frequency locks the in terannual signal to a quasi-biennial period. The seasonal cycle in the eastern Pacific is well simulated by the coupled model. Wind stress f eedbacks are an important part of the cycle near the equator but are n ot the sole factor in producing westward propagation along the equator . The seasonal cycle in the western Pacific shows great sensitivity to the mixing scheme. With the PP scheme, small errors in the uncoupled simulation are amplified by coupling; with the modified scheme, great improvements are obtained, These differences also provide an example o f nonlinear interaction between ENSO and the coupled seasonal cycle. W ith the PP scheme, the amplitude of the ENSO signal increases with cou pling, but at strong coupling competition with ENSO can decrease the a mplitude of the seasonal cycle in the cold tongue region. However, wit h the modified scheme, although the irregularity of interannual variab ility is increased, stronger coupling does not affect the amplitude of the coupled seasonal cycle in equatorial SST. Simulating the seasonal cycle on the same basis as interannual variability thus provides much stronger constraints on subgrid-scale parameterizations than simulati ng ENSO alone.