SHORTWAVE FEEDBACKS AND EL-NINO-SOUTHERN-OSCILLATION - FORCED OCEAN AND COUPLED OCEAN-ATMOSPHERE EXPERIMENTS

Changes in tropical sea surface temperature (SST) can produce changes in cloudiness that modify incoming solar shortwave (SW) radiation, whi ch in turn affects SST. The effects of this negative feedback on Pacif ic interannual variability are examined in forced ocean model and hybr id coupled ocean-atmosphere model simulations. Two empirical schemes a re used to model the large-scale, low-frequency response of surface SW to SST anomalies. The first scheme attempts to account for the nonloc al nature of the atmospheric response to SST based on patterns of cova riability analyzed through singular value decomposition. In the observ ations the primary coupled mode of variability is composed of a SW ano maly in the central Pacific that covaries with anomalous SST in the ea stern Pacific. This is applied in the model as a nonlocal SW feedback. The second scheme examines the effects of a purely local feedback wit h a spatially varying coefficient of magnitude chosen similar to the f irst scheme. In almost all cases the second scheme behaved similarly t o the first, presumably because the correlation scale of SST is large enough for El Nino-Southern Oscillation (ENSO) dynamics that there is little sensitivity to the local approximation in the SW feedback. In s imulations forced by time series of observed wind stress the SW feedba ck induced very minor SST damping. Results from a simplified heat budg et analysis showed that while the SW feedback increased the local heat flux damping on SST, it also induced a mean shallowing of the mixed l ayer. The resulting changes in both the local mean vertical temperatur e gradient and the zonal velocity response to the wind stress acted to oppose the local heat flux damping effects. When observed SW anomalie s were applied to forced simulations, the simulated SST anomalies were modified as expected, and agreement with observed SST improved. In co upled simulations the SW feedbacks had greater impact than in the case of specified stress. The main effects were to decrease the magnitude of the warm and cold SST anomalies in the central Pacific, while leavi ng the pattern and evolution of ENSO anomalies essentially unchanged e lsewhere. The SW feedbacks thus produce a modest improvement of the mo del ENSO SST pattern compared with observations, although they tended to shorten the period of the model ENSO cycle. Overall the results sug gest that large-scale SW feedbacks are of quantitative importance to s imulating some aspects of the ENSO cycle but are not critical to the o verall occurrence of the phenomenon.