Enso and climatic signals across the Indian Ocean Basin in the global context: Part I, interannual composite patterns

This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after st rong, near-global El Nino and La Nina episodes. It then examines MSLP and S ST anomalies in the 2-2.5-year quasi-biennial (QB) and 2.5-7-year low-frequ ency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between r ainfall and joint spatiotemporal empirical orthogonal function (EOF) time s eries band pass filtered in the QB and LF bands. The seasonal composites indicate that the El Nino-1 (La Nina-1) pattern ten ds to display a more robust and coherent (weaker and less organized) struct ure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an ev ent, when El Nino events tend to collapse quite quickly. Climatic variables over the Indian Ocean Basin linked to Fl Nino and La Nin a events show responses varying from simultaneous, to about one season's la g. In general, SSTs tend to evolve in response to changes in cloud cover an d wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker cir culation is found over the African continent (central Indian Ocean) during La Nina phases, and that the opposite configuration occurs in El Nino event s. These alternations are linked to distinct warm-cool (cool-warm) patterns in the north-south SST dipole over the western Indian Ocean region during the Fl Nino (La Nina) events. An examination of MSLP and SST anomaly patterns in the QB and LF bands show s that signals are more consistent during El Nino-1 and El Nino sequences t han they are during La Nina-1 and La Nina sequences. The QB band has a tend ency to display the opposite anomaly patterns to that seen on the LF band d uring the early stages of event onset, and later stage of event cessation, during both El Nino-Southern Oscillation (ENSO) phases. Fl Nino events tend to be reinforced by signals on both bands up to their mature phase, but ar e then seen to erode rapidly, as a result of the presence of distinct La Ni na anomalies on the QB band after their peak phase. During La Nina events, the opposite is observed during their cessation phase, Both QB and LF bands often display SST dipole anomalies that are not clearl y evident in the raw composites alone. An eastern Indian Ocean SST dipole s hows a tendency to occur during the onset phase of particular El Nino or La Nina episodes, especially during the austral autumn-winter (boreal spring- summer) and, when linked to tropical-temperate cloud bands, can influence A ustralian rainfall patterns. Analyses of seasonal correlations between rainfall and joint MSLP and SST E OF time series on QB and LF bands and their dynamical relationship with MSL P and SST anomalies during El Nino and La Nina events, show that the interp lay between atmospheric circulation and SST anomalies dictates the observed rainfall response. Instances where either, or both, QB and LF bands are th e prime influence on observed rainfall regimes are evident. This ability to discriminate the finer structure of physical relationships, correlations a nd patterns provides a deeper insight into Indian Ocean responses to ENSO p hases. Copyright (C) 2000 Royal Meteorological Society.