Coupled dynamics over the Indian Ocean: spring initiation of the Zonal Mode

Atmosphere and ocean model assimilated products, in conjunction with observed precipitation and ocean model estimates of Indonesian Throughflow (ITF) transport and barrier layer thickness, are analyzed to elucidate the role of external (ENSO and ITF) and internal (monsoon) factors in the initiation of the Indian Ocean Zonal Mode (IOZM). The diagnostics show that there exists a natural mode of coupled variability in the eastern equatorial Indian Ocean (EEIO) that is weak on its own but intensifies in boreal spring/early summer, usually when ENSO-like conditions exist in the western Pacific, as implied by the Southern Oscillation Index (SOI). In the EEIO, there exists a ‘time window’ in the annual cycle—boreal spring—during which the ocean–atmosphere system is particularly sensitive to external forcing. At interannual timescales, spring atmospheric conditions in the EEIO are remotely controlled by SST in the equatorial western-central Pacific. Warm SST anomalies there cause changes in the Pacific Walker circulation and induce subsidence over the EEIO that results in negative precipitation anomalies: (i) Forced by this heat sink, an anticyclone develops in the lower atmosphere over the southeastern Indian Ocean as a Rossby-wave response, and the alongshore upwelling-favorable winds off Java–Sumatra are enhanced. (ii) The reduced surface fresh-water flux and enhanced upwelling reduce the barrier layer in the upper ocean. These processes along-with the reduction of ITF help trigger the IOZM. Once triggered, IOZM grows in summer by the Bjerknes feedback. Its interactions with the monsoon heat source result in enhanced precipitation along the monsoon trough in July–August. This north–south heating gradient favors a local meridional circulation with increased alongshore winds off Sumatra, implying the potential role of the monsoon background cycle. The hypothesis that the equatorial western-central Pacific SST anomalies control the spring precipitation variations in the EEIO/maritime continent is demonstrated by sensitivity experiments with an atmospheric general circulation model. During the spring initiation stages of the IOZM, an analysis of the mixed layer heat budget in an ocean general circulation model indicates that cooling off Java is primarily due to entrainment and also due to latent cooling, both caused by enhanced upwelling-favorable winds. r 2003 Elsevier Science Ltd. All rights reserved.