Contrasting Indian Ocean SST variability with and without ENSO influence: A coupled atmosphere-ocean GCM study

Jin-Yi Yu,K M Lau

doi:10.1007/s00703-004-0094-7

Abstract

In this study, we perform experiments with a coupled atmosphere-ocean general circulation model (CGCM) to examine ENSO’s influence on the interannual sea-surface temperature (SST) variability of the tropical Indian Ocean. The control experiment includes both the Indian and Pacific Oceans in the ocean model component of the CGCM (the Indo-Pacific Run). The anomaly experiment excludes ENSO’s influence by including only the Indian Ocean while prescribing monthly-varying climatological SSTs for the Pacific Ocean (the Indian-Ocean Run). In the Indo-Pacific Run, an oscillatory mode of the Indian Ocean SST variability is identified by a multi-channel singular spectral analysis (MSSA). The oscillatory mode comprises two patterns that can be identified with the Indian Ocean Zonal Mode (IOZM) and a basin-wide warming/cooling mode respectively. In the model, the IOZM peaks about 3–5 months after ENSO reaches its maximum intensity. The basin mode peaks 8 months after the IOZM. The timing and associated SST patterns suggests that the IOZM is related to ENSO, and the basin-wide warming/cooling develops as a result of the decay of the IOZM spreading SST anomalies from western Indian Ocean to the eastern Indian Ocean. In contrast, in the Indian-Ocean Run, no oscillatory modes can be identified by the MSSA, even though the Indian Ocean SST variability is characterized by east–west SST contrast patterns similar to the IOZM. In both control and anomaly runs, IOZM-like SST variability appears to be associated with forcings from fluctuations of the Indian monsoon. Our modeling results suggest that the oscillatory feature of the IOZM is primarily forced by ENSO.

Highlights

The recent interests in the observed east–west contrast pattern in Indian Ocean sea-surface temperature (SST) anomalies have prompted the suggestion that the Indian Ocean has its own unstable coupled atmosphere-ocean mode like El Ni~no-Southern Oscillation (ENSO) (e.g., Saji et al, 1999; Webster et al, 1999)
By contrasting SST variability of the tropical Indian Ocean in these two simulations and their associated ocean-atmosphere coupling, we examine whether the Indian Ocean Zonal Mode (IOZM) is an intrinsic variability of the Indian Ocean or a forced variability influenced by ENSO
The comparisons indicate that an oscillatory mode with IOZM-like features, followed by a basin-wide warming=cooling pattern, can be identified in the Indian Ocean when the ENSO influence is included in the coupled model

Summary

We perform experiments with a coupled atmosphere-ocean general circulation model (CGCM) to examine ENSO’s influence on the interannual sea-surface temperature (SST) variability of the tropical Indian Ocean. The timing and associated SST patterns suggests that the IOZM is related to ENSO, and the basin-wide warming=cooling develops as a result of the decay of the IOZM spreading SST anomalies from western Indian Ocean to the eastern Indian Ocean. In the Indian-Ocean Run, no oscillatory modes can be identified by the MSSA, even though the Indian Ocean SST variability is characterized by east–west SST contrast patterns similar to the IOZM. In both control and anomaly runs, IOZM-like SST variability appears to be associated with forcings from fluctuations of the Indian monsoon. Our modeling results suggest that the oscillatory feature of the IOZM is primarily forced by ENSO

Introduction

Model and simulation

Interannual variability in the Indo-Pacific Run

Interannual variability in the Indian-Ocean Run

Conclusions and discussions