A Theory for the Indian Ocean Dipole–Zonal Mode

Abstract

Four fundamental differences of air?sea interactions between the tropical Pacific and Indian Oceans are identified based on observational analyses and physical reasoning. The first difference is represented by the strong contrast of a zonal cloud?SST phase relationship between the warm and cool oceans. The in-phase cloud?SST relationship in the warm oceans leads to a strong negative feedback, while a significant phase difference in the cold tongue leads to a much weaker thermodynamic damping. The second difference arises from the reversal of the basic-state zonal wind and the tilting of the ocean thermocline, which leads to distinctive effects of ocean waves. The third difference lies in the existence of the Asian monsoon and its interaction with the adjacent oceans. The fourth difference is that the southeast Indian Ocean is a region where a positive atmosphere?ocean thermodynamic feedback exists in boreal summer. A conceptual coupled atmosphere?ocean model was constructed aimed to understand the origin of the Indian Ocean dipole?zonal mode (IODM). In the model, various positive and negative air?sea feedback processes were considered. Among them were the cloud?radiation?SST feedback, the evaporation?SST?wind feedback, the thermocline?SST feedback, and the monsoon?ocean feedback. Numerical results indicate that the IODM is a dynamically coupled atmosphere?ocean mode whose instability depends on the annual cycle of the basic state. It tends to develop rapidly in boreal summer but decay in boreal winter. As a result, the IODM has a distinctive evolution characteristic compared to the El Niño. Sensitivity experiments suggest that the IODM is a weakly damped oscillator in the absence of external forcing, owing to a strong negative cloud?SST feedback and a deep mean thermocline in the equatorial Indian Ocean. A thermodynamic air?sea (TAS) feedback arises from the interaction between an anomalous atmospheric anticyclone and a cold SST anomaly (SSTA) off Sumatra. Because of its dependence on the basic-state wind, the nature of this TAS feedback is season dependent. A positive feedback occurs only in northern summer when the southeasterly flow is pronounced. It becomes a negative feedback in northern winter when the northwesterly wind is pronounced. The phase locking of the IODM can be, to a large extent, explained by this seasonal-dependent TAS feedback. The biennial tendency of the IODM is attributed to the monsoon?ocean feedback and the remote El Niño forcing that has a quasi-biennial component. In the presence of realistic Niño-3 SSTA forcing, the model is capable of simulating IODM events during the last 50 yr that are associated with the El Niño, indicating that ENSO is one of triggering mechanisms. The failure of simulation of the 1961 and 1994 events suggests that other types of climate forcings in addition to the ENSO must play a role in triggering an IODM event.