International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HawaiiC.-P. Chang
Department of Meteorology, Naval Postgraduate School, Monterey, CaliforniaYongsheng Zhang
International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii
Four fundamental differences of airsea 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 cloudSST phase relationship between the warm and cool oceans. The in-phase cloudSST 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 atmosphereocean thermodynamic feedback exists in boreal summer.
A conceptual coupled atmosphereocean model was constructed aimed to understand the origin of the Indian Ocean dipolezonal mode (IODM). In the model, various positive and negative airsea feedback processes were considered. Among them were the cloudradiationSST feedback, the evaporationSSTwind feedback, the thermoclineSST feedback, and the monsoonocean feedback. Numerical results indicate that the IODM is a dynamically coupled atmosphereocean 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 cloudSST feedback and a deep mean thermocline in the equatorial Indian Ocean.
A thermodynamic airsea (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 monsoonocean 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.