Structure of SST and Surface Wind Variability during Indian Ocean Dipole Mode Events: COADS Observations

Abstract

A study of the detailed spatiotemporal characteristics of the Indian Ocean dipole (IOD) mode in SST and surface winds using available observations from 1958 till 1997 is reported. The analysis is used to address several of the controversial issues regarding the IOD. One key finding of this study is that interdecadal fluctuations contribute strongly to tropical Indian Ocean (TIO) SST variability; in SST anomalies (SSTA) interdecadal variance is as strong as interannual variance. Over both the western and eastern TIO, an accelerated warming of SST after the mid-1970s is apparent. The lack of anticorrelation between western and eastern TIO SSTA occurs only in this latter half of the analysis period. In order to examine the hypothesis that the IOD is a part of ENSO evolution in the TIO, the temporal characteristics of IOD indices have been compared with Niño-3. On the basis of several quantitative comparisons that include wavelet and cross-wavelet analysis, several important differences between the two phenomena are reported. These differences are highlighted to argue that the IOD is not a part of ENSO evolution in the TIO. On the other hand, a striking similarity is found in the temporal structure of atmospheric and oceanic variability within the TIO that is suggestive of IOD arising from inherent coupled air?sea interactions in the TIO. ENSO events that do not co-occur with IOD have been isolated and their impacts on TIO SSTA and winds described. Similarly, the characteristics of IOD events that occur independently of ENSO are described. Based on the characteristics of these two groups a hypothesis is suggested through which both phenomena may interact. It is noted that ENSO events co-occurring with IOD events are much stronger compared to non-co-occurring events. On the other hand, IOD events that are independent of ENSO as well as those that co-occur with it appear to have the same strength.