Indian Ocean Dipole Modoki (IODM) and Its Responses to Diabatic Heating and Circulation

Tropical oceans play major roles in the natural variability of the world climate. Anomalous coupled ocean–atmosphere phenomena generated in the tropical Indian oceans produce global atmospheric and oceanic circulation changes that influence regional climate conditions even in remote regions. Based on the importance and the impact of the Indian Ocean Dipole events on Indian precipitation and circulation, we introduce a new categorization of such events. The endeavour of the present research is to propose a new classification method to distinguish different aspects of Indian Ocean Dipole (IOD) and extract the main features in terms of the Canonical (IODC) and the Modoki (IODM) type IOD . The canonical IOD is associated with warming or cooling anomalies over western tropical Indian ocean, whereas the Modoki IOD is associated with central tropical Indian ocean heating or cooling. These two groups of IOD are mainly based on different structure of sea-surface temperature anomalies (SST): the canonical IOD is associated with a dipolar structure of SST anomalies where as Modoki IOD is associated with tri-polar structure over the Indian ocean. Estimates of the structure of three-Dimensional diabatic heating structure supports the new classification. The diabatic heating is estimated as a residual in the thermodynamic equation using the full resolution 37-level ERA-Interim 6 hourly reanalysis. The diabatic heating is associated with radiative fluxes, phase changes of water substance and turbulent flux of sensible heat from the earth’s surface, so it includes precipitation. Precipitation characteristics examined during both IOD classes from observed data are consistent with the different type of IOD events. The methodology employed in this study is Singular Value Decomposition Analysis (SVDA) for identifying the temporally synchronous spatial structure. SVDA isolates the leading pairs of the coupled signals that mainly explain the relationship between the coupled fields. The investigation suggests that not only the prediction of occurrence of IOD but also, the types of IOD are equally important. The assessment of the skill of coupled general circulation models to simulate and predict the types of the IOD is the important topic of future studies.