Climate Variability over the Tropical Indian Ocean Sector in the NSIPP Seasonal Forecast System
Authors: Wajsowicz Roxana C. 
Published: December 2007

Prospects for forecasting Indian dipole mode (IDM) events with lead times of a season or more are examined using the NASA Seasonal-to-Interannual Prediction Project (NSIPP) coupled-model forecast system. The mean climatology of the system over the sector is reasonable, as determined from an almost-century-long run without data assimilation. However, the system presents biases, for example, too cool sea surface temperatures (SSTs), too shallow thermocline, and too strong southeasterlies along the Sumatra–Java coast in the east, and too warm SSTs, too deep thermocline, and too weak extension of the southeast trades into the Findlater jet in the west. These suggest coupling between the ocean and atmosphere is stronger, and the SST–clouds–shortwave radiation negative feedback less effective, than observed in the east with the opposite holding true in the west. Also, the negative zonal gradient in SST in the eastern equatorial basin, in contrast with the positive observed, suggests that equatorial Kelvin and Rossby coupled modes may have a different character from observed. Biases identified in the seasonal cycle, which may affect the strength and timing of IDM events, include a delayed onset of the boreal summer monsoon in the west, and a prolonged boreal summer monsoon in the east.

Eight major positive IDM events occur during the almost-century-long run over a range of El Niño–Southern Oscillation phases with a tendency to occur post–El Niño/pre–La Niña. Consistent with the identified air–sea interaction biases, the cold (warm) anomaly at the east (west) pole tends to be stronger (weaker) than observed. Also, the cold anomaly extends much farther westward and is more equatorially trapped than observed; its slow westward propagation and the structure of the associated fields is reminiscent of an unstable, coupled Rossby mode with SST governed by lateral advection due to the westward displacement of the convective anomaly from the heat source. Otherwise, the life cycle of the eight-event composite is similar in seasonal phase locking and mechanisms of evolution and decay to the canonical event.

For the decade from 1993 to the present, there were major positive IDM events in 1994 and 1997/98. Monthly mean SST anomalies over the western pole are well hindcast by the ocean component of the NSIPP system forced by observed surface fluxes with SST damped to observed values, and in which subsurface temperature data available in real time are assimilated; these data are very sparse over the Indian Ocean. Over the eastern pole, the SST anomalies are well hindcast except for the 1997/98 event, when it is too cool. The ensemble mean hindcast of the zonal surface wind anomaly of the central basin by the atmosphere component of the NSIPP system forced by observed SST is too weak during both events. These hindcasts provide initial conditions for the coupled system forecasts. Forecast ensembles for the decade 1993 onward, generated by the coupled system, give monthly mean SST anomalies averaged over the east and west poles of the IDM in agreement with observations at lead times of three months. The cool anomaly at the eastern pole is slightly too large in 1997/98, and the onset of the warm anomaly in 1997 is delayed by a month or so; its peak and decay are correctly timed. At lead times of six months, there is a significant deterioration in the forecast at the eastern pole with either false positive or negative alarms generated annually in boreal fall; that at the western pole remains good. These results are very encouraging and suggest that major IDM events have the potential to be forecast a season or more in advance.