The influence of Indian Ocean convection on cold season Central-Southwest Asian precipitation extremes

Drought has continued to plague Central-Southwest Asia (CSWA) in recent years, with severe drought returning to the region in 2007-2008 and the early season of 2008-2009. Here we: 1) analyze these drought patterns relative to previous events, especially the exceptionally severe 1999-2001 period; 2) provide a preliminary assessment of a possible increasing trend in droughts; and 3) investigate subseasonal changes in drought pattern and intensity.

The recent winters of 2007-2008 and 2008-2009 mostly resemble the previously identified pattern of extratropical Rossby wave formation as a result of enhanced or suppressed Indian Ocean tropical convection. Rossby waves interact with the mean flow, resulting in changes to the moisture and thermodynamic budgets over Southwest Asia, thereby forcing enhanced or suppressed precipitation. As in the extreme 1999-2001 event, a combination of warm western Pacific Sea Surface Temperatures (SSTs) and La Nina conditions appear to be the driving force in increasing the tropical Indian Ocean convection. The demise of La Nina-like conditions in 2009 corresponded with a recovery to wet conditions for the end of the 2008-2009 cold season. While the occurrence of drought in the region is episodic, consistent with the link to La Nina, there appears to be an overall trend to drier conditions. This is consistent with the upward trend in western Pacific SSTs and may be an example of the dynamical influence of global warming.

We have also begun an analysis of the monthly evolution of the drought pattern and of the role of intraseasonal variability in enhancing or suppressing the drought pattern. For the 2007-2008 boreal cold season over CSWA, where there was a precipitation deficit of 34%, six distinct extratropical patterns existed, and were closely linked to Indian Ocean tropical convection. Periods of strongly suppressed (enhanced) CSWA precipitation were linked to Indian Ocean convection (descent) associated with MJO periods, and exhibited strong thermodynamic forcing of precipitation. The MJO need not be present for CSWA precipitation to be influenced as organized convection (suppression) over the Indian Ocean excited weaker extratropical Rossby waves that influence both the thermodynamic and moisture budgets, having impacts upon precipitation.