Representation of thermodynamical processes in CFSv2 for the prediction of extended monsoon episodes over south Asia

During the peak monsoon season (July-August), breaks lasing for 7 days or more (termed extended breaks) and active conditions prevailing for 7 days or more (termed extended active phase) often lead to seasonal drought and flood conditions. These extended monsoon episodes are hypothesized to arise due to nonlinear interaction between intraseasonal variability and slowly varying boundary forcing. Thus, in CFSv2, skillful prediction of such episodes requires realistic representations of boreal summer intraseasonal variability and ENSO-monsoon teleconnection.

In ensemble hindcasts performed with CFSv2, first, moist static energy (MSE) budget diagnostics is applied to elucidate processes that initiate and maintain dryness (or lack thereof) over south Asia during the developing phase of El Nino and La Nina. Second, similar diagnostics are performed during extended monsoon episodes. A particular focus is paid on identifying processes (e.g., moisture-convection feedbacks) that preclude the revival of the monsoon after an extended break episode. Third, the applicability of weak temperature gradient approximation over south Asia is discussed. Finally, robustness of MSE budgets obtained from a suite of reanalysis products is used for model validation.

Direct implications of the process-based diagnostics obtained here include: (i) a test-bed for assessing representation of physical processes employed in climate models that are used for monsoon prediction; (ii) lays a pathway for improvements in model physics and (iii) justifies the need for direct observations of moisture and other thermodynamical variables over the monsoon region.