The leading mode of Indian Ocean convection during the boreal cold season and associated circulation variability

The leading pattern of Indian Ocean convection between November and April and the associated circulation variability at regional and hemispheric scales is investigated through a series of observational analyses, including separating the intraseasonal and interannual contributions, calculation of the thermodynamic budget, analysis of the West Pacific sea surface temperature (SST) gradient in modifying the ENSO pattern, and ray tracing. Both intraseasonal and interannual Indian Ocean tropical convection are linked to forced subsidence and precipitation decreases over Western Asia through the interaction of forced Rossby waves with the mean climate. The Asian circulation response to intraseasonal convection consists of a single baroclinic Rossby wave while the interannual response is mixed between zonally oriented baroclinic and barotropic Rossby waves. The interannual pattern strongly resembles the subset of ENSO events that are associated with a stronger-than-average SST gradient in the western Pacific.

Indices of intraseasonal and interannual Indian Ocean convection were created by filtering area averaged outgoing longwave radiation for a low-pass filter of greater than 105 days and a band-pass filter of 33-105 days, respectively, over the region 80E-110E and 15S-5N, and are in good agreement with a monthly EOF analysis. Interannual Indian Ocean convective variability is related to a convection seesaw between the Maritime Continent and West Pacific Ocean, and is similar to the observed ENSO convection response. The similarly between the ENSO and interannual Indian Ocean convection responses improves when the SST gradient over the region 5S to 5N and 140E to 150E is strong, suggesting that the West Pacific SST gradient is a link between atmospheric convection over the Pacific and Indian Oceans. Barotropic ray tracing shows a stationary barotropic Rossby wave propagation mechanism through the North Pacific, North America, Atlantic, North Africa and Middle East into Asia originating over the Indian Ocean. Comparison of intraseasonal and interannual variability suggests that a convectively active (inactive) Indian Ocean on intraseasonal time scales corresponds to a convectively inactive (active) Indian Ocean on interannual time scales. However, intraseasonal convective activity appears to be a stronger indicator of interannual convective activity than the reverse.