Influence of Convective Systems on Intraseasonal to Interannual Variability of the Intra-American Monsoon

We propose to examine the contribution from large-scale organized transient disturbances to intraseasonal and seasonal total rainfall of the intra-Americas Sea region (IAS). The focus will be on disturbances with periods of less than 30 days affecting the tropical regions from Mexico to northern South America. Kelvin waves, cold surges and easterly waves are of primary interest. A visual inspection of Hovmoeller diagrams reveals many examples of Kelvin disturbances that form near the dateline and propagate eastward. When they encounter South America, convection – as depicted by outgoing longwave radiation (OLR) – typically increases over tropical regions. These disturbances propagate into the Atlantic, where they substantially affect the intertropical convergence zone there (Wang and Fu 2007). Kelvin waves can also form in-situ over the Amazon basin during southern summer when cold surges originating in the middle latitudes of South America force convection at the Equator (Liebmann et al. 2008). During southern winter, when cold surges are stronger and Amazon convection is weak, they frequently propagate across the Equator and into the Caribbean. Mesoscale convective activity is seen to be significantly modulated by equatorial waves, and this will also be examined in some detail. Our initial analyses indicate that convective activity is enhanced when westward propagating disturbances encounter Kelvin waves and cold surges. It is likely that problems in representation of the climate of the IAS by models suffer from an incorrect representation of subseasonal disturbances. In addition to analyzing the aforementioned disturbances in observations, a similar analysis will be made for the coupled general circulation models that will constitute the basis of the Intergovernmental Panel on Climate Change 5th Assessment Report.

The questions we propose to address are the following:

1) What is the seasonal cycle and interannual variability of Kelvin wave activity, and how is Kelvin wave activity related to large-scale, slowly varying phenomena?

2) How do Kelvin and other equatorial waves modulate mesoscale variability and the diurnal cycle?

3) Although transients are prominent in satellite-based observations of cloud top such as OLR, to what extent do they account for seasonal totals in the IAS?

4) Do westward propagating disturbances amplify or diminish when they encounter Kelvin waves or cold surges from either hemisphere?

5) Where are the transients that affect the IAS initiated?

6) Do coupled atmosphere-ocean models represent well the subseasonal transients, and to what extent do errors in their climatology result from problems in representing these disturbances?

7) Can rainfall be predicted with skill for several days in advance from trajectories of disturbances as they approach the IAS?