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The Influence of Superparameterization on the Simulation of the West African Monsoon

Rachel Rose McCrary

The West African Monsoon (WAM) is a complicated system involving many interactions between the atmosphere, ocean and land surface on a range of temporal and spatial scales, from individual rain events to global atmospheric dynamics. Coupled general circulation models (CGCMs), which are used to make future climate change projections, have difficulty representing the annual cycle of precipitation over West Africa. Many models place the summertime precipitation maxima over the Gulf of Guinea rather than over the continent (Cook and Vizy, 2006).

One reason why CGCMs have difficulty representing the monsoon is their inability to represent key rain making weather systems, such as African easterly waves (AEWs). AEWs are synoptic-scale disturbances have periods of approximately 3-5 days, wavelengths of 2000-6000 km, and move at about 7-9 m s-1. They are the dominant mode of atmospheric variability over West Africa during the summer (June - September; JJAS) and are important for organizing precipitation over this region. Current theory suggests that AEWs are initiated by convective heating in central and eastern Africa (Berry and Thorncroft 2005; Hsieh and Cook 2008) and propagate westward feeding off of the barotropic-baroclinic instability associated with the African easterly jet (AEJ; Hall et al. 2006). While there is a clear connection between AEWs and convection (Mekonnen et al. 2006: Kiladis et al. 2006), our understanding of specific interactions and potential feedbacks for these waves is incomplete.

CGCMs which use traditional parameterization techniques to represent convection are unable to capture the complex multi-scale interactions between AEWs and convection. A simulation done with a CGCM with explicit convection the so called "Superparameterized" CCSM or SP-CCSM (Stan et al. 2010) shows that the inclusion of explicit (rather than parameterized) convection improves the overall representation of the West African monsoon. Adding superparameterization improves the position and intensity of the summer maximum in precipitation which is shifted from over the Gulf of Guinea in the standard CCSM (not realistic), to over the continent in SP-CCSM which is in keeping with the observations (Figure 1). AEWs and their relationship with convection are also improved in the SP-CCSM: In the standard model, little to no easterly wave activity occurs over West Africa, and the relationship with convection is tenuous at best. SP-CCSM on the other hand produces strong AEWs over the region that exhibit similar horizontal and vertical structures to observations (Figure 2). AEWs in SP-CCSM are strongly coupled to convection, more so than is supported by observations. An examination of the energetics of the simulated AEWs suggests that convection drives the generation and propagation the waves in SP-CCSM.

This study also suggests that in the superparametrized model, intraseasonal precipitation variability over West Africa may be modulated by variations in precipitation over the Indo-Pacific regions corresponding with the Madden Julian Oscillation and the Indian summer monsoon. These results support observational evidence of a link between the Indo-Pacific and West Africa. The SP-CCSM is currently being used to investigate teleconnections between West African climate and the Indo-Pacific.

References

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