11.2
Mechanisms of land rainfall anomalies associated with tropical Atlantic variability
J. David Neelin, University of California, Los Angeles, CA; and H. Su
Observational studies show that the rainfall anomalies in equatorial South America (Nordeste Brazil) and equatorial Africa (Guinean regions) have a positive correlation with tropical southern Atlantic sea surface temperature (SST) anomalies. Such relationships are reproduced in model simulations with the quasi-equilibrium tropical circulation model (QTCM), which includes a simple land model. A suite of model ensemble experiments is analysed using observed SST over the tropical oceans, the tropical Atlantic and the tropical southern Atlantic (30S-0), respectively (with climatological SST in the remainder of the oceans). Warm tropical south Atlantic SST anomalies yield positive precipitation anomalies over the Nordeste and Guinean coast regions and the southern edge of the Atlantic marine intertropical convergence zone (ITCZ). Several mechanisms associated with moisture variations are responsible for the land precipitation changes. Increases in moisture over the Atlantic cause positive anomalies in moisture advection, spreading the moisture increases. The increased moisture supply due to advection is enhanced by increases in low-level convergence required by moist static energy balances. The increased moisture has a further effect due to decreases in the gross moist stability which also enhance convergence. Over equatorial Africa, wind anomalies also contribute positively to moisture advection. Concurrent with positive rainfall anomalies in the adjacent land regions, the ITCZ over the Atlantic is also strengthened by Atlantic warm anomalies. This results from combined effects of increased evaporation from the warm water and moisture advection anomalies. The effects of Atlantic SST anomalies on land regions differ from those of Pacific SST anomalies in part because the proximity of Atlantic ocean to surrounding land regions puts them within range of moisture transport, rather than the dominant teleconnection pathway occuring via wave dynamically induced tropospheric temperature anomalies. However, the tendency of Atlantic SST repsonse to partially cancel Pacific response is potentially consistent with part of the Atlantic SST variability being induced by teleconnections from the Pacific. Further analysis is underway to separate such effects from Atlantic SST anomalies induced by atmospheric internal variability. .
Session 11, Observed Seasonal/Interannual Variability (Room 608)
Thursday, 15 January 2004, 11:00 AM-3:00 PM, Room 608
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