Summer-Time Rainfall Variability in the Tropical Atlantic

Abstract

Convection and rainfall in the tropical Atlantic basin exhibit intense variations on various time scales (e.g., Nobre & Shukla, 1996; Giannini et al., 2001; Chiang et al., 2002). The Atlantic Intertropical Convergence Zone (ITCZ) stays always north of the equator with the trade winds converging into it, representing a prominent climate feature on the seasonal time scale. The ITCZ, though generally over the open ocean, extends to the northeast coast of South America during boreal spring and to the West African continent during boreal summer. Longer-than-seasonal time scale variability is also evident and has been extensively explored in the past through both observational analyses and modeling studies (e.g., Lamb, 1978a, b; Carton & Huang, 1994; Nobre & Shukla, 1996; Sutton et al., 2000). The Atlantic Nino and an interhemispheric sea surface temperature (SST) gradient mode are discovered to be the two major local forcing mechanisms (e.g., Zebiak, 1993; Nobre & Shukla, 1996), in addition to the two other remote large-scale forcings: the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) (e.g., Curtis & Hastenrath, 1995; Chiang et al., 2002; Wang, 2002). The most intense year-to-year variability in the tropical Atlantic is usually observed during boreal spring [March-April-May (MAM)], specifically in the western basin and over the northeastern portion of South America, when the marine ITCZ approaches the equator (e.g., Hastenrath & Greischar, 1993; Nobre & Shukla, 1996). Therefore, most of past studies have been primarily focused on this season (e.g., Chiang et al., 2002; Gu & Adler, 2006). Intense interannual variability has also been seen during boreal summer [June-July-August (JJA)] in the tropical Atlantic (e.g. Sutton et al., 2000; Gu & Adler, 2006, 2009). The Atlantic Nino becomes mature during this season, and the impact of the interhemispheric SST gradient mode and ENSO can still be felt in the equatorial region (e.g., Sutton et al., 2000; Chiang et al., 2002). Particularly, evident interannual variations exist in various distinct severe weather phenomena such as African easterly waves (AEW) and associated convection, and Atlantic hurricane activity (e.g., Thorncroft & Rowell, 1998; Landsea et al., 1999). These severe weather systems frequently appear during boreal summer and fall, and