Influences of the Atlantic Warm Pool on Western Hemisphere Summer Rainfall and Atlantic Hurricanes

The Atlantic warm pool (AWP) of water warmer than 28.5°C comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic (TNA). The AWP reaches its maximum size around September, with large AWPs being almost 3 times larger than small ones. Although ENSO teleconnections are influential on the AWP, about two-thirds of the large and small AWP variability appears unrelated to ENSO. The AWP is usually geographically different from the TNA; however, the AWP size is correlated with the TNA SST anomalies. During August to October, large AWPs and warm TNA are associated with increased rainfall over the Caribbean, Mexico, the eastern subtropical Atlantic, and the southeast Pacific, and decreased rainfall in the northwest United States, Great Plains, and eastern South America. In particular, rainfall in the Caribbean, Central America, and eastern South America from August to October is mainly related to the size of the AWP. Large (small) AWPs and warm (cold) TNA correspond to a weakening (strengthening) of the northward surface winds from the AWP to the Great Plains that disfavors (favors) moisture transport for rainfall over the Great Plains. On the other hand, large (small) AWPs and warm (cold) TNA strengthen (weaken) the summer regional Atlantic Hadley circulation that emanates from the warm pool region into the southeast Pacific, changing the subsidence over the southeast Pacific and thus the stratus cloud and drizzle there. The large AWP, associated with a decrease in sea level pressure and an increase in atmospheric convection and cloudiness, corresponds to a weak tropospheric vertical wind shear and a deep warm upper ocean, and thus increases Atlantic hurricane activity.