Variations in Low-Latitude Circulation and Extreme Climatic Events in the Tropical Americas

Departure characteristics of the general circulation in the tropical Atlantic and eastern Pacific are studied in relation to extreme climatic events identified from collectives of long-term rainfall stations and other hydrometeorological parameters, with emphasis to the Central American-Caribbean region (CARIB). Ship observations during 1911–72 compiled with a 1° square latitude-longitude resolution and extending between 30°N–30°S from the African coast to the eastern Pacific form a major observational basis. The quality of the rainy season in CARIB has a large negative correlation with annual precipitation in the U.S. Central Great Plains, and with rainfall and sea surface temperature along the Ecuadorian/Peruvian littoral; in addition there are weak negative correlations with northeastern Brazil rainfall and with discharge and water level in northern tributaries of the Amazon, and a strong positive correlation with hydrometeorological events in Subsaharan Africa. Northeastern Brazil rainfall is strongly negatively correlated with sea surface temperature along the Eucador/Peru coast, but not with rainfall. During extreme dry years in CARIB the North Atlantic high expands equatorward, meridional pressure gradients steepen, and the trades are stronger, albeit in a somewhat more southward location; at the same time, the ITCZ over the eastern Pacific stays farther south and the South Atlantic high contracts on its equatorward side. For extreme wet years, the reverse departure patterns from the 1911–70 mean maps are characteristic. Sea surface temperature anomalies reflect a response to variations in the subtropical highs and major ocean currents: advection of cold waters in the eastern part of the oceans is favored by equatorward expansion of the subtropical high in the respective hemisphere; the wintertime Gulf Stream system has a distinct signature concomitant with departures in the equatorial and South Atlantic; and in the equatorial eastern Pacific departures reverse from the winter preceding toward the height of an extreme rainy season, warm waters in July/August being characteristic for drought in CARIB. In view of the strong spatial correlations, departure patterns constructed from stratification according to extreme events in CARIB are expected to have more general validity. Anomalous rainy season conditions are signaled in advance by large-scale departure patterns in January/February, thus offering the prospect of foreshadowing extreme rainy season behavior from the setup of low-latitude circulation during the preceding northern winter.