On Modes of Tropical Circulation and Climate Anomalies

Large-scale departure maps of sea level pressure (SLP) and sea surface temperature (SST) are presented for the tropical Atlantic and eastern Pacific Oceans, as obtained by stratification with respect to extreme climatic events in key regions of the tropical Americas. Drought in the Central American-Caribbean region is characterized by an equatorward expansion of the North Atlantic high, a band of anomalously cold water extending across the North Atlantic and a positive SST anomaly in the eastern Pacific. Drought in northeast Brazil is associated with high SLP over the South Atlantic and low SLP over the North Atlantic, cold water in the South Atlantic, a band of positive SST anomalies across the North Atlantic, and positive SST departures in the eastern Pacific. During the Ecuador/Peru El Nin̄o, SLP in the eastern Pacific is low and SST high, and positive SLP departures dominate the tropical Atlantic. Independently, preferred modes of departure configurations are identified from principal component analysis of SLP (1942–71) and SST (1948–71). The first four principal components of SLP explain 43, 26, 14 and 10%, and the first four SST components 42, 24, 10 and 6% of the variance. The first principal components of SLP and SST, and the sea temperature along the Ecuador/Peru coast are highly correlated. This ensemble of departure configurations closely replicates the ones characteristic of the Ecuador/Peru El Nin̄o. The second principal components of SLP and SST are correlated, as is the third SLP with the fourth SST component. However, the departure patterns obtained by stratification with respect to regional climate anomalies provide no overall analogy to these pattern ensembles. The fourth principal SLP and the third SST components are highly correlated. Both possess a high correlation of one sign with rainfall in the Central American-Caribbean region, and a high correlation of opposite sign with precipitation in northeast Brazil. This pattern ensemble offers an excellent replication for the two sets of departure patterns obtained by stratification with respect to drought in the Central American-Caribbean region and north-east Brazil. An additional principal component analysis was performed in which SLP, SST, and the three aforementioned regional hydrometeorological time series simultaneously served as input. Results corroborate the separate SLP and SST principal component analyses. The stratification and principal component analyses are complementary approaches, in that they yield realistic and physically plausible patterns. It is hypothesized that mass exchanges on the scale of the near-global tropics dominate the pressure pattern and are related to regional circulation changes and climate anomalies.