Characterization of the 1970s climate shift in South America

Abstract

The 1976-1977 cold-to-warm sea surface temperature (SST) shift in the tropical Pacific Ocean, which has been associated with a phase change of the Pacific Decadal Oscillation (PDO) index, separated a 'La Nina-like' decadal regime from an 'El Nino-like' one. In this article, we analyse the differences of mean of annual and austral-summer (DJF) temperature, precipitation, and sea-level pressure (SLP) over South America (SA) between 1961-1973 and 1978-1990, and explore the occurrence of significant shifts in their time series. Our sources are instrumental records, gridded interpolated data, and reanalyses. Although major regional differences in the intensity of the signal are detected, the climate shift is identified in all variables. In the mid-1970s at annual level, reanalysis SLP data reveal the onset of a step-like anticyclonic circulation anomaly in the southern tip of SA and an abrupt weakening of the Southeast Pacific Subtropical Anticyclone (SEPA). This latter feature may have partly induced the rapid warming observed along the tropical-extratropical west coast of the continent through the weakening of the cold Humboldt current system. An abrupt warming was also detected in surface air temperature (SAT) composites located along the coast of the northern part of SA and in Southeastern SA (SESA). During summer, we found a particularly conspicuous shift-like warming over Southern South America (SSA, comprising Patagonia). Besides, a shift-like increase (decrease) in annual mean precipitation is observed over Central Argentina and in the tropics, to the south (north) of 10 degrees S. In line with previous studies, we conclude that both the interannual (El Nino-Southern Oscillation, ENSO) and the interdecadal (PDO) variability modes seem to have had an incidence in the manifestation of the 1970s climate shift, and that its magnitude appears to be unprecedented during the 20th century, as shown in particular by century-long SAT composites from northern Chile and SSA.