Seasonal Dependence of ENSO Teleconnections over South America and Relationships with Precipitation in Uruguay

Abstract

The El Nino-Southern Oscillation (ENSO) has an established impact on precipitation in Uruguay during austral spring (October-December). This impact is absent in peak summer (January-February), and returns weakly in fall- winter (March-July). Interannual and intraseasonal variability of the atmospheric circulation over South America and the South Pacific is investigated using the NCEP-NCAR reanalysis data for these seasons. The leading empirical orthogonal function (EOF) of seasonally averaged 200-hPa winds over South America is found to be associated with ENSO through a pronounced Walker cell component in all seasons. However, during spring, this pattern acquires an extratropical teleconnection that links the circulation over southeastern South America (SESA) with ENSO. This extratropical teleconnection disappears in summer, when the circulation over SESA is dominated by variability in the South Atlantic convergence zone. In fall, extratropical South America again becomes affected by a wavelike pattern that extends over the South Pacific, but it is uncorrelated with ENSO. On intraseasonal timescales, a cluster analysis of daily geopotential height fields over the South Pacific sector reveals three wave train-like circulation regimes with similar structures in all seasons. During the transition seasons (but not summer), the frequencies of occurrence of two of these regimes are found to be significantly different from normal in years when the interannual wavelike EOFs are pronounced. Interannual anomaly patterns are constructed from the intraseasonal regimes according to these changes in their frequency of occurrence, and shown to resemble quite closely the interannual EOFs over the South Pacific sector. These results provide evidence that the interannual teleconnection patterns seen over the South Pacific in austral spring and fall-winter are due to changes in the frequency of occurrence and amplitude of intraseasonal circulation regimes. The Rossby wave source composited over ENSO years suggests that ENSO heating anomalies are able to trigger these changes in regime occurrence and amplitude during October-December through Rossby wave propagation, leading to the known ENSO teleconnection in austral spring. By contrast, the interannual teleconnection over the South Pacific during fall-winter appears to be due to essentially random changes in the frequency of occurrence of the intraseasonal circulation regimes, which are found to be much larger than during austral summer when no extratropical teleconnection pattern exists.