ENSO-related Climate Variability on Precipitation and Temperature in Southeastern South America (Uruguay)

ENSO-related Climate Variability on Precipitation and Temperature in Southeastern Southamerica

Mario Bidegain

Dirección Nacional de Meteorología, Montevideo, Uruguay

Guillermo Podesta

Rosenstiel School of Marine & Atmospheric Science, Univ. Of Miami, U.S.A.

Abstract

This paper describes results in the project "Climate variability and agriculture in Argentina and Uruguay: Assessment of ENSO effects and perspectives for the use of climate forecasts", supported by IAI’s Initial Science Program Round 2. The project addresses the effects of climate variability related to the El Niño-Southern Oscillation (ENSO) phenomenon on the agricultural sectors of Argentina and Uruguay. Long series (66 years) of climate data for meteorological stations in Uruguay and Argentina have been compiled. Meteorological variables gathered include daily maximum and minimum air temperature, daily total precipitation, and daily sunshine hours. We categorized ENSO events according to an index developed by the Japan Meteorological Agency (JMA), which selects well the known events. Composite monthly mean height and winds and anomalies at 850 hPa and 200 hPa, are taken from the NCEP/NCAR reanalysis archive for the period 1958 to 1997. 10 El Niño events and 8 La Niña events are included in the circulation anomalies features.

A general increase of rainfall mainly in last seventies and eighties is observed over region. This behavior is due (in part) to increased of Niño events in last years like 1976-1977 and 1982-1983, when the positive anomalies in rainfall affect mainly the northern region. In general the increase in rainfall is related to increased northeasterlies winds and higher levels of moisture. The general warming that occorred in the Northern Hemisphere up till 1940 was also present in the Southern Hemisphere but as yet there is evidence of the subsequent cooling during sixties that has been felt in many places in the Northern Hemisphere. Since last years in sixties temperatures rose. The changes in temperature have been slight. The mean maximun temperature and the mean maximum original and smoothed temperatures and illustrated the similarity in the maxima except during forties, cooling during sixties and increase to present. The mean minimim original and smoothed temperatures, a general increase is observed since fifthies.

General features of the climatic anomalies of precipitation, temperature and circulation over Southeasthern Southamerica are described in the context of the ENSO phases. The evolution of the anomalies in precipitation shows an increase in the late spring and early summer that is generally observed during warm ENSO phase, nevertheless during cold ENSO phase a sistematically negative anomalies overall the period was generally observed in precipitation. The anomalies associated to warm ENSO phase on maximum mean temperature shows positive anomalies during early winter and on minimum mean temperature shows significative positive anomalies during autumn and early winter. The anomalies associated to cold ENSO phase on maximun mean temperature shows a significant negative anomalies during winter and on minimum mean temperature shows a little significant negative signal during same period and in the core of summer period a significant positive anomalies. The atmospheric circulation leading to precipitation and temperature anomalies in SSA during El Niño and La Niña events are: enhacement o weakening of moisture advection from the Atlantic Ocean, enhance and/or shift of the subtropical jet, changes in the northely advection and moisture.