Simultaneous relationships between SST anomalies in the southwestern subtropical Atlantic Ocean and precipitation in a basin in Uruguay

INTRODUCTION

This is an exploratory study of the simultaneous relationship between sea surface temperature (SST) anomalies in the Atlantic Ocean and precipitation in Rincon de Bonete basin in Uruguay, for the period 1947-1986. A special focus is made on the relationships found for the austral summer, which involve SST anomalies in the southwestern Atlantic Ocean.

DATA AND METHODOLOGY

The database consists of monthly SST data from a 4.5 latitude by 7.5 longitude grid in the Atlantic Ocean between 37 S and 28 N and monthly precipitation data from 5 rainfall stations in Rincon de Bonete basin in Uruguay. The latter data were used to build a precipitation index through a spatial average.

For both SST and precipitation, 40 year-long sliding bimonthly time series (i.e., January-February, February-March,...December-January) were constructed. For each bimonthly period and for each gridpoint in the Atlantic Ocean, the simultaneous correlation coefficient between the SST and precipitation time series was calculated.

For periods selected according to the relevance of the results, a SST index was built and the simultaneous relationship between SST and precipitation indices was studied by means of correlation and stratification analysis, on bimonthly an quarterly bases.

PRELIMINARY RESULTS

Three periods were found for which a significant relationship associated to a region of considerable extension in the Atlantic Ocean persisted for more than one bimonthly period. For two of them, there are previous studies suggesting that they may be related to El Nino- Southern Oscillation (ENSO) phenomenon. However, this does not seem to be the case for the December-January-February (DJF) period, which shows a significant relationship between SST anomalies in a region in the southwestern subtropical Atlantic Ocean and precipitation. This preliminary study is focused on this austral summer period, both on bimonthly (DJ and JF) and quarterly (DJF) bases.

Based on the strength of the relationships found for DJF, a SST index was built by averaging the SST values corresponding to the gridpoints which gave rise to higher correlation scores. These covered a region in the Atlantic between 18 W and 48 W, and 18 S and 36 S. The correlation values between SST and precipitation indices were 0.46, 0.61 and 0.59 for DJ, JF and DJF respectively.

A stratification analysis was made in order to assess the simultaneous response of precipitation to extreme anomalies of the southwestern Atlantic SST index. Threshold values of +0.5 C and -0.5 C were selected to define "warm" and "cold" years respectively. The results show that "warm" ("cold") years are strongly associated to positive (negative) rainfall anomalies. This relationship is significant or highly significant for 3 of the 6 considered cases. It is worth noting that, for all cases, all the "cold" years belong to the first half of the period (1947-1966), and most of the "warm" years to the second half. This is associated with the existence of a trend in the SST index time series, which does not seem to have a counterpart in precipitation.