A late 1980s shift in El Niño influence on rainfall over Uruguay during austral spring

Abstract

Rainfall over Uruguay is characterized by large interannual variability partially due to the influence of El Niño–Southern Oscillation (ENSO), such that El Niño (La Niña) events on average lead to increased (decreased) rainfall during austral spring (October–December) particularly in the northern portion of the country. Here we document a clear shift in the late 1980s of the influence of ENSO on rainfall over Uruguay during this season. Analyses based on observed station data and a reanalysis show that before the late 1980s ENSO influence was relatively weak and only in the northern portion of the country. After the late 1980s the ENSO signal became stronger and affected both the southern and northern portions of Uruguay. Moreover, we found that this shift was mainly related to the impact of El Niño events, not of La Niña. Before the 1980s there is a negative relationship between the magnitude of El Niño events and rainfall anomalies over Uruguay, while after this date the relationship became positive so that rainfall anomalies are largest for strong El Niño events. The change in the impact of El Niño events is traced back to distinct extratropical teleconnections. To explore this, we defined a South American Teleconnection Index (ITSA), which characterizes the geopotential height anomalies over South America that determine the positive rainfall anomalies during El Niño over Uruguay. It is shown that after the late 1980s the ITSA is well connected to ENSO, while before late 1980s ITSA is more dependent on other variability modes such as the Southern Annular Mode.