The Role of the Indian Ocean Basin-Wide and El Niño–Southern Oscillation Modes in Interannual Rainfall Variability over South America during Austral Summer

Mary T Kayano,Rodrigo A F Souza,Wilmar L Cerón,Rita V Andreoli

doi:10.3390/atmos12091094

Abstract

This paper examines the relative role of the Indian Ocean basin-wide (IOBW) mode and El Niño–Southern Oscillation (ENSO) in the atmospheric circulation and rainfall interannual variations over South America (SA) during southern summer of the 1951‒2016 period. The effects of the warm IOBW and El Niño (EN) events, and of the cold IOBW and La Niña (LN) events are examined using partial correlations. The ENSO and IOBW modes, through the associated large-scale and regional anomalous circulation patterns, induce contrasting effects on the rainfall in northeastern SA. The EN without the warm IOBW effect induces anomalously dry conditions over eastern Amazon and part of northeastern Brazil (NEB) through anomalous sinking motions of the EN-related anomalous Walker and Hadley cells and strong moisture divergence associated with a vigorous anticyclone over tropical South Atlantic (TSA) and SA. The warm IOBW without the EN effect induces anomalously wet conditions in NEB, which is marginally related to the anomalous Walker and Hadley cells but is modulated by an anticyclone over SA between the equator and 20° S, and a cyclone in the southwestern Atlantic between 20° S and 40° S. The results here might be relevant for climate monitoring and modeling studies.

Highlights

As a natural climate forcing, the El Niño–Southern Oscillation (ENSO) is the most prominent phenomenon that modulates interannual climate variability on a global scale. in South America (SA), ENSO drives rainfall anomalies in the tropical sector through changes in the Walker cell and local Hadley cell [1,2] and in the subtropical and extratropical sectors through ENSO-induced Rossby wave train teleconnection patterns [3,4].The El Niño (EN) related eastward-shifted Walker cell reduces rising motions over equatorial SA, creating favorable conditions for droughts [5,6,7]
Since the rainy season in large portions of SA peaks during southern summer [38], and the largest variability of the Indian Ocean basin-wide (IOBW) mode [19] and ENSO mature stage [39] occur in this season, the present analysis focused on southern summer
For positive principal component (PC) values, this mode shows the largest positive sea surface temperature (SST) anomalies in the centraleastern tropical Pacific area limited at the 10◦ N–10◦ S band between the dateline longitude and 120◦ W (Figure 1)

Summary

Introduction

As a natural climate forcing, the El Niño–Southern Oscillation (ENSO) is the most prominent phenomenon that modulates interannual climate variability on a global scale. Taschetto and Ambrizzi [17] provided modeling and diagnostic evidence on the warm IOBW events’ contribution to strengthening the EN-related atmospheric circulation and rainfall anomalies over SA through the Walker cell and Rossby wave train teleconnection pattern during southern autumn. They argued that the warm IOBW event is responsible for the persistent dryness over northeastern Brazil (NEB). As the IOBW and ENSO modes are closely connected [18,19], partial correlation analyses were used to examine the relative role of the IOBW and ENSO modes in the rainfall and atmospheric circulation variations over SA

Data and Methods

EOF Modes of the SST Anomalies in the Tropical Indian and Pacific Oceans

Partial

Longitude

Latitude

Discussion and Conclusions