The multi-scale nature of SAM influence on South America climate

CAROLINA S. VERA; MARIANO S. ALVAREZ ; GABRIEL SILVESTRI

Queenstown

Congreso; SPARC 2014 General Assembly; 2014

SPARC

Southeastern South America (SESA) is one the continental regions in the Southern Hemisphere (SH) significantly influenced by the activity of the Southern Annular Mode (SAM), as it has been shown by previous works. However, little attention has been paid to the multi-scale nature of that influence. The objective of this work is then to describe and discuss SAM influence on climate variability in SESA from intraseasonal, to interannual and longer timescales. The study is motivated by the fact that a better understanding of such influence might contribute to improve climate predictions in that particular region. SESA encompasses the La Plata Basin, which is the fifth largest in the world and second only to the Amazon Basin in South America in terms of geographical extent, it includes one of the most agricultural productive regions of the world, it is home of more than 100 million people including the capital cities of 4 of five countries (Argentina, Brazil, Uruguay, Paraguay, and Bolivia), generating more than 70% of the five countries GNP. Intraseasonal variability in SESA explains a significant percentage of variance all year around. In particular, during JJA, the leading pattern of intraseasonal variability, as described from band-pass filtered OLR anomalies, is characterized by a monopole centered over SESA with a northwest-southeast orientation. It has been found that 10 days before the leading pattern induces positive precipitation anomalies in SESA, a quasi-stationary anticyclonic anomaly develops over Antarctica Peninsula,. The stationary behavior of the anticyclonic center seems favored by a hemispheric circulation anomaly pattern associated with a negative SAM phase and a wavenumber 3-4 pattern at middle latitudes. Case studies have shown that such hemispheric pattern can be promoted by the early development of SAM negative phase at lower stratosphere, which in terms of weeks, promotes similar circulation anomalies at the troposphere. Precipitation also exhibits over SESA considerable interannual variability, influenced mainly by the El Niño-Southern Oscillation (ENSO). However, during austral spring, SAM influence is also relevant. SAM positive (negative) phases are associated with decreased (increased) precipitation over SESA. Moreover, SAM activity can produce a large modulation of ENSO influence in climate variability over SESA. It was found that the joint occurrence of a warm ENSO event with a negative SAM phase induces larger precipitation anomalies in SESA. However, SAM influence on SESA climate variability is not that stable as the ENSO influence is. Significant change in the spatial circulation anomaly pattern typically associated with SAM influence was found in the vicinity of South America between the 1960s?70s and 1980s?90s. In the first decades, SAM positive phase is associated with an anomalous anticyclonic circulation developed in the southwestern subtropical Atlantic that promotes precipitation increase over SESA. On the other hand, during the last decades the anticyclonic anomaly induced by the SAM?s positive phase covers most of southern South America and the adjacent Atlantic, producing decreased precipitation over SESA. The origin of such multi-decadal changes is not clear yet.