Climate variability in Southern South America associated with El Niño and La Niña events.
Part II: circulation anomalies.
Alice M. Grimm
Department of Physics, Federal University of Parana, Caixa Postal 19081 - CEP 80531-990 Curitiba-Brazil; e-mail: grimm@fisica.ufpr.br
Vicente R. Barros
Department of Atmospheric Sciences - University of Buenos Aires
ABSTRACT
The circulation anomalies over Southern South America and adjacent oceans, related to El Niño (EN) and La Niña (LN) events during their entire cycle, are presented through composite fields. Their consistency is assessed and their connection with the precipitation anomalies is discussed. These circulation anomalies are responsible for processes that lead to the observed precipitation anomalies in different regions of SSA. Those processes act in opposite ways during large periods of EN and LN cycles, causing opposite precipitation anomalies in these periods. Processes responsible for precipitation anomalies include changes in the subtropical jet, changes in the northerly advection of humidity, the establishment of an equivalent barotropic cyclonic (anticyclonic) anomaly over Chile and the enhancement (weakening) of advection from the Atlantic Ocean.
From May to November of the year in which EN starts (May (0)-November (0)) the 200 hPa anomaly pattern comprises cyclonic anomalies over the South Pacific high, anticyclonic ones to the south, anticyclonic anomalies in the subtropical Atlantic and cyclonic ones to the south, southeast of SSA. This pattern is nearly equivalent barotropic and dipole-like both in the Pacific and in the Atlantic, but with reversed polarity. The action centers of the pattern undergo some changes of magnitude and shifts in position during the EN cycle. It is strongest and more stable in the Pacific. This pattern strengthens the westerlies over the east Pacific in the subtropics and over the west Atlantic in the middle latitudes, causing an anomalous cyclonic curvature over SSA. In the spring, this pattern moves to the north and to the east. In December (0) and January (+) it weakens and even tends to reverse the polarity of the dipoles. In winter of the following year there is a clear displacement of the Pacific low anomaly to the east and then to the north, strengthening the subtropical jet over Southern Brazil in July (+).
During the year (0), the symmetry of the geopotential anomaly fields with opposite signs between LN and EN cases is remarkable. This symmetry indicates the importance of the linear component of the atmospheric response to EN and LN associated anomalous heating