Large-scale dynamical processes associated with summer intraseasonal variability over South America

The most distinctive feature of rainfall summer variability on intraseasonal timescales over South America is a dipolar pattern known as the South American Seesaw (SASS). Enhanced precipitation over the subtropics is associated with increased southward moisture flux from the Amazon region, while in the opposite phase; a SACZ enhancement is accompanied by decreased rainfall at the subtropical plains. SASS is usually identified as the first leading pattern from an EOF analysis of band-pass filtered OLR anomalies over South America. Observed SASS spectral analysis exhibits two distinctive and separate bands of maximum variability, one around periods between 35 and 60 days, and a sharp peak at around 22 days. The objective of the paper is to understand of the most relevant mechanisms associated with the remote forcing of SASS development.

The SASS activity in the 10-30 day and 30-90 day intraseasonal bands of variability was isolated performing two separate EOF analyses of the corresponding filtered OLR anomalies over South America. Lagged regression maps show that the SASS variability in the 30-90-day band is related to an OLR anomaly pattern characterized by an eastward traveling wavenumber 1 structure in the Tropics similar to that for MJO, and a wave train propagating downstream from a convectively active area in the tropical Pacific. On the other hand, the SASS variability in the 10-30-day band does not seem to be associated with changes in the convection over western tropical Pacific. Rossby-like wave trains emanating from the subpolar jet exit regions moving eastward towards South America characterize the circulation anomaly pattern for that variability band.

Through an energy balance, the dominant processes related to how the large-scale circulation anomalies associated with both variability bands constructively or destructively contribute to induce the local conditions for SASS development over South America, are also explored.