Rossby waves as key drivers of the South American Monsoon high-frequency variability

Abstract

The seasonal cycle of precipitation in South America (SA) peaks in intensity during austral summer (DJF), contributing up to 80% of the total annual rainfall. This seasonality is influenced by variability at different time-scales. At the subseasonal range, there is a lack of understanding of the mechanisms controlling highfrequency transients (8-20 days) of precipitation, which accounts for similar to 60% of the total DJF subseasonal variance. Our results show that the observed high-frequency variability features a dipole pattern in SA, whose phase is controlled by the complex interplay between upper-tropospheric Rossby waves, remotely triggered by variability in the South Pacific Convergence Zone (SPCZ) convective activity, and regional low-tropospheric moisture transport and topography. The nature of the remotely-generated signal is further identified in sensitivity experiments with a Linear Baroclinic Model. Our results indicate that this teleconnection pattern can be used as a potential tool for subseasonal precipitation predictability, and that addressing model biases in simulating the spatio-temporal characteristics of SPCZ precipitation is important for near-hemispheric climate projections.