A Lagrangian characterization of convergence zones in South America

Convergence zones (CZs) contribute to the hydrological cycle on subseasonal, seasonal and interannual scales in the South American continent. The South Atlantic Convergence Zone and the Intertropical Convergence Zone have been shown to influence the variability and extremes of precipitation, affecting water availability in densely populated areas, particularly in Northeast and Southeast Brazil. However, the definition of CZ events is frequently subjective and inconsistent among authors, preventing systematical cross-scale identification. We explore Eulerian and Lagrangian definitions of convergence zones as coherent structures in the moisture flux flow. The coherence is identified through invariants of the velocity gradient and Lagrangian deformation tensors. For each proposed definition we employ a classification and tracking algorithm over the ERA5 reanalysis horizontal moisture flux data (1980-2010). We evaluate the contribution of precipitation associated to CZ events to the hydrological cycle and compute its interannual and decadal variabilities. We evaluate the ability of HadGEM3 climate simulations in different spatial resolutions to reproduce the CZ climatology and discuss trends and variabilities from future climate simulations using the same model.