Water Vapor Budget Evaluation in Atmospheric Rivers Associated with Heavy Rainfall Events in the Southern Andes

Atmospheric rivers (AR) are transient, narrow, and elongated channels in the atmosphere through which poleward transport of water vapor takes place from the tropics. In central-southern Chile, ARs contribute about 50% of the annual precipitation. ARs also produce extreme rainfall events, which may occasionally result in compound events. The water vapor associated with an AR originates from evaporation from the ocean, local mass convergence, and direct poleward moisture transport from the tropics. One way to understand the sources of moisture along an AR is the analysis of column integrated water vapor (IWV) budget, as suggested in previous studies. In this study, we have evaluated moisture and heat budgets for 50 land-falling zonal AR between 1980 and 2023 that caused heavy to extreme rainfall in the Southern Andes. The ARs are identified based on a global AR detection algorithm included in the ARTMIP (Guan and Waliser, 2018). The budget terms for all the ARs are calculated using the ERA5 reanalysis datasets along a backward trajectory obtained from the HYSPLIT trajectory model at 6-hour intervals. The budget analysis suggests that while mass convergence dominates the vertically integrated water vapor transport (IVT), the advection of moisture is significantly enhanced near the coast and the landfalling region. It is shown that the enhanced IWV bands from the tropics and subtropics can merge along the AR channel to enhance moisture convergence. The convergence of the tropical IWV bands occurs in a region between a stationary subtropical anticyclone and a midlatitude trough which migrate towards the continent. This region of convergence further acts as a source of moisture and heat that ends up being transported to the subtropical Andes.  Over the land-falling regions, the IVT convergence and precipitation rate remain tightly associated (R2 =0.79), suggesting stronger IVT convergence (intense AR) may produce heavy rainfall events in Chile. It is also shown that synergistic action between moisture and heat enhances IVT convergence along an AR. An enhanced IVT convergence along AR channels produces intense AR. These intense AR can produce extreme rainfall events when they encounter coastal mountains and the Andes through the orographic precipitation enhancement mechanisms.