Zonal Atmospheric Rivers impinging the West Coast of Extratropical South America: A silent threat

Atmospheric Rivers (AR) landfall in the extratropical west coast of South America about 35-45 days per year -one of the largest frequencies worldwide- and contribute 45%-60% of the annual precipitation in south-central Chile. As in other regions, most ARs are located ahead of a cold front impinging the coast with its long axis oriented from NW to SE (~45° with respect to the coastline) causing moderate precipitations under cold conditions. This is a generally beneficial condition that builds up a much-needed snowpack over the Andes cordillera. A few ARs, however, develops ahead of stationary fronts extending thousands of kilometers across the South Pacific with a zonal direction and little displacement in the cross-front direction. This pattern is referred to as a Zonal Atmospheric River (ZAR).

In this work we present the ZAR structure at the synoptic and local scale. A distinctive subtropical low and blocking high are generally present over the central Pacific several days before the storm in Chile (see attached figure). Poleward flow downstream of the low transports moist, warm air from its tropical reservoir into midlatitudes, and equatorward flow downstream of the blocking high steepened the north-south temperature contrast over the eastern Pacific. These two ingredients provide the conditions for the formation of the ZAR that rapidly moved eastward to reach South America. Once the ZAR landfalls, substantial prefrontal precipitation (up to 100 mm) can accumulate at the coast, inland valleys and western slope of the Andes over periods of 24-72 hours under relatively warm conditions thus increasing the pluvial area in the mountain basins. Periods of high rainfall intensity have also been observed during ZAR events. Thus, Zonal Atmospheric Rivers pose significant hydro-meteorological hazards for south-central Chile.