The Influence of ENSO Conditions on Orographic Convection and Storm Structure in Subtropical South America

El Niño-Southern Oscillation (ENSO), with its primary impact in the tropical Pacific Ocean, is known to have teleconnections to atmospheric circulations and weather patterns around the world. Previous studies have examined connections between ENSO and rainfall in the Amazon rainforest and mountainous regions of tropical South America, but little work has been done connecting ENSO phases with convection in subtropical South America. While ground-based observations of convective storms are difficult in remote regions due to the frequent lack of radar and upper air networks, the Tropical Rainfall Measuring Mission (TRMM) satellite’s Precipitation Radar (PR) has provided novel observations of convection in these areas. Analysis of these TRMM PR observations has shown that convection in the lee of the Andes Mountains is among the deepest and most intense in the world. Orographic forcing from the South American Low-Level Jet (SALLJ) impinging upon the Sierras de Cordoba (SDC) and Andes mountains causes topographic convective initiation in western Argentina. This convection can grow upscale into large mesoscale convective systems due to back-building of convection over the mountainous terrain as the SALLJ continues to advect tropical air southward toward the SDC mountains. The convection can become very intense and produce frequent large hail, flooding rains, and occasional tornadoes. With ground-based radars only being installed in Argentina over the last couple years, there is a dearth of observations for climatological studies of convection in this region, making the TRMM PR data extremely valuable to this study. A 16-year dataset from the TRMM PR was used to analyze deep and wide convection during austral spring, summer, and autumn in combination with ERA-Interim reanalysis storm composites. Results from the study show that deep and wide convection in this region occurs in all phases of ENSO in all study regions, with only some modest variations in frequency between ENSO phases. However, the most evident and consistent differences between ENSO phases, as derived from TRMM PR data, occur in the three-dimensional storm structure. Deep and wide convection that occurs during El Niño tends to be taller and contain stronger convection, while La Niña storms contain stronger stratiform echoes, as evidenced by an enhanced bright band within their stratiform regions. The synoptic and thermodynamic conditions supporting the deeper storms during El Niño, as analyzed from ERA-Interim reanalysis composites, is related to increased surface-based convective available potential energy, a farther southward extent of the SALLJ into central Argentina, and a stronger upper-level jet stream, often with a left-entrance region directly over the convective storm locations. These enhanced synoptic and thermodynamic conditions provide insight into how the frequency and structure of some of the most intense convection on Earth varies with phases of ENSO.