Modulation of Convective Activity over South America by the MJO

Convective storms are an important component in the hydrological cycle over the Amazon and La Plata basins in South America. The convective population over these regions ranges from frequent shallow convection to sporadic intense and organized convection. However, the predominant storm mode can vary on a day-to-day basis, which may have implications for precipitation amount or storm severity. Here, we utilize the TRMM satellite Precipitation Feature database (1998-2014) to analyze different parameters related to convective activity in South America and how they are modulated by the Madden-Julian Oscillation (MJO) during austral summer (DJF). The convective variables utilized are classified into two groups to represent different storm modes: variables related to storm horizontal extent (volumetric rain [VOLR], number of rain pixels [NPIX]) and storm vertical development (flash count [FLASH] and maximum height of 40dBZ echoes [MXHT40]). The area of study comprises four different subregions: northern Amazon, southern Amazon, eastern Brazil, and subtropical South America.

Composites of the different convective parameters per MJO phase, based on the RMM index (Wheeler and Hendon 2004), reveal that for the Amazon subregions parameters related to storm vertical development exhibit a dissimilar modulation pattern than those associated with horizontal development. An enhancement (suppression) in VOLR and NPIX is observed for days under active MJO phases 1-2 (3-5), while for FLASH and MXHT40, an enhancement (suppression) is observed for days under active MJO phases 3-5 (6-8). This suggests that the MJO modulates the storm mode over the Amazon region, favoring the formation of deep isolated storms under some MJO phases but allowing mesoscale convective systems to grow more under different MJO phases. However, the subregions in eastern Brazil and subtropical South America show a different storm mode modulation by the MJO: the same MJO phases either enhance or suppress both horizontal and vertical related parameters in tandem. This suggests that the MJO modulations of intensity of convection and rainfall amount are in phase at these locations.

The mechanisms that lead to these various modulations in convective activity are explored by looking at composites of vertical soundings and regional-scale composite maps of atmospheric circulation and moisture flux derived from ERA5 reanalysis data. In the Amazon basin, the vertical profiles show that the atmosphere is drier at all levels and warmer at low-levels during those MJO days that promote deep isolated storms, consistent with an enhancement of CAPE over those regions, especially in the southern Amazon. However, in the subtropics and eastern Brazil, phases with enhanced convective intensity show an increase in atmospheric moisture that coincides with an increase in CAPE, especially in the subtropics. Regional MJO phase composite maps for precipitable water and CAPE reinforce what was observed from the vertical profiles. An analysis of planetary-scale anomalies is also presented to explore the teleconnection mechanisms that lead to the regional and storm-scale modulation observed by the MJO. These results could provide useful insights for sub-seasonal prediction of storm modes in South America.