Influence of the cross-equatorial Chaco-northwest Africa pressure gradient on the South American monsoon

Abstract

In December and January, the pressure contrast between the subtropical plains of South America (CHA) and northwestern Africa (WEM) reaches its maximum. This happens because of simultaneous and opposite surface pressure variations in these two regions. The low tropospheric winds flow from the tropical Atlantic into Amazonia and then divert southwards into the Gran Chaco region, suggesting a possible strengthening of this circulation associated with the CHA-WEM gradient. This article analyses the effect of a cross-equatorial CHA-WEM gradient intensification on the monsoon system of South America. The reinforcement of the gradient is associated with increased easterlies in the eastern equatorial Atlantic, a stronger low-level jet east of the Andes, increased moisture transport towards the CHA region, and reduced precipitation over much of Brazil. The large quasi-stationary wave in the upper troposphere, consisting of the Bolivian High and the Nordeste Low, weakens in amplitude, altering the return flow to the North Atlantic. Extratropical dynamics, such as Rossby wave trains that reach southern South America and the North Atlantic Oscillation, affect the variability in the CHA and WEM regions and modulate the CHA-WEM gradient intensity.