Transient-mean flow interactions over the northern tropical Americas

Abstract

The Mid Summer Drought (MSD) over the Mesoamerican region constitutes a unique feature of its precipitation seasonal cycle. The MSD is a relative minimum in convective activity during July and August that coincides with an intensification of the Caribbean Low Level Jet (CLLJ) (mean flow at 925 hPa). There is not a unique theory on what maintains the CLLJ is, but the effect of the mean meridional convergence of easterly momentum related to tropical extratropical interactions over the Caribbean Sea, appars to play an important role. The barotropicaly unstable nature of the CLLJ shows that when this mean circulation is intense the amplification of high frequency transients (eg, easterly waves) is inhibited. Empirical observational evidence shows that as the CLLJ intensifies above a certain magnitude, transient activity decreases. Such transient activity is related to easterly waves, a key element in producing precipitation over the tropical Americas. Therefore, the CLLJ acts as a modulator of tropical convection in relation to the MSD, as the period of relatively minor tropical convection coincides with a decrease in high frequency Perturbation Kinetic Energy (PKE). Its role in the modulation of convective activity over the tropical Americas on interannual and even on interdecadal time scales makes it one of the key elements to understand climate variability over the tropical Americas.