Tropical cyclones (TCs) are an important element of the climate dynamics in the tropical Americas. They produce intense precipitation during a few days of the rainy season. The contribution of tropical cyclone precipitation to seasonal accumulated rainfall may be as large as fifty per cent, particularly in the arid and semi-arid regions of northern Mexico. A positive trend in the number of tropical cyclones over the eastern Pacific, has resulted in more of these systems approaching the Baja peninsula and a positive trend in annual precipitation. However, the contribution of TCs to regional accumulated may be positive or negative depending on the trajectory followed by the system. If the TC is not close enough to the coastal region, it may induce atmospheric moisture divergence over land, reducing the chances of tropical convective activity and rainfall. Years of large but “distant to continent” TC activity result in negative anomalies in precipitation for some regions of the tropical Americas. Seasonal regional climate predictions or regional climate change scenarios provide information on TC activity but not on preferred trajectories. By means of TC cluster analysis, the preferred trajectories of TCs around the tropical Americas are explored in relation to quasi-stationary circulations at the steering level. Some ideas on how to estimate preferred TCs trajectories for a season are given.
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.