The midsummer drought: A review of the its dynamic elements

The Mid Summer Drought (MSD) constitutes a peculiarity of the summer Mesoamerican climate and the Intra Americas Seas (IAS). The MSD corresponds to a relative minimum in precipitation during the summer rainy season. Several theories have been proposed to explain such consistent signal in the monsoonal climate of the northern tropical Americas climate, but t is shown that there are several aspects of this phenomenon that need to be considered for its adequate description and representation in climatic models. The MSD should include air-sea interaction processes over the northeastern Pacific warm pool and the Caribbean Sea. In addition, the transient - mean flow interaction over the Caribbean Sea should be should be considered, since the role of the Caribbean Low Level Jet (CLLJ) is crucial in explaining the various spatial and temporal aspects of the MSD. The barotropically unstable CLLJ may amplify easterly wave type disturbances that approach the Caribbean region. However, if the CLLJ is to strong or weak, the disturbances do not amplify. This is the case of the July August period, when the CLLJ magnitude is close to 15 m/s, out of the magnitude range in which the disturbance may be amplified as to result in an intense easterly wave. By means of Perturbation Kinetic Energy (PKE) analysis, it is shown that when the CLLJ is intense, the PKE over the Caribbean region diminishes, and convective activity over the Mesoamerican region and the northeastern tropical Pacific diminishes as well. In addition, a slight cooling of the eastern Pacific warm pool reduces the intensity of tropical convection over the intertropical convergence zone (ITCZ). As the CLLJ weakens in september, the PKE increases as a result of more easterly wave activity that leads to more convective activity over the region. The signal of the MSD is also present in the air sea interaction processes. The complexity of the various dynamic and thermodynamic processes involved in the MSD are not always pprosent event in regional climate models that apper to generate a sorth of MSD signal in the regional climate.