Climate Modeling Study of Variability about the Seasonal Cycle in the Tropical Eastern Pacific and the Caribbean Sea

A climate modeling study to analyze the variability in the Tropical Eastern Pacific (TEP) and the Caribbean Sea is realized using two different climate model runs with different precipitation sensitivities to free tropospheric humidity. The climate model runs were then compared to daily 850 hPa observational data from NCEP/NCAR reanalysis for the years 1998-2006. In order to perform this, the months from June through October were separated from the years and then the variance was taken over. Usually, climate models produce too much precipitation over the Caribbean, and these biases may affect their ability to simulate important events such as the Caribbean low-level jet and the easterly wave variability.

This study shows that the simulation with the enhanced moisture trigger creates more precipitation and exhibits stronger variance across the east Pacific as compared to the control simulation, with a better variance distribution compared to observations. The sensitivity of climate models to tropospheric moisture is critical because the more moisture that is present, the higher the potential for precipitation if uplift mechanisms are in place. The relevant season to look for sensitivity to humidity above the boundary layer was during the boreal summer, because for both regions; the TEP and the Caribbean, important climatological events occur during this period. Having plots in this study representing separately the seasonal cycle runs and the yearly data helped to demonstrate how active are both regions during the summer months in contrast to the annual cycle.

The results of high amounts of precipitation near the TEP and Central America; as shown in the plots, were as expected since previous studies have established that during the boreal summer the inter tropical convergence zone (ITCZ) is shifted northward, leading to intense precipitation over the tropical North Atlantic, Central America, and the tropical northeastern Pacific. However, over the Caribbean Sea the precipitation is not as intense as in the previously mentioned regions. This study also shows that substantial model biases remain even with the enhanced moisture trigger employed, including excessive variance over southern Mexico, which may be contributed by the coarse representation of North and Central American topography.