The annual precipitation pattern in the Caribbean basin shows a distinct bimodal behavior, where the first mode is called the Early Rainfall Season (April–July), and the second mode the Late Rainfall Season (August–November). The brief, relatively dry, period in July is usually referred to as the midsummer drought (MSD). It has been hypothesized that the migration through the Caribbean basin of the Intertropical Convergence Zone (ITCZ) and increases in aerosols due to the passing of Saharan Dust across the Caribbean in the summer months may result in the observed precipitation pattern. This paper focuses on determining the origins of the Caribbean MSD. Multiple regression analysis was carried‐out to determine if the ITCZ, the North Atlantic Oscillation (NAO) index, the Vertical Wind Shear (VWS), and different atmospheric particle (AP) concentrations transported from northern Africa correlate with the Caribbean MSD. It is shown that the ITCZ and NAO are weakly correlated with the Caribbean precipitation variability; however, the VWS and aerosol particles revealed an important contribution to rainfall during the summer months. Numerical experiments were then performed to quantify the influence of different VWS scenarios and different AP concentrations on the Caribbean precipitation bimodal behavior. The numerical approach uses the Regional Atmospheric Modeling System coupled with a new cloud microphysics module that allows discrimination between small and giant particles, as well as Cloud Concentration Nuclei (CCN) and Giant CCN activation. These numerical experiments support the statistical result that the VWS and the AP influence the rainfall production and pattern during the MSD. Results indicate that cloud microphysics play an important role in producing the observed climatological bimodal pattern, while variations in large‐scale atmospheric dynamics (like the VWS) help explain variations in the strength and pattern of the bimodal events and behavior.

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Abstract:Since the 1800s the global average CO 2 mixing ratio has increased and has been related to increases in surface air temperature (0.6 ± 0.2°C) and variations in precipitation patterns among other weather and climatic variables. The Small Island Developing States (SIDS), according to the 2001 report of the Intergovernmental Panel on Climate Change (IPCC), are likely to be among the most seriously impacted regions on Earth by global climate changes. In this work, three climate change scenarios are investigated using the Parallel Climate Model (PCM) to study the impact of the global anthropogenic CO 2 concentration increases on the Caribbean climate. A climatological analysis of the Caribbean seasonal climate variation was conducted employing the National Center for Environmental Prediction (NCEP) reanalysis data, the Xie-Arkin precipitation and the Reynolds-Smith Sea Surface Temperature (SST) observed data. The PCM is first evaluated to determine its ability to predict the present time Caribbean climatology. The PCM tends to under predict the SSTs, which along with the cold advection controls the rainfall variability. This seems to be a main source of bias considering the low model performance to predict rainfall activity over the Central and southern Caribbean. Future predictions indicate that feedback processes involving evolution of SST, cloud formation, and solar radiative interactions affect the rainfall annual variability simulated by PCM from 1996 to 2098. At the same time two large-scale indices, the Southern Oscillation Index (SOI) and the North Atlantic Oscillation (NAO) are strongly related with this rainfall annual variability. A future climatology from 2041 to 2058 is selected to observe the future Caribbean condition simulated by the PCM. It shows, during this climatology range, a future warming of approximately 1°C (SSTs) along with an increase in the rain production during the Caribbean wet seasons (early and late rainfall seasons). Although the vertical wind shear is strengthened, it typically remains lower than 8 m/s, which along with SST > 26.5°C provides favorable conditions for possible future increases in tropical storm frequency.

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Nineteen scorers from seven Cuban laboratories participated in this slide exercise designed to test the influence of the scorer on the accuracy, sensitivity and variability of the comet assay when a visual method of DNA damage evaluation is used. The assay was performed using human lymphocytes from a single donor exposed in vitro for 5 min at 0 degrees C to doses of 0, 5, 10, 25, 50, 100 and 200 microM of hydrogen peroxide. Each participant scored the same set of 14 coded slides with silver stained comets. The comets were classified visually into five categories according to the appearance resulting from the relative proportion of DNA in the tail. The extent of DNA damage was expressed in arbitrary units. At zero dose the median values of 12 scorers out of 19 were included between the values of the overall 25 and 75 per thousand. This proportion remains practically the same as the dose increases. The lowest dose detected by this method for the majority of scorers (11) was 10 microM. The coefficient of variation at the control dose was the highest (median value 26%), progressively declined to 20%, and starting from 25 microM, values are around 10%. The results of the exercise show the reliability of the silver staining and visual scoring for the comet method.