Characterization of the Rainy Season of Mesoamerica

The timing and intensity of rainfall in Mesoamerica (Mexico and Central America) is a key component to the economies and agriculture of the region. The mid-summer drought (MSD) is a climatological reduction in precipitation, which usually occurs around July-August, in the middle of the rainy season. The lack of precipitation during the MSD plays a critical role in the coffee rust fungus that destroys coffee bean plants. Therefore, understanding the seasonal predictability of the MSD phenomenon is of great interest to farmers. The goal of this research is to objectively define the onset, demise and length of the rainy season and similarly the onset, demise, and length of the MSD, using different observational datasets. The rainy season onset is defined as the day when the daily cumulative anomalies reach a minimum in the year for the domain as a whole. The rainy season demise is defined as the day when the daily cumulative anomalies reach a maximum in the year. In order for this objective criterion to work over the Mesoamerican region, the choice of the domain for computing the area average becomes important. Otherwise, we will show how false onsets and demises of the rainy season could dominate the results. Based on the onset and demise of this domain centered over Mesoamerica, we will show the local evolution of the rainy season. The advantages of using this definition of onset and demise for operational use will become self-evident from the results of this analysis.

Using the Ensemble Empirical Mode Decomposition (EEMD) on the rainfall datasets, we separate the intra-seasonal time scales (30-90 days) from the observed time series and analyze the intra-seasonal modes relating MSD. Using the NOAA Climate Prediction Center (CPC) Unified Gauge-Based Analysis of Global Daily Precipitation retrospective version data, NASA's Tropical Rainfall Measuring Mission (TRMM), and the CPC Merged Analysis of Precipitation (CMAP), we will examine the relationship on the interannual and longer-term variations of the onset, demise and length of the rainy season and MSD over Mesoamerica. In addition, the relationship between the evolution of the rainy season and MSD will also be examined. We will also characterize the onset and demise of the rainy season and the MSD based on the large-scale circulation anomalies of NASA's Modern Era Reanalysis for Research and Applications Version-2 (MERRA-2), air-sea fluxes of the Woods Hole Oceanographic Institute ocean-atmosphere flux, and soil moisture anomalies of NCEP Land Data Assimilation System -3 (NLDAS-3).