Abstract/Summary

Seasonal rainfall over Middle America is crucial for human and wildlife welfare, agriculture, industry, and hydroelectric infrastructure. The biophysical and social conditions in Middle America make this region particularly vulnerable to rainfall variability. Extreme events, such as tropical cyclones (TCs), can exacerbate seasonal variability of regional rainfall. This thesis aims to understand the causes of interannual variability of seasonal rainfall in Middle America, the TC contribution to the regional hydrological cycle, and the causes of interannual variability of seasonal TC-related rainfall. Regions of coherent interannual variability of seasonal rainfall are identified using the objective Empirical Orthogonal Teleconnections (EOT) technique. El Ni˜no-Southern Oscillation (ENSO) is responsible for the largest fraction of interannual variability of seasonal rainfall, with canonical ENSO events driving more variability than ENSO Modoki. The Atlantic Meridional Mode (AMM) and local land-atmosphere processes are secondary drivers of interannual rainfall variability. TCs are important rainfall sources in Middle America during boreal summer, particularly on the Gulf coastal plain, the Mexican western coast and the southernmost states of Mexico. TCs contribute ≈ 10–30% of monthly accumulated rainfall (June-October) in those regions, with the most substantial contribution in Baja California Peninsula of up to 90% in September. TCs contribute 40–60% of extreme daily rainfall over Middle American coasts. TCs are a significant moisture source for the regional water budget; TC vertically integrated moisture flux (VIMF) convergence can turn regions of weak VIMF divergence by the mean circulation into regions of weak VIMF convergence. Interannual variability of TC-related rainfall is mainly driven by ENSO, through variations in East Pacific and North Atlantic TC activity. The AMM is the secondary driver, through changes in North Atlantic TC activity. Changes in location and strength of the North Atlantic Subtropical High and the Caribbean Low-Level Jet are important circulation features that influence TC rainfall variability.