Diversity of Mesoamerican Midsummer Drought

The Mesoamerican midsummer drought (MSD) is a distinctive precipitation feature characterized by a mid-season rainfall reduction within the boreal summer wet season. Despite its socioeconomic significance, most previous studies have emphasized its canonical bimodal structure, leaving the diversity of MSD expressions less explored. Using ERA5 reanalysis (1979–2019), we objectively identify MSD events for each grid cells over the domain and classify them into four distinct clusters via K-means analysis. These clusters reveal diverse temporal structures, intensities, and spatial preferences, spanning southern Mexico, Central America, the Caribbean basin, and adjacent oceans. Composite analyses of sea surface temperature (SST), cloud fraction, winds, and moisture flux convergence (MFC) indicate that low-level circulations—particularly the Caribbean and Chocó low-level jets, along with eastern Pacific winds—play a central role in shaping MSD diversity. In contrast, eastern Pacific SST anomalies exhibit only weak and inconsistent associations, suggesting a secondary role of SST–cloud feedbacks. Decomposition of MFC further highlights the combined zonal and meridional moisture transport as the primary driver of bimodality, with meridional fluxes being especially important for Caribbean MSD events. An evaluation of 33 CMIP6 models shows that while most capture the overall MSD frequency, they underperform in reproducing asymmetric precipitation structures, particularly those over the Caribbean. These results emphasize the need to incorporate MSD diversity into model evaluation frameworks to improve regional precipitation projections. Our findings provide a new perspective on the mechanisms underlying MSD variability and establish a foundation for more reliable seasonal prediction and climate change assessments in the Intra-Americas Seas region.