The spatial pattern of midsummer drought as a possible mechanistic response to lower-tropospheric easterlies over the intra-Americas seas

Abstract

Following the idea that large-scale wind perturbations cause repeatable rainfall patterns over small tropical islands, the spatial pattern of the midsummer drought (MSD) is investigated as a repeatable rainfall pattern over the Intra-Americas Seas (IAS). For that, statistical techniques, including linear regressions, canonical correlation analysis, and variance budgets, were applied to the Tropical Rainfall Measuring Mission and ERA-Interim datasets to assess 1) the MSD pattern repeatability and 2) its explained variance in different time scales. As shown by the results, the MSD pattern is not a unique feature of the boreal summer intraseasonal variability in the IAS: it is more robust during summer but it exists in all rainy seasons on daily, intraseasonal, and interannual time scales. On diurnal time scales, the MSD pattern explains a negligible part of the total variance during summer (,2%), but on interannual scales it explains up to 20% and it captures the spatial features of ‘‘El Niño’’ rainfall anomalies. On all time scales, the MSD pattern is accompanied by repeatable wind and pressure patterns: anomalous lower-tropospheric (925 hPa) easterlies over a domain-wide meridional northward pressure gradient. These results provide evidence for the hypothesis that the MSD pattern manifests an underlying geographically determined, mechanistic pattern. Also, they suggest that the repeatable MSD-shaped rainfall and wind patterns could be extrapolated in time to better understand the climatic conditions behind droughts and pluvials, and to diagnose the causes behind rainfall trends.