Spatial variability of the Caribbean mid-summer drought and relation to north Atlantic high circulation

Annual rainfall in the Caribbean exhibits a bimodal structure with two rainfall maxima (May–June and September–October) separated by what has been termed a mid-summer drought (MSD) (July–August). Despite general acceptance of the intensification and expansion of the North Atlantic High Pressure (NAHP) as the cause of the Caribbean MSD, it has been noted in several studies that the influence of the NAHP may not be consistent across the region. The purpose of this research is to better understand the Caribbean MSD by mapping the spatial co-variability of the Caribbean MSD and then determining the association between these spatial patterns and NAHP circulation. The spatial variability of the Caribbean MSD is identified through mapping of Principal Component Analysis (PCA) loadings of Caribbean MSD season rainfall time series. A correlation analysis was completed between monthly MSD region rainfall time series and measures of the NAHP to assess the degree of association between the spatial variability in the Caribbean MSD and NAHP circulation. The PCA identified six MSD regions, the northwestern Caribbean, the interior Caribbean, the eastern rim Caribbean, Coro Venezuela, Grantley Barbados, and a transition zone. The spatial pattern of the MSD regions suggests that the NAHP impacts the eastern Caribbean first and then progresses westward as the summer develops. In regard to the association between NAHP circulation and MSD region precipitation time series, correlation analysis indicates that overall, a modified Bermuda High Index (BHI) is a more effective tool in evaluating the association between the NAHP and Caribbean MSD as compared to the traditional indices, NAO EOF and BHI. Lagged correlations support previous findings that the winter preceding a particularly dry summer in the Caribbean is characterized by a strong pressure gradient on the southeastern flank of the NAHP; implying stronger Trade Winds, cooler sea surface temperatures, and reduced convective rainfall. Copyright © 2007 Royal Meteorological Society