Mid-summer droughts of the Carribean Region

Although summer is the wet season in the Caribbean region (including Miami), the heart of summer (centered in July) is a period of relatively less rain. This period is called "veranillo" or "canicula" in Spanish, and can have both positive and negative societal impacts in the area, with damage to agriculture, but also reduced tropical cyclone activity.

Because of its strength and repeatability, the MSD may be a useful test signal for improving our understanding of the climate dynamics of this region. At the same time, it varies enough from year to year that predictions (based on this improved understanding) could have significant value.

There are 2 main types of hypotheses for why the MSD occurs. 1. Local land - sea temperature contrasts in the region, or 2. regional-global dynamics mechanisms which may cause the atmospheric subtropical high pressure cell to change its configuration (as seen in Fig. 1). We are working to test these 2 main hypotheses, using local and global observations as well as model experiments like those described at

http://www.rsmas.miami.edu/users/bmapes/pagestuff/JClim.note_IPRCmodel.html

Fig. 1: Climatological sea level pressure maps for early summer (top) and during the Mid-Summer Drought (MSD) experienced in the Caribbean region.

Fig. 2: Mean annual cycle data from Key West, FL.  Two different epochs are shown (1965-1979 at left, 1990-1996 at right) to illustrate the robustness of the midsummer drought (seen in rainfall plots at bottom). Dry conditions in July are associated with high surface pressure (middle panels) and the interruption of the west-to-east wind in the upper troposphere which prevails during the rest of the year (solid contours in top panels) .