Climate Fact: Delayed Deluge
During the summer, the hot and dry interior of the North American Continent sucks moist air from the Pacific Ocean onto the land. This process drives the summer monsoon season, making summers the wettest time of the year in the Southwest. In the last fifty years, a warming of the Pacific Ocean and an increase in the frequency and intensity of El Niño events has increased winter and spring precipitation in the Region. Researchers noticed that the more winter and spring rainfall there is, the later in the year that the summer monsoon arrives. The most likely explanation for this phenomenon is that the extra winter and spring rainfall has increased the amount of soil moisture present when the sun starts baking the Region during the late spring and early summer. Heat that would be stored in rocks, which is the type of heat necessary to establish the pressure gradient between the land and the ocean and thus drive the monsoon winds, is instead being stored as latent heat in water vapor. Thus, a few weeks of extra “baking time” are necessary for the land to become hot and dry enough for the winds to come. The monsoon cycle is now happening about 10 to 20 days later than it was fifty years ago; the peak is usually around early August.
Season: Summer
(Source: Grantz, K et al. “Seasonal Shifts in the North American Monsoon.” Journal of Climate. May 1, 2007. Accessed Online 30 July 2007 <http://cadsweb.colorado.edu/PDF/RiverWare/GrantzEtAl2006SeasonalShifts_JClim.pdf>)
