Climate Fact: Southwest Monsoon Influences
In Brief: Decadal variability of the North American monsoon is affected by corresponding decadal sea surface temperature variability in the North Atlantic.
Southern Arizona and southern New Mexico lie just at the northern extent of the North American monsoon region. Driven by the temperature contrast between the Pacific Ocean and the desert that peaks between July and September, which causes the hot deserts to suck cool and moist air into their interiors, monsoon precipitation accounts for about half of the total annual precipitation and most of the region’s liquid precipitation (snow is solid). The 2012 monsoon rains are off to a strong start and seasonal totals are expected to be among the ten wettest totals of the 1981-2012 period. Because it is such an important source of precipitation, improving seasonal monsoon forecasts has been a key priority, yet the complexity of the monsoon system makes forecasting a challenge. Sea surface temperature distributions in the tropical Pacific Ocean (controlled by the El Niño-Southern Oscillation), water temperatures elsewhere in the Pacific and in the North Atlantic, snow cover across the Rockies, soil moisture conditions in the Midwest United States and even tropical storm activity have been known to influence the strength, duration and northern extent of the North American monsoon. One key factor affecting the monsoon strength on decadal time scales is the Atlantic Multidecadal Oscillation (AMO), the period shift to and from predominately warm to predominately cool conditions in the North Atlantic. Warm AMO phases boost moisture transport from the Gulf of Mexico into the central United States, which works to increase the atmospheric pressure over the Southwest and inhibit the monsoon rains. Cool phases tend to do the opposite. From 1948-1970 and from 1991-2005, predominately weak and dry monsoon conditions corresponded to warm North Atlantic sea surface temperatures. From 1971-1990, predominately strong and wet monsoon conditions corresponded to cool North Atlantic sea surface temperatures.
Source: Arias, PA et al. “Decadal Variation of Rainfall Seasonality in the North American Monsoon Region and Its Potential Causes.” Journal of Climate 25 (2012): 4258-4274 and University of Arizona Climate Assessment for the Southwest. “Monsoon.” Accessed Online 20 July 2012 <http://www.climas.arizona.edu/sw-climate/monsoon>