Will the Tropical Pacific Impact This Year’s Monsoon?

The La Niña of 2010-2011 brought extreme drought to many parts of the Southwest. Luckily, the respite of monsoon rains is (hopefully) only a month or so away. But could last year’s La Niña impact this year’s monsoon? We’ll have to wait a little while longer to get an official forecast for this season, but it seems timely to discuss the potential impacts of El Niño and La Niña on the summer monsoon here in the Southwest.

The relationship between the tropical Pacific and winter precipitation in the Southwest is pretty clear: La Niña conditions bring drier winters and El Niño conditions bring wetter winters. But climate scientists have also found a few intriguing links between summer precipitation in the Southwest and conditions in the tropical Pacific. Wet winters in the Southwest and dry winters in the Pacific Northwest, a pattern usually associated with El Niño, usually precede drier Southwest summers (Higgins et al. 1998).  Likewise, dry winters in the Southwest and wet winters in the Pacific Northwest, a pattern usually associated with La Niña, often precede wetter Southwest summers. Some scientists think this has to do with the ‘memory’ of the landscape (Lo and Clark, 2002): When winter snowfall is abundant, it takes extra energy and time to melt it all in the spring and dry out the soil. This delays the warming of the surface, which helps initiate monsoon circulation. Once the land heats up and the temperature contrast between the warmer land and the colder ocean increases, winds switch direction and bring more lower-level moisture into the Southwest, primarily up through the Gulf of California.

El Niño and La Niña can also influence the development of the monsoon ridge in the upper atmosphere (Castro et al. 2001). Typically this ridge forms over Mexico in June and moves north into the southwest U.S. by July, bringing extra moisture into the upper atmosphere of the region from the Gulf of Mexico (in addition to lower-level moisture from the Gulf of California). During an El Niño, the jet stream extends further south than normal over North America, and this can hinder the northward movement of the monsoon ridge into the Southwest, delaying the onset of the monsoon. During a La Niña, the jet stream stays farther north. Without this blockage, the ridge can move north more quickly and the monsoon can begin earlier.

So can we apply any of this information to this year? The forecast calls for the development of neither El Niño nor La Niña conditions this summer (“ENSO neutral”), so perhaps the monsoon onset will be right on time?

Last winter’s La Niña and the drought in the Southwest would maybe hint that the monsoon would be strong--dry winter, wet summer, right? But this year was odd. Regions to the north--Colorado, Utah, and the Sierra Nevada, for example, received abundant precipitation and now have extremely high snowpack.  Could all this extra snow delay the surface warming and stall the monsoon?

Stay tuned for upcoming information on this year’s monsoon from CLIMAS. And for more information on how the monsoon works, check out last year’s monsoon roundtable discussion and additional information on SWCCN.