When I was younger, I thought that Arizona was a desert. Not just an arid region, but a desert like the Sahara or the Gobi. As I grew older, and especially when I moved here, I realized that it isn’t. There’s green plants all around. There are a lot of Sahuaro cactus and other desert scrub which nicely green up the landscape.

The precipitation here comes in two seasons: summer and winter. We receive almost no rain during the spring or fall (autumn). The reason is that our winter rain comes from passing mid-latitude cyclones, and our summer rain comes from the monsoon. The AMS defines a monsoon as “A name for seasonal winds. It was first applied to the winds over the Arabian Sea, which blow for six months from northeast and for six months from southwest, but it has been extended to similar winds in other parts of the world.” During the summer months, the direction of the winds changes in the desert Southwest which brings us much-needed moisture.

Most Americans don’t realize that we have our very own monsoon. While ours is not as famous as the Indian monsoon, it is still important to the residents of the Southwest US. For instance, in the Tucson area the monsoon is responsible for over half of our yearly precipitation. As one gets further North, such as in Las Vegas, the monsoon gets less important to local precipitation. And as one gets further south, such as in Hermosillo, Mexico, the monsoon provides nearly all of the yearly rainfall.

This is a cartoon of the weather systems during a typical monsoon event, which is characterized by a thermal low near the four corners region. Note, in the cartoon the low is almost directly over Las Vegas, Nevada. Due to the differences in heat capacity, the ocean takes longer to warm than the land does. (This also means it takes longer to cool down.) Therefore, in the summer, the land surface is much warmer than the ocean surface. It is this differential heating that causes the differences in pressure - low pressure over the land, and high pressure over the water.

The pressure differential forces the air to move from the high pressure towards the low pressure. In this case, it will move from the Gulf of Mexico towards Las Vegas. This brings the warm, moist air from the Gulf to the desert Southwest.

In the context of climate change, this is what the IPCC WG1 has to say about the future climate projections of the Southwest.

The southwest region is very arid, under the general influence of a subtropical ridge of high pressure associated with the thermal contrast between land and adjacent ocean. The North American Monsoon System develops in early July (e.g., Higgins and Mo, 1997); the prevailing winds over the Gulf of California undergo a seasonal reversal, from northerly in winter to southerly in summer, bringing a pronounced increase in rainfall over the southwest USA and ending the late spring wet period in the Great Plains (e.g., Bordoni et al., 2004). The projection of smaller warming over the Pacific Ocean than over the continent, and amplification and northward displacement of the subtropical anticyclone, is likely to induce a decrease in annual precipitation in the south-western USA and northern Mexico.

As the IPCC notes, a warming climate will likely lead to a smaller warming over the Pacific Ocean than over the continent. All else being equal, this will lead to a stronger monsoon winds during the summer months. A stronger monsoon will lead to more moisture being advected into the Southwest. Because the summer rains are primarily caused by terrain-forced convection, it is presumed that the increased moisture flux will lead to more precipitation.

The opposite side of the coin is that there may be a decrease in winter precipitation due to the northward displacement of the subtropical anticyclone. Assuming this is true (I have no reason to suspect otherwise), precipitation during the winter will decrease. What I don’t understand is their assertion that the warming will lead to a net decrease in the amount of annual precipitation.

References:
Bordoni, S., et al., 2004: The low-level circulation of the North American Monsoon as revealed by QuikSCAT. Geophys. Res. Lett., 31, L10109, doi:10.1029/2004GL020009.

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