Precipitation in the Southwest United States comes in two flavors: winter and summer. During the winter, the precipitation is mostly from the passage of mid-latitude storms. And during the summer, the precipitation comes from what is colloquially called “the monsoon”.
What is a monsoon?
The word monsoon derives its name from the Arabic word ’season’. Sailers in the Indian Ocean would use the word to describe the change in wind direction throughout the year. They had noticed that during the summer the winds blow from a different direction than during the winter. The word monsoon refers to the seasonal reversal of winds, and not to changes in the precipitation (National Weather Service).
The most famous monsoon is the Indian Monsoon, which not surprisingly occurs over the Indian subcontinent. The formation of a monsoon is similar to the formation of the land- or sea-breeze. Because of the difference in heat capacities, the land surface heats faster than the ocean during the day. This causes the formation of a thermal low. The resulting surface pressure difference between the land and ocean cause the wind to blow from the ocean to the land (during the day), or from the land to the ocean (during the night).
The monsoon is formed on the same principles as the sea-breeze, except over larger areas and longer times. Instead of a diurnal variation in wind direction as occurs with the sea-breeze, the changes in the monsoon winds occur on seasonal time scales. During the summer months, the Mojave Desert becomes extremely hot, and a semi-persistent thermal low develops over the region. This is what causes the winds to change directions from northeasterly in the winter months to southwesterly in the summer months.
So What is the Monsoon?
If we already know what the monsoon is and why it forms, then what’s the big idea? Part of the problem is in the predictability of its onset and duration. Because roughly one-half of the precipitation in the Southwest occurs during the monsoon, there are important implications of changes in the monsoon due to climate change. With the increase in temperatures, is the desert Southwest going to turn into a dust-bowl or will the changes be more subtle and complicated (see e.g. Seager, 2007).
Kursinski et al. use the Global Positioning System (GPS) satellites to determine the amount of precipitable water vapor (PWV) in the atmosphere. The main purpose of the paper is to describe the use of the GPS technique to allow high temporal resolution water vapor measurements to help determine the source of PWV in an effort to increase the skill in predicting summer precipitation.
They introduce an innovative approach to defining the start of the monsoon. The National Weather Service defines the start date of the monsoon as “when the average daily dewpoint is 54 degrees or greater for 3 consecutive days.” However, since the monsoon describes a seasonal shift in winds, this is not the best approach. Kursinski et al. “define the monsoon as beginning during the 5 to 7 day onset period commencing on July 1 over which PWV values grew to sustained high values and the PWV-weighted winds at Empalme shifted from westerly to southeasterly and remained there.” This definition has one major advantage to the NWS onset date: it incorporates the direction of the wind.
The analysis of Kursinski et al. show that during the 2004 North American Monsoon there were two distinct phases which they label as “sub-synoptic scale “and “synoptic scale”. Each of these phases has different statistical characteristics. For instance, during the sub-synoptic scale phase there is increased moisture variability at scales smaller than 90 kilometers. They attribute this variation to changes in moist convection and mountain circulation.
During the synoptic scale phase, the statistical nature of the monsoon changes. The variations are no longer on small scales, but on the large scales. They postulate that this could be due to the advection of moist air into (or out of) the North American Monsoon region. One major question unable to be answered by this study is whether the two-regime nature of the monsoon is typical of the North American Monsoon or whether it was simply a one-year occurance.
Implications
The North American Monsoon system is not well researched, and not well understood. While the basic physical principles for the existance of the monsoon have been known for some time, the inability to predict the onset and duration of the monsoon point to significant gaps in the scientific understanding of an important precipitation source for the Southwest United States. With the likely northward progression of mid-latitude winter storms due to global warming, it is important to fully understand the consequences of changes in the monsoon.
References:
Kursinski, E.R., Bennett, R.A., Gochis, D., Gutman, S.I., Holub, K.L., Mastaler, R., Minjarez Sosa, C., Minjarez Sosa, I., van Hove, T. (2008). Water vapor and surface observations in northwestern Mexico during the 2004 NAME Enhanced Observing Period. Geophysical Research Letters, 35(3) DOI: 10.1029/2007GL031404
National Weather Service (NWS), Flagstaff Forecast Office, The Monsoon.
Seager, R., et al. (2007), Model predictions of an imminent transition to a more arid climate in Southwestern North America, Science, 316, 1181–1184, doi:10.1126/science.1139601.