Resource: National Weather Service
The word “monsoon” is derived from the Arabic word “mausim” which means season. Ancient traders sailing in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer. Thus, the term monsoon actually refers solely to a seasonal wind shift, and not to precipitation.
Even though the term monsoon was originally defined for the Indian subcontinent, monsoon circulations exist in other locations of the world as well, such as in Europe, Africa, and the west coasts of Chile and the United States. Arizona happens to be located in the area of the United States that experiences a monsoonal circulation. During the summer months, winds shift from a west or northwest direction to a south or southeasterly direction. This allows moisture from the Gulf of California and the Gulf of Mexico to stream into the state. This shift in the winds, or monsoonal circulation, produces a radical change in moisture conditions statewide.
This monsoonal circulation is typically referred to here in Arizona as the Arizona monsoon. What we experience during the summer months, however, is only a small part of a much larger circulation that encompasses not only Arizona, but much of the southwestern United States and northwestern Mexico. Thus, it sometimes is also known as the Mexican monsoon. Others call it the North American Monsoon.
The Causes of Wind Shift
This change in wind direction is the result of two meteorological changes:
The movement northward from winter to summer of the huge upper level subtropical high pressure system, specifically known as the Bermuda High, and the intense heating of the Mohave Desert creates rising air and surface low pressure, called a thermal low.
These two features combine to create strong southerly flow over Arizona. The southerly low-level winds help to bring in moisture from Mexico, originally coming from a combination of moisture from the Gulf of Mexico, the Gulf of California, and the Pacific Ocean. When this moisture encounters the higher terrain of Arizona, it gets lifted and forms thunderstorms. These thunderstorms can contain very heavy rainfall, hail, strong gusty winds, or a combination of these conditions.
By the way, the term “monsoons” as typically used in the phrase “when the monsoons arrive…”, or “we had some monsoons move through town last night…” is wrong. There is no such thing as monsoons. The word should be used the same way that the word “summer” is used. Thus, the proper terminology is “monsoon thunderstorms” not “monsoons”. Remember, the monsoon is just a seasonal shift in wind direction, the thunderstorms occur because of the moisture moving over the state from the south.
The months of May and June tend to be very dry and warm months over Arizona. By the end of June, however, afternoon clouds can be seen building over the higher terrain of northern Arizona, and by the beginning of July, the skies typically begin to release its store of moisture. Thus, the monsoon represents the second rainy season for Arizona, with the first rainy season occurring during the cooler months of November through April.
The monsoon circulation does not produce thunderstorms every day during the months of July-September, but rather occurs in a pattern that has what are known as “bursts” and “breaks”. During the “bursts” (below, left), weak disturbances in the upper atmosphere act to focus thunderstorm activity over the state for a period of a few days to more than a week. Occasionally, however, the Bermuda high will become a bit stronger and develops over northwestern Mexico. This leads to “breaks” (below, right) in the monsoon, where the southerly winds decrease and the atmosphere becomes much less likely to allow thunderstorms to develop. This leads to a significant decrease in thunderstorm activity, and may last from a day or two to close to a week. This cycle of “bursts” and “breaks” will continue from the onset of the monsoon circulation (typically becoming established near the end of June or the beginning of July), until the time when cold fronts begin to move across the state of Arizona (typically during the month of September), which returns our winds to a westerly or northwesterly direction.
What is a Wash?
An arroyo also called a wash is usually a dry creek or stream bed – gulch that temporarily or seasonally fills and flows after sufficient rain.