Gary Archibald, Weathercaster

Often times the term “monsoon” is used to describe – incorrectly – heavy consistent rain precipitation as a product of seasonal change and dynamics. Here’s what you need to know about what it actually entails.

Derived from the Arabic language, the term "monsoon" is derived from the Arabic word "mausim" which is defined as “season”. Navigating their vessels in the Indian Ocean and Arabian Sea, ancient traders 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, Asia, 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.

Now, the change in wind direction itself is determined by 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.

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", 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" in the monsoon, where the southerly winds decrease and the atmosphere becomes much less likely to allow thunderstorms to develop.

So there you have it. Keep in mind that the repercussion of the monsoon season here at home in the USA and abroad are often times tragic. In southeast Asia, the resultant heavy rains from constant thunderstorm activity over an elongated period of time contributes to high incidences of mudslides, flash flooding, damaging of crops, homes, property. In addition, and most importantly, many deaths are linked to the aforementioned effects of monsoonal episodes. Local economies tied directly to agriculture and industry is often adversely affected as well. Here in the USA, flash flooding is often a major problem in the southwestern region once the season begins – especially for Arizona, New Mexico, Nevada and southeastern California.

GA

Data and portions of this report are courtesy of the NOAA