Special Feature: The North American Monsoon System

A monsoon is generally defined as a seasonal variation of wind, cloud cover and precipitation that is controlled by the annual cycle of the sun. In climates that are strongly influenced by monsoons, most of the annual precipitation is received during the monsoon season.

Portions of the southwest United States, including New Mexico, are influenced by the North American Monsoon System (NAMS), which is also referred to as the Southwest Monsoon. Many locations in New Mexico receive 40 to 50 percent of the annual precipitation during the period from July 1 through August 31 and much, but not all, of the summer rainfall can be attributed to the Southwest Monsoon. This paper will provide a short description of the North American Monsoon System, its onset, and characteristics most commonly observed across New Mexico.

While monsoon circulations are strongest in southeast Asia, the regime associated with monsoons is observed in many tropical locations across the globe. Important elements include a temperature contrast between the land and adjacent sea surface, low pressure at the surface with high pressure aloft, and a transport of moisture at low levels of the atmosphere.   Monsoon climates have a distinct and dramatic increase in precipitation associated with the onset of monsoon circulations.

Although New Mexico is not impacted by the NAMS until July, development of the system takes place over Mexico during May and June. The animation to the right depicts the mean pressure pattern at 500 mb (middle tropospheric levels) for the months May through July for the years 1950 through 2000. Note that in May, the pressure distribution is nearly zonal with westerly flow at most latitudes. By June, pressure increases over northern Mexico such that a high pressure cell, or a monsoon high, is readily apparent. This strengthening and northward movement the high is one sign of the development of the NAMS. By July, the monsoon high migrates north with a mean position over New Mexico. This animation illustrates the "wind reversal" over Mexico generally associated with a monsoon circulation, that is, flow from the west in May and from the east by July.

At the same time, the distribution of precipitation changes dramatically, as illustrated in an animation of mean precipitation for the same months and years. In May, precipitation is maximized from eastern Texas southward through eastern Mexico. In general, the distribution is meridional precipitation is greatest over the U.S. great plains and decreases towards the west. By June, precipitation increases over south central Mexico. In July, the northward push of the precipitation occurs along and to the west of the elevated terrain of the Sierra Madre Occidental in Mexico. The south to north flow along the west side of the monsoon high transports the moisture/precipitation from Mexico into the U.S. desert southwest, including New Mexico. As precipitation increases across much of Arizona and New Mexico in July, a decrease occurs across the U.S. great plains.

July and August are considered the mature phase of the NAMS. During this time the northward extent is maximized and extends into Arizona and New Mexico, as noted by the precipitation increases and position of the monsoon high over the west/central U.S. Depending on location, the NAMS ends during September and October as the circulations associated with the NAMS patterns decay.

The onset of the NAMS in New Mexico varies, but on average is around July 3 for the southwest corner of the state and around July 9 for the central Rio Grande valley, including Albuquerque. The daily mean precipitation plot for Deming, in southwest New Mexico (shown below), depicts the rather dramatic increase in precipitation in early July. Similar mean precipitation data is also shown for Tulsa, OK, and illustrates the corresponding decrease in precipitation over the Great Plains in July. While portions of Mexico receive as much as 70% of their annual precipitation in July, August and September, the southwest corner of New Mexico receives about 50% of the annual precipitation for the same period. (Other differences in the precipitation patterns can be noted - while the plot for Deming indicates a single precipitation maximum for the year, occurring in July and August, a double peak in spring and autumn occurs in Tulsa.)

Flash floods across New Mexico are associated with the NAMS. The heart of thunderstorm season is generally from July 20 through August 20, and flash floods peak the last week of July and first week of August. The return of the dry westerlies occurs first near the Four Corners region, bringing an abrupt end to thunderstorm season at the beginning of September. The southwest monsoon lasts longest over southern New Mexico, usually dissipating more slowly during mid September.

The thunderstorm season that develops over New Mexico and Arizona during July and August is generally a result in changes in the circulations associated with the NAMS, but other features and circulations can support summer thunderstorms across the state. Many of the storms are fueled by moisture that surges northward from the southwest monsoon. On other days, the storms may be related to tropical waves that move from east to west across the region. At times the storms are fueled by east winds that are created when atmospheric waves move from west to east across the central Rockies and push surges of cooler, wetter air southward that end up moving westward through New Mexico.

There are two main weather patterns that favor bursts of shower and thunderstorm activity over New Mexico. The first pattern, shown to the right,  is the main pattern forecasters look for when forecasting activity related to the southwest monsoon. High pressure develops in the middle and upper troposphere and is centered east of New Mexico, and a low pressure trough is well to the west. This large circulation pattern is able to send volumes of moist air northward from the monsoon over Mexico to New Mexico or Arizona, as shown in the figure to the right. And though the northward extent of the NAMS in generally depicted in central New Mexico and Arizona, this plume undeniably affects many more regions of the country. The moisture plume helps fuel massive thunderstorm complexes that move eastward across the central and even northern United States. These complexes can grow to as large as 100,000 square miles in scope after beginning as individual afternoon thunderstorms along the lee slopes of the Rocky Mountains on summer afternoons. In a typical scenario, they begin around mid-day or early afternoon along the eastern slopes of the Rocky Mountains. By dark they are large clusters of storms over the high plains, perhaps covering an area greater than 50,000 square miles. They typically reach peak mass in the middle of the night before dying over the middle of the country as sunrise approaches the next day.

A second pattern occurs when high pressure is to the northeast of  New Mexico, resulting in east flow over much of the state.  Weak, easterly waves can travel west all the way from near the African coast to New Mexico.

Across the desert southwest, the monsoon moisture plume helps fuel some of the more intense, most efficient rainfall producers of the summer. However, thunderstorm activity in New Mexico during the summer can result with other circulation patterns. When the plume is not available, and high pressure in the middle and upper troposphere is centered north of New Mexico, the door is open for features to come across the state from the north. In the figure to the right, thunderstorms formed over Colorado and moved southward into New Mexico.

In any case, it is important to note that while July and August are considered as months dominated by the NAMS, the features are transitory occasionally producing increases in moisture that only last a couple of days before dry air settles back into the region.  When the air circulating around the high pressure system is subsiding over New Mexico, the atmosphere dries, decreasing or eliminating showers and thunderstorms.

Note, the climate graphics depicted here were generated using the Climate Diagnostics Center homepage: http://www.cdc.noaa.gov .