Photo by Todd Shoemake
IAn Introduction to the North American Monsoon System

The term monsoon generally refers to the seasonal reversal in atmospheric low-level circulations, particularly the surface winds and associated precipitation, resulting in a pattern of wet summers and dry winters. Monsoon circulations are present over many regions of the globe including Asia, Australia, North and South America and Africa.  Many years ago, these monsoons were considered to be regional in scale.  However, it is now known that monsoons are an important component of the large-scale global circulation.  In terms of "weather", the monsoon is associated with a dramatic in increase in summer precipitation, mostly in the form of thunderstorms.  The basic forcing of the monsoons is derived from a seasonal contrast in the heating of the land continent versus the ocean.

New Mexico is impacted by the North American Monsoon System (NAMS), which is also referred to as the Southwest Monsoon.  While precipitation increases in New Mexico associated with the monsoon are generally noted in July, the initial changes in the circulation patterns take place over Mexico during May and June. The animation below depicts the mean pressure pattern at 500 mb (middle tropospheric level) and moisture content at 700mb (lower tropospheric level) for the months May through September. 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 (dark orange shading), or a monsoon high, is readily apparent. This strengthening and northward movement of the high is one sign of the development of the North American Monsoon. By July and into August, the monsoon high (red shading) migrates north with a mean position over New Mexico. Typically during September, the high weakensand shifts equatorward, with atmospheric moisture decreasing over New Mexico.

animation of climatological monsoon circulations

Both upper level and surface circulations associated with the North American Monsoon transport moisture from the Pacific Ocean, the Gulf of California and the Gulf of Mexico into Mexico and the Southwest U.S. As the upper level ridge shifts to the north, the prevailing winds change from westerly to south or southeasterly over much of Mexico and the southwest U.S.  While this switch is dramatic across much of Mexico, it is also evident in vertical wind profile at Albuquerque.

The shift to southerly flow results in increased atmospheric moisture, and without strong westerlies aloft the moisture is not scoured out and remains available for thunderstorm development. Atmospheric moisture within the column above the surface can be measured by our radiosonde, resulting in a value of precipitable water, or PW. Climatologic values of PW for Albuquerque are shown to the right. Atmospheric moisture increases dramatically in July and remains high in August.

precipitable water climatology for NM
example of monsoon circulation

A "typical" monsoon burst pattern is illustrated in the image to the left. In the satellite image, the gray and blue shades represent moist areas while the red colors depict a very dry atmosphere.  The upper high is centered over Texas, and the clockwise circulation around the high (yellow streamlines) results in a south to north transport of moisture from Mexico into portions of New Mexico. Thunderstorms will form where there is sufficient atmospheric moisture, with the necessary lift provided by afternoon heating. Thunderstorms that form under this regime will move from south to north or southwest to northeast. In this example, from  August of 2013, portions of the east central plains received four to eight inches of rain while western New Mexico remained dry. 

The position of the high determines the orientation of the moisture, and spatial variability  in the resulting precipitation can be quite large.

Once monsoon moisture is in place, a number of synoptic patterns can support daily rounds of thunderstorms, with the position of the high determining the direction of movement.

The evolution of the monsoon from Mexico to the United States is noted by the northward progression of convective precipitation from southern Mexico in early June that spreads northward into the southwest U.S. by early July. The onset of the NAMS in New Mexico varies, but average onset occurs 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 associated with the migration of the upper level high and monsoon onset. Similar mean precipitation data is also shown for Tulsa, OK, and illustrates a 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 to 60% of their annual precipitation for the same period with a decrease to 40 to 50% across the central and northern portions of the state.

  annual precipitation of Deming, NM and Tulsa, OK
Seasonal Variability

Although a large scale circulation change supports the monsoon with an increase of atmospheric moisture, once onset occurs there is considerable variability in the day to day precipitation. Active periods of precipitation are known as monsoon "bursts", and are separated by drier, less active monsoon "breaks". The bursts are supported by a variety of patterns of low level moisture surges while the breaks often occur with an increase in westerly winds.  An example is shown below, with 2013 monsoon season precipitation for Deming clearly illustrating active or wet "burst" periods followed by dry "break" episodes.

daily precip at Deming in 2013 monsoon season 

The season variability associated with the North American Monsoon System translates to parameters associated with precipitation.  Here we show the maximum precipitable water (dark blue line) for much of the 2006 monsoon seaon.  Also shown is peak channel flow at the north floodway channel in Albuquerque (red bars), as well as the peak channel flow on days when flash flooding was reported in the Albuquerque Metro Area. The first item of note is that high flows occurred when the PWAT value was greater than 1.2 inches.  The second item that is likely more meaningful to the operational forecaster is the occurrence of the individual peaks in channel flow are closely related to the transition of PWAT values from high to low, and vice versa. PW vs mean channel flow in ABQ monsoon 2006
Annual Variability

Interannual variability of the North American Monsoon System is controlled by both ocean and land conditions, including sea surface temperature, soil moisture and snow cover.

As illustrated in the graph below, precipitation at Albuquerque for the monsoon season of June 15 through September 30 shows substantial variability from year to year.  While the average from 1950 through 2014 is 4.27inches, more than double that amount was received in 2006 with only 1.46 inches in 2003. The variability noted in Albuquerque is common for all areas affected by the North American Monsoon System and has long been a topic of research.

seasonal precipitation at ABQ

Several studies have shown an inverse relationship with winter precipitation and monsoon precipitation for the following season. The figure below depicts the distribution of a precipitation index for Arizona and New Mexico for seasons starting with a wet winter (dashed line) versus a dry winter (dotted line) with a comparison to the mean value for all years (solid line).  This study, published in 1998 was one of the first to document the inverse relationship such that wet winters are often followed by a drier than normal monsoon while dry winters are associated with a wetter than normal monsoon.

Fig. 3 from higgins et al 1998


Recent Research
Field experiments, often supported by an enhanced platform of observations, can provide an opportunity for researchers to investigate ocean-atmospheric phenomenon. The North American Monsoon Experiment, or NAME, was an international effort designed to better understand the key physical processes associated with monsoon. A major component of NAME was a field campaign during the summer of 2004. This effort supplied a basis for recent research supporting the monitoring, diagnostics and modeling of the North American Monsoon. Additional areas of recent research include the impact of tropical easterly waves, the roles of upper tropospheric troughs, and numerical simulations of the North American Monsoon system. Holle and Murphey (2015, Montly Weather Review) recently published a lightning climatology for the North American Monsoon, and the figure below illustrates the monthly change in lightning frequency for the core monsoon area. It is interesting to note the how change in frequency changes from May and June, when lightning frequency favors the eastern plains of New Mexico, to July and August when lighting impacts much ov Arizona and New Mexico, with maxima over the higher terrain. Additionally, Holle and Martin illustrated the diurnal variability in lightning frequency, which was shown to be greatest between 2pm and 6pm MDT across Arizona and New Mexico.
 Holle and Marting 2015 MWR fig 5
Additional Information

You can track the monsoon at the WFO Tucson site.  More details regarding the science of the North American Monsoon System, including research on the variability of the monsoon, are also available. is the U.S. government's official web portal to all federal, state and local government web resources and services.