Monsoon storms deliver blinding dust storms, powerful winds, and raging floods that can wreak havoc on people and property. The intense rains also snuff out fires, ripen pastures, temper temperatures, and breathe life into dry landscapes. This summer the monsoon will play a vital role in reversing or amplifying widespread drought conditions in the Southwest and quelling fires that already have burned more than 900,000 acres in the region.
To help the Southwest stay informed of observations, insights from experts, and cutting-edge science related to the monsoon and climate, the Climate Assessment for the Southwest will publish the Southwest Monsoon Tracker each month through September. This issue will feature the latest understanding of the monsoon, and subsequent issues will focus on current conditions.
The sun is the engine of the monsoon. As summer progresses, solar rays warm the land faster than the Pacific Ocean off the coast of Mexico, and the monsoon ridge migrates north. When conditions are right, the winds shift from a southwesterly to a more southeasterly direction, marking the onset of the Southwest monsoon, also known as the North American Monsoon.
The monsoon season brings a rapid increase in rain in June in southern Mexico and in early July in the southwestern U.S. (Figure 1a). In Arizona and New Mexico, the monsoon typically ends around mid-September (Figure 1b). Recently, the National Weather Service adopted a static start and end date to the monsoon season–June 15 and September 30—to simplify communicating the onset of monsoon hazards, such as floods.
Several ingredients are vital to the monsoon. Moisture is the most important, while mountains and winds play a supporting role. Winds at altitudes around 15,000 feet carry water vapor in from the Gulf of Mexico. Monsoon storms begin only when this moisture enters the region. Near the Earth’s surface, moisture rushes into the region from the Gulf of California, often in fits and starts called gulf surges that occur when tropical storms form. This moisture usually amplifies monsoon storms.
Mountains play a key role by initiating storms because high topography heats up quickly during the day and destabilizes the atmosphere above them, helping to instigate and maintain thunderstorms. Storms usually stay glued to the mountains unless winds aloft blow them into the valleys. If storms do move off the mountains, they can help kick-start additional storms in a chain reaction, causing more widespread rainfall. For regions far from high elevation areas, like the Colorado River Valley in western Arizona, monsoon precipitation is lower because storm formation depends on this chain reaction.
A defining characteristic of the monsoon is variability, both in space and time. The epicenter of monsoon activity is the Sierra Madre Occidental in northwest Mexico, where about 70 percent of the area’s annual precipitation falls during the monsoon season. New Mexico and Arizona are on the northern fringes of the monsoon region and receive between about 30 and 50 percent of their yearly precipitation between July and September (Figure 2). Because Arizona and New Mexico lie on the northern margin of the monsoon region, precipitation is influenced by many climate processes such as the position of the monsoon ridge, sea surface temperatures (SSTs), and tropical storms near the Gulf of California. North of the Sierra Madre mountains, the monsoon intensity decreases while the variability increases.
Year to year variability results from conditions in the landscape and Pacific Ocean left over from the previous winter, as well as longer term climate fluctuations. La Niña events, coupled with cool North Pacific Ocean SSTs or low values of the Pacific Decadal Oscillation correlate with an early monsoon arrival and above-average rain because they enable the monsoon ridge to migrate north sooner.
Monsoon variability also appears to relate to winter precipitation. Studies have demonstrated that wet winters with high snowpack often are followed by dry summers, and vice versa. This may occur because the melting of copious snow absorbs more solar energy, reducing the temperature contrast between the land and ocean and causing the monsoon ridge to migrate north more slowly. This in turn delays the onset of the monsoon. Because El Niño and La Niña events influence winter precipitation in the Southwest, they often fine-tune this winter-summer link.
Trends in the monsoon are evident. Rainfall totals between 1948 and 1999 in Arizona and New Mexico have decreased in July and have increased in August and September; these trends have been higher for New Mexico than Arizona. Lower July precipitation translates into a significant delay in the entire monsoon cycle over this period by about 10 to 20 days, depending on location.
While much is known about the Southwest monsoon, there are more questions than answers. First and foremost, monsoon forecasts still cannot reliably capture how robust the monsoon will be. Models have shown some ability to predict June precipitation, or the onset of the monsoon, but have not been consistently accurate in forecasting precipitation in the following months. Topography and convection are not well represented in most climate models used for seasonal forecasting, and both play a major role in instigating storms. Related to this, little is known about the principal climate influences on precipitation variability in the second half of the season.
Also, weather observations in the West started only about 100 years ago. A monsoon record that extends farther in the past, perhaps constructed using the widths of tree rings, would help diagnose causes of monsoon variability. Another critical question relates to human-caused climate change. How the monsoon will react in a warmer world is unknown. Some theories propose it will intensify, while others suggest it will weaken.