The North American Monsoon Season

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25 Sep 2021 5:55 AM
[Arizona monsoon cloud with lightning striking the beautiful Sonoran desert in North Scottsdale. Photo by James Bo Insogna. Title: Arizona Monsoon Thunderstorm. Taken on August 15, 2016. Used under a Creative Commons license.] [From NOAA Climate written by Emily Becker]  If you live in the U.S. Southwest or northwestern Mexico, you may already be familiar with the annual climate phenomenon called the North American Monsoon, especially since rainfall in some spots has been way above average this summer. In fact, this monsoon may turn out to be the wettest on record for some places! More on that later… Now, let’s take a sojourn through some North American Monsoon basics (1).

What is the North American Monsoon?

The North American Monsoon is a seasonal change in the atmospheric circulation that occurs as the summer sun heats the continental land mass. During much of the year, the prevailing wind over northwestern Mexico, Arizona, and New Mexico is westerly (blowing from the west) and dry. (Going forward, to avoid having to say “northwestern Mexico, Arizona, and New Mexico” over and over, I’ll refer to this area as “the monsoon region.”)
[Water vapor animation for the afternoon of August 22, 2018 showing the monsoon circulation and thunderstorm formation (dark blue, green, dark red). Dry air is shown in orange. NOAA Climate image of original from Albuquerque, NM National Weather Service office.]
As the summer heat builds over North America, a region of high pressure forms over the U.S. Southwest, and the wind becomes more southerly, bringing moisture from the Pacific Ocean and the Gulf of California. This circulation brings thunderstorms and rainfall to the monsoon region, providing much of their annual total precipitation.
The monsoon starts to develop in Mexico in June, and moves into the U.S. Southwest in July. By early to mid-September, wind patterns have generally reverted back to the westerly pattern, bringing an end to the monsoon. There is some variability in the onset and demise of the monsoon.

What impacts does it have on Mexico and the U.S. Southwest?

Rainfall associated with the monsoon is very important for the region. Northwestern Mexico receives upwards of 75% of its average annual precipitation from it, and Arizona and New Mexico more than 50%, during July–September. [Percent of total annual precipitation occurring during July–September, based on 1979–2020 using CPC Unified rain-gauge-based data. Figure by NOAA Climate.] For many of us, the word “monsoon” conjures images of heavy rain lasting for months. While that’s often the situation for the Indian monsoon, the monsoon in North America behaves a bit differently. The rainfall generally has a strong diurnal cycle, meaning a daily pattern of mostly dry mornings, storms developing through the day, and most rainfall occurring in the afternoon and evening. Some of these thunderstorms can be strong, delivering heavy rain and frequent lightning. Monsoon rainfall activity tends to be grouped into bursts, with periods of rainy days interspersed with drier periods, rather than rain every day. Also, the occasional eastern Pacific tropical storm can increase monsoon moisture and rainfall. The impacts of the monsoon go beyond just rainfall amounts. There is also an important relationship between rainfall and temperature: usually, more rain leads to cooler conditions, and less rain leads to hotter conditions. One recent study explored the relationship between the monsoon and wildfires in the Southwest and northern Mexico, finding that monsoon rains were important for ending wildfires.

When was the North American Monsoon discovered?

The rainy season would have been critical for Native Americans for thousands of years, and, for some Native American tribes, continues to be so. Scientists first noted the seasonal rainfall patterns in the Southwest in the early 20th century, with the circulation pattern being understood as monsoonal by midcentury. A couple of field campaigns, including the Arizona-based South-West Monsoon Project (SWAMP, 1993) and the international North American Monsoon Experiment (NAME, 2004), provided a lot of observational data and resulted in a better understanding of the mechanics of the monsoon. [The thunderstorm begins. Photo by Udo S. Title: Monument Valley - Arizona / USA. Taken on September 23, 2017. Used under a Creative Commons license.]

What affects the North American Monsoon?

Although there has been a fair amount of research into the monsoon, there are still far more questions than answers about how it works, and if the seasonal amount of rain, potential start date, or other characteristics can be predicted. I listened to the Southwest Climate Podcast from CLIMAS, the Climate Assessment for the Southwest, to learn more about what affects the monsoon and its rainfall, and how Monsoon 2021 is shaping up, and reached out to the podcast co-hosts, Zack Guido and Mike Crimmins, for help with this post. Rainfall, as anyone who has read the ENSO Blog before will know, is an extremely complicated thing to predict! When you add in the sparse rain-gauge observations available in the U.S. Southwest and Mexico, it becomes even more difficult to make confident statements about the effects of the monsoon and how it can be predicted. In the podcast episode “2021—a generational monsoon?” Zack listed some of the factors that influence how much moisture is available to the monsoon, including the position of the high-pressure area, wind patterns, and transient weather features. Another player is land-surface feedbacks—wetter soils provide more moisture to the air through evaporation. Zack also mentioned our good friend El Niño! He pointed out that ENSO does influence Pacific tropical storms, which can supply moisture to the monsoon. I did a quick comparison of the average July–August rainfall in the monsoon region with the Niño-3.4 index, using 70 years of records. Some earlier studies suggested that El Niño may be related to lower July–August rainfall, and La Niña related to higher rainfall, due to large-scale atmospheric circulation changes. We can see some hints of this relationship in my scatter-plot here. [July–August rainfall anomaly averaged over North American Monsoon region for every year 1950–2019 (y-axis) versus Niño-3.4 index (x-axis). Green dots show years with greater-than-average monsoon rainfall while brown dots show less than average. Monsoon region averaged over all land gridpoints, 20°N–37°N, 102°W–115°W. Data from Global Precipitation Climatology Centre (GPCC) and ERSSTv5. Figure by Emily Becker.] “Hey! But El Niño leads to ­more tropical storms than average,” you’re saying, because you’re not new here. “How would that result in less total July–August rain?” Good question! And yet another element of the monsoon system that needs more study to resolve.

What about this year?

It depends where you are! Zack and Mike described this year’s monsoon for southern Arizona as “generational,” meaning once in a generation. It’s not over yet, but possible that the overall monsoon rainfall in Arizona will end up being the highest on record. Some regions have received more than 200% of the average rainfall, and Tucson recorded its wettest month ever this July. On the other hand, New Mexico and northern Mexico are near or a bit below average. [July 1–August 22, 2021 precipitation shown as a percent of the average July 1–August 22, based on 1979–2020. Brown indicates where precipitation has been less than average; green is greater than average. Figure by NOAA Climate; data from CPC Unified data.] While this will help with the ongoing drought in the southwest, in many regions the precipitation deficit has been building for a long time. The current drought outlooks expect that the drought in Arizona and New Mexico will improve in the short term, but persist. Check out a recent post on the drought in Arizona to understand more about how drought works in this region.
[Precipitation accumulation over the past 12 months, shown as a percent of the average mid-August through mid-August total. Despite the monsoon rainfall this year, much of the region is still in a precipitation deficit. Average is based on 1979–2020 using CPC Unified data. Figure by NOAA Climate.]


(1The North American Monsoon, published in the Bulletin of the American Meteorological Society by David Adams and Andrew Comrie, provides a comprehensive overview of the North American Monsoon and related research through the late 20th century. (2) In fact, comparing 1955, a year with very similar total rainfall in Tucson as this year, to this year shows temperature between July 1 and August 23 were on average more than 2 degrees F warmer. (3) There is a whole lot of interesting detail in this report—about everything, but about the North American Monsoon specifically. I’ve summarized their conclusions above, and include the quotes here, but I suggest you head over to the full science report if you’re in the mood for some specifics. In chapter 8.3, How is the water cycle changing and why?, the report states “In summary, both paleoclimate evidence and observations indicate an intensification of the NAmerM in a warmer climate (medium confidence). The intensification recorded since about the 1970s has been partly driven by greenhouse gas emissions (medium confidence).” In 8.4, What are the projected water cycle changes?, the summary statement is “there is low agreement on a projected decrease of NAmerM precipitation, however there is high confidence in delayed onsets and demises of the summer monsoon.” Edited for WeatherNation by Mace Michaels
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