Hi, I’m Peter Clift, Professor of Geosciences at Louisiana State University.
In earlier videos, we investigated the origins and impacts of the Asian monsoon.
We emphasized the role of the high Himalayas and Tibet Plateau in controlling the climate.
However, any continent can generate monsoon
where there is a seasonal contrast between the temperature of the lands and oceans
which forces reorganization of the winds and draws moisture deep into the continental interior.
In this video we’re going to look at the monsoon in North America, and see what causes
it and how it affects people.
The North American monsoon affects the south west part of the continent where it brings
a significant amount of (summer) rainfall.
The area of maximum summer precipitation is focused in NW Mexico in the (states of) Sonora,
Chihuahua, Sinaloa, and Durango, but also extends north into the SW United States, where
it especially affects Arizona and New Mexico.
although wider areas across the western United States also receive summer rain related to
the seasonal reversal of regional wind patterns.
Over this region more than half of the annual precipitation falls between June and September,
In contrast, the region to the east has little summer rainfall.
Like its Asian twin, the North American Monsoon is characterized by a summer season of strong
precipitation in what is otherwise an arid region.
The monsoon starts in late May to early June in Mexico and in July (further north) in the
SW United States.
Onshore winds bring moist air from warm tropical eastern Pacific Ocean and the Gulf
of California, while the Gulf of Mexico contributes some upper-level moisture.
Unlike the Asian monsoon where rainfall continues to days or even weeks,
the North American Monsoon is characterized by dry mornings
when the air heats up, followed by afternoon and evening thunderstorms, with the cycle
repeating the next day.
In addition, the North American Monsoon has intervals when there is little rain.
The monsoon season is marked by violent thunderstorms and flash flooding accompanied
by lightning that can be a serious hazard in the arid canyonlands of the region.
Fires now are more part of our ordinary experience.
The prediction is that things would become drier and hotter, are occurring, and that will continue
On the other hand, summer rains waters these desert regions and reduce the risk of bush
or forest fires.
As the SW North America (continent) heats up in the spring,
a strong low-pressure region forms over NW
Mexico and the SW United States that draws in moist air from the SW, especially from
warm waters in the Gulf of California.
The low-pressure region is blocked to the east by a summer high pressure system around
the Four Corners region and by cold air masses caused by upwelling cold water in the eastern
Pacific Ocean along the California coast.
This stops the North American Monsoon extending all the way to the Pacific Ocean.
The low pressure and the rainfall over NW Mexico and SW US are less intense than seen in Asia
largely because the Tibetan Plateau and the Himalayas is much higher than
the Mexican and Colorado Plateaus, especially the Sierra Madre Occidental.
Nonetheless, in both areas the rainfall is focused in (steep) mountainous regions.
The North American monsoon is also subject to feedback in which rains foster
growth of vegetation in the Sierra Madre Occidental,
which evaporate soils and plants
to add moisture to the atmosphere.
Furthermore, tropical cyclones in the eastern Pacific sometimes add rain,
as their remnants move inland causing destructive flash flooding.
There have been significant historical variations in the strength of the monsoon
season but unlike in Asia these do not seem to be tightly coupled to oceanic cycles
like El Nino.
Because the monsoon is linked to the mountains in this regions, the North American Monsoon is
believed to have been active since the late Cretaceous, around 80 million years ago,
when the Sevier Orogeny began forming mountains in the region.
Climate models predict that the monsoon would have been intensified especially following the
younger Laramide deformation.
and uplift, as the Interior Seaway of North America closed and the continent became drier.
The North American monsoon weakens following the last glacial cycle
and intensified after around 8000 years ago when the waters
of the Gulf of California became warmer and the cold waters of the Coastal California
current weakened.
This resulted in a more moisture flowing from the Gulf
of Mexico and the Pacific Ocean,
and more widespread monsoon region.
After 7500 years ago, a stronger California Coastal Current pushed the rain back towards
the east and into the more restricted area we see now.
Climate records derived from analysis of the stalagmite mines in New Mexican caves
show that before 4,000 years ago, the climate was drier than today,
but became more like the present between four and three thousand years ago.
After that time, the climate became cooler and wetter,
which allows the Anasazi people to start planting corn and settling in towns.
A dry period from 300 to 700 AD was followed by a wet phase that lasted until 900 AD.
After which, the climate slowly dried again.
It was during the wet times, the famous cliff dwelling was established.
Such as the Mesa Verde,
and Manitou Springs at Colorado.
As the region dried, the cities were abandoned.
And the Pueblo cultures moved to the low elevations and thrived near rivers
such as the Chaco Canyon in New Mexico.
Climate modeling suggests that future rainfall in the SW US and NW Mexico
will weaken due to the global warming.
Most of the precipitation decline is linked to uniform sea-surface warming which
stabilizes the atmosphere, resulting in weakening of the convection that brings moist air inland.
Because this region is already arid, such models imply a need for long-term planning
to offset the harm to water resources in this rapidly growing region:
Phoenix, Arizona, was the fastest growing big city in the US from 2010 to 2020.
I hope you have enjoyed learning about the North American monsoon and how it affects
and is influenced by the environment, the Pacific Ocean and the geology of the region.
Check out the following references if you would like to learn more about the North American monsoon.
And don't forget to look at the earlier videos about the Asian Monsoon!