Let’s face it. The 2020 Southwestern Monsoon was a bust. Summer temperatures broke record highs, while hot, destructive fires raged across the region. And now the American Southwest may continue to face hard times as we head into yet another weather phenomenon, La Nina. La Nina could, by winter’s end, render the region parched as a bone.
What gives? First, we’ll discuss the Southwestern Monsoon, which is a seasonal flow of winds that brings tropical moisture from the Gulf of California and northern Mexico northward to the American Southwest. A ridge of high pressure hovering over the Mexican Plateau must first move out of the way, which it usually does by early July.
Most years, this high-pressure ridge then parks itself over the Four Corners region and stays for weeks at a time, making way for moist air from the south and southeast to enter the southwestern states. Once this moisture-laden air starts climbing the higher elevations, like the Mogollon Rim, it releases its load in the form of thunderstorms. Flagstaff normally receives about 42% of its annual precipitation in the form of monsoonal thunderstorms.
Several times during the summer of 2020, however, the high-pressure system hovered over Arizona rather than the Four Corners region, keeping most moisture to the south and southeast.
To illustrate just how consequential this “nonsoon” is, I’ll cite some statistics.
Flagstaff recorded 2020 as its driest summer ever, measuring a mere 1.78” of precipitation in comparison to its normal 7.6”, Las Vegas had no rain for at least 164 days, and Phoenix experienced its hottest season ever. As of this writing, the National Forest in Coconino County continues stringent fire restrictions, while all of Arizona, New Mexico, Colorado, California, and Utah are in droughts. Added to that is the fact that the American Southwest underwent a “nonsoon” in 2019, as well. The combination of heat and tinder-dry forests and brush continue to fuel fires everywhere.
Well then, what happens next, and how does La Nina play into it? In a sense, La Nina is the opposite of El Nino. Both El Nino and La Nina are complex weather patterns, and the two together are known as the El Nino-Southern Oscillation (ENSO) Cycle. El Nino occurs when surface waters in the central and eastern equatorial Pacific Ocean heat up more than normal. In contrast, La Nina happens when this region’s ocean surface temperatures cool below normal by at least a half-degree Celsius. Both cycles occur about every three to five years and may impact weather patterns around the world.
La Nina events are triggered when trade winds generate an upwelling of deep, cool water in the central and eastern Pacific, thereby shifting warm water to the western Pacific. This causes the jet stream to retract to the west while curving and shifting to the north and steering low pressure systems, cool air, and precipitation away from southern United States. At the same time, La Nina may induce cooler weather in the northern plains, southeastern Alaska, and parts of Canada. The Pacific Northwest may experience a wetter winter than usual. And because La Nina reduces mid- and upper-level winds and wind shear across the tropical Atlantic, hurricanes are more likely to form and strengthen.
On the world stage, La Nina’s effects may be of a grander scale. Some areas, such as southeastern China, may see severe drought, while others, like Indonesia, may experience severe flooding.
A little closer to home, La Nina may not amount to much here in Northern Arizona. We’ve become somewhat accustomed to dry spells, even severe ones, and we’ve managed. We can count ourselves fortunate that we have enough water to maintain our lifestyles, we’re not having to deal with raging fires or hurricanes, and the prospect of a flood any time soon is unlikely. However, it behooves us as gardeners to become educated on any weather-related matters. After all, being well-informed is the first step towards being well-prepared.
Cindy Murray is a biologist, Arizona Cooperative Extension Coconino Master Gardener, and co-editor of Gardening Etc.
Even though the Coconino Cooperative Extension Office is closed, we are still here to answer your gardening questions. Call 928-773-6115 and leave a message or email CoconinoMasterGardener@gmail. A Master Gardener will return your call or message.
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