Each summer, Arizona gets stormy. Winds shift and bring moisture in from the south which helps form thunderstorms in the heat of the day. This is the North American Monsoon.
Much of Arizona typically receives 30 to 50 percent of its annual rainfall during the monsoon months of June, July, August, and September which helps replenish reservoirs and diminish wildfire threats.
But, the monsoon as we know it is changing. As our atmosphere gets hotter with climate change, it’s able to essentially hold more water vapor and that’s changing how our monsoon behaves.
“We don't get as many storms, but when they do come, they're tending to produce higher rainfall amounts, more intense rainfall, and more intense winds,” says Dr. Christopher Castro with the Hydrology and Atmospheric Science department at the University of Arizona.
That’s exactly what we saw play out here in Phoenix last summer. Ninety percent of our monsoon rainfall came in less than two hours on August 20.
“From the water supply perspective, it's bad news for recharge of the groundwater table. If you're drying out the soils and then realizing your precipitation with only a few intense events, more of that precipitation is likely to run off. There’s also a greater danger for wildfires to happen and that totally changes the characteristics of the permeability of the soil. Then, when it rains, that water will immediately wash off very similar to a landslide,” Castro says.
Castro and his team at UArizona are leading the way when it comes to research on how climate change is impacting our monsoon. Although not yet published in scientific peer-reviewed journals, they have run experiments simulating how storms move down into the Valley.
They have discovered that a greater number of green spaces, like parks, golf courses, and green landscaping, are leading to fewer storms making it down into the city.
“The presence of all these watered areas is suppressing the amount of instability in the atmosphere. So, it's harder to initiate storms from the outflow boundaries,” he says.
So, as storms try to move down off of the Mogollon Rim, they are now often getting diverted around the periphery of the Phoenix metro instead of rolling right through it like they used to.
Scientists like Castro have also seen a broadening of our monsoon ridge, which is impacting our monsoon, too.
It’s the ridge of high pressure, which sets up near the Four Corners during the summer, that allows monsoon moisture to flow in. As it expands and gets more intense, we see greater descending motion in the atmosphere which makes it more stable and less likely to form storms.
Castro says, “That’s precisely what happened last year. It suppresses the convection and it also makes the atmosphere hotter. Last year was one of the hottest and driest summers we’ve ever experienced here in Arizona. And not just in our typical period of late June, but also into July and August, which was really unprecedented.”
But, that’s not the only impact. A bigger, broader monsoon ridge also deflects atmospheric disturbances, known as inverted troughs, from tracking through Arizona.
Those disturbances often serve as triggers to get widespread rain and thunderstorm activity going during the monsoon.
“The expanded monsoon ridge is suppressing the path of those inverted troughs more to the south, so the number of inverted troughs is decreasing.
So, the frequency of big precipitation events is decreasing. But when they do occur, they're occurring in a more moist environment that is favorable for heavier amounts of precipitation,” says Castro.
We’re seeing similar impacts across the country as the number of heavy downpours increases in this warming world.
While downpours are increasing, periods of drought are increasing too.
So we are increasing both of these extremes as our climate changes.
Castro says, “We’re going into a world where we have these more dramatic swings in climate variability, whether it's winter or summer. There's been this term coined climate whiplash, you go from one extreme to another, either between seasons or within a season. This is devastating from the standpoint of climate and our resiliency to the natural climate because if we're exceeding the ranges of which our natural and human systems can cope with these whiplash swings, you're going to go from one year where a dam nearly fails because it's flooded to a year of devastating wildfires and the ecosystem cannot recover.”