Summer monsoon storms provide a significant share of the water that allows the southwest United States to thrive. These storms pushing into the desert from more tropical regions to the south can deliver more than half a year’s precipitation.

But how do they traverse a region that is also bisected by tall mountain ranges – mountains that suck most of the moisture out of a storm?

Thomas Galarneau thinks he has an answer. An assistant professor of atmospheric sciences at the University of Arizona, he has published a new study identifying a previously unrecognized terrain feature that may be responsible for passing most of the monsoons that reach Arizona.

Galarneau has named it the Chiricahua Gap, after a nearby mountain range. The gap – 150 miles wide and nearly a mile deep – is basically a break between mountain ranges. It lies at the border between New Mexico, Arizona and Mexico, and it may be the primary reason that wet summer storms from the Gulf of Mexico manage to find their way into Arizona.

Water Deeply: How do monsoons function in the Southwest?

Thomas Galarneau: There has actually been a large body of work in the literature on what are the mechanisms by which enhanced water vapor gets into the southwest U.S. during the monsoon.

There are generally two different arguments. One says moisture is coming from the southwest up the Gulf of California into southwest Arizona and turns more to the east and gets into Phoenix and Tucson. The other argument is that water vapor slightly higher up in the atmosphere actually comes in from the east, all the way from the Gulf of Mexico, and comes up the Rio Grande River valley and over the Rocky Mountains and the Sierra Madre and into southern Arizona.

For the low-level water vapor, it’s important that it comes from over the Gulf of Mexico, which has a very warm and moist flow. But part of the issue with that is that a lot of the air has to descend back down the west side and into southern Arizona. What happens when air descends is it warms and dries out.

Water Deeply: How did you get interested in the Chiricahua Gap?

Galarneau: We noticed as you drive along Interstate 10, you’re actually driving through this low region of the Continental Divide across North America. We thought it would be interesting to investigate whether moisture sneaks in through this gap and leads to heavier rain. So we got curious to look at the datasets that are available now at higher resolution, and started to resolve some of these terrain features.

We put together an analysis using upper-air observations, or wind observations, in the lowest 12km (7.5 miles) of the atmosphere. Then we used available rainfall data to identify the heaviest rain days in southern Arizona. So we manually flipped through these cases and found the heaviest rain days in Arizona were associated with this easterly wind over southwest New Mexico and southeast Arizona.

We were able to determine [that], for the heaviest rain days in southeast Arizona, nearly three-quarters of them are associated with this flow of moisture in the atmosphere from the east side of the Continental Divide.

If you look at the top 10 percent of all rain days during the monsoon – the wettest days – three-quarters are associated with this surge through the gap.

What’s important about the gap is that it allows moisture to sneak into southeast Arizona without having to rise up over the Rocky Mountains and Sierra Madre, so you don’t have the descending and drying problem.

Water Deeply: Were you surprised to find such a close connection between the gap and these wet monsoon events?

Galarneau: In a sense I was. I didn’t go in with any real preconceived notions or expectations. We knew the gap was there, and just from looking at weather maps every day, we just happened to notice there are frequent instances in which flow is easterly in this gap. But it was a bit surprising to see how strong the signal was for the heaviest rain days.

This is an initial study, and we certainly would like to do more where we take more observations of wind in the gap and use numerical models to actually simulate the flow structure within the gap to gain a deeper understanding of how that moisture moves through the gap.

Water Deeply: Can you describe the Chiricahua Gap? What does it look like?

Galarneau: The gap itself is about 250 km (150 miles) across and about 1 km (0.6 miles) to about 1.5 km (0.9 miles) deep. Essentially, it begins in northern Mexico, on the north side of the Sierra Madre mountain range, and extends into southwest New Mexico to the southern end of the Rocky Mountain range. So there’s essentially this gap region between the Rockies and the Sierra Madre.

If you’re familiar with southwest New Mexico, it’s about near Silver City, and it extends across the international border. Along Interstate 10, you don’t have such a dramatic increase in your altitude, and that’s because you are in this gap. But visually it’s not like when you’re in the middle of the gap you can see each end of the gap, because it is a fairly wide feature.

Water Deeply: How important are monsoonal storms to this region?

Galarneau: Well, certainly the monsoon itself makes up nearly 60 percent of the annual rainfall here in the southwest U.S. So it’s certainly very important. Tucson, for instance, gets about 11-12in (28-30cm) of rainfall per year, and over half of that is due to monsoon rains. It is an important contributor.

Certainly it can recharge the groundwater, so it’s important in that sense. And it also maintains some of the rivers locally. The washes are usually active here during the monsoon season, whereas they’re dry most other times of year. In terms of raw precipitation amounts, the monsoon is really important.

Water Deeply: What would this area of the Southwest be like if the Chiricahua Gap didn’t exist?

Galarneau: If we were to make a hypothesis, it may be we would lose the easterly water vapor transport, because then the air would be moving over steep terrain and descending on the west side, and then drying out a bit. So that could potentially reduce the number of heavy rain events over southern Arizona.

That’s certainly something we can at least attempt to test in our computer models and is on the list of research that we would like to do moving forward. But we haven’t done that yet.

Water Deeply: Are there other places along the Continental Divide that can pass monsoonal moisture?

Galarneau: When forecasting for thunderstorms and heavy rainfall, one of the key ingredients meteorologists look for is enhanced water vapor at low altitudes of the atmosphere. For the summer monsoon season in southeast Arizona, meteorologists track the transport of low-altitude water vapor, which is defined as altitudes below 700 millibars or 3,000m (9,900ft) in elevation above mean sea level.

If you look along the Continental Divide from central Mexico north to the U.S.-Canada border, the Chiricahua Gap represents the lowest region of the divide. If you look at the amount that’s open to the transport of air and water vapor at low altitudes, the Chiricahua Gap itself represents 80 percent of the total water vapor transport through the Continental Divide. That’s a huge contribution at extremely low altitudes. So the argument is that the Chiricahua Gap is really the only good location for low-altitude water vapor transport from the east side of the divide to the west side.

Water Deeply: Are there any other management implications to consider?

Galarneau: What we did notice is that, if there’s an easterly flux of moisture through the gap, it appears to impact the moisture on the next day. There’s a one-day lag. So there may be some predictive skill to work on, at least on that one-day timeframe. If we see a surge of moisture through the gap, the following day may be an active monsoon day.

In terms of water management, that is an area we’re still looking at. But certainly, there appears to be a link with this one-day lag that could be useful for prediction.