East Pacific hurricanes bring rain to Southwest

Pacific hurricanes and tropical cyclones can have a profound influence even on the landlocked Southwest—and, arguably, their impact may increase as the oceans warm.

“It turns out that there’s quite a lot of tropical cyclone activity that actually impacts the Southwest,” explained Elizabeth Ritchie, a climatologist who joined The University of Arizona faculty this summer. September is the peak month for this activity, which has resulted in some serious floods in years past.

An average of 2.2 remnants from East Pacific hurricanes and named tropical cyclones ventured into the Southwest each year between 1992 and 2004, representing 15 percent of the region’s named storms, Ritchie found in an analysis she conducted with a colleague. A tropical cyclone must reach sustained wind speeds of 39 miles per hour (mph) to qualify for a name, and 74 mph to attain hurricane status.

All but two of the 29 cyclones brought at least some rain to the Southwest. The researchers defined the Southwest as Arizona, California, and New Mexico.

“The main story is Albuquerque really does the best out of all these sites,” Ritchie noted, adding, “Tucson is not far behind.”

During this 13-year time frame, Albuquerque received a total of 20 inches of rainfall from tropical cyclone remnants, while Tucson received 12 inches and Phoenix collected 4 inches. Compared to the annual average rainfalls for these three cities, the values amount to half a year’s worth for Phoenix, a year’s worth for Tucson, and more than two years’ worth for Albuquerque.

About 1.3 tropical cyclone remnants affected the Southwest each year during the time frame 1966–1984, according to an earlier analysis by Walter Smith of the National Oceanic and Atmospheric Administration (NOAA) that was published in 1986 as a NOAA Technical Memorandum.

However, it’s unlikely the studies by Ritchie and Walter are directly comparable. Detecting remnants of tropical cyclones remains more of an art than a science, researchers noted, as official long-term tracking data ends when wind speeds fall below tropical storm status.

From 1974 through 2004, the number of intense East Pacific hurricanes increased by about a third, according to a study by Peter Webster of the Georgia Institute of Technology and several colleagues. Intense hurricanes have sustained winds above 130 mph. Webster and his colleagues compared data based on satellite imagery and found 49 intense hurricanes forming from 1990 through 2004 compared to 36 forming from 1974 through 1989.

Their finding and its perceived link to global warming via rising sea surface temperatures remains controversial among some researchers (see June 2006 Southwest Climate Outlook). Although climate experts agree rising ocean temperatures strengthen individual hurricanes, they disagree on whether past data is reliable enough to reveal a trend directly connected to global warming.

More intense East Pacific hurricanes won’t directly translate into more rainfall in the Southwest, at any rate, as David Gutzler, a climatologist at the University of New Mexico, pointed out. That’s because storms are more likely to become intense when contacting the warm waters of the open sea, he noted, while those heading into the Southwest must swing toward cooler coastal waters. The current from Alaska typically keeps U.S. coastal sea surface temperatures in the 70s and below even in August.

Tropical storms generally must encounter sea surface temperatures (SSTs) of 83 degrees Fahrenheit or more to attain the sustained 111 mph wind speeds of major hurricanes, based on a study of 270 Atlantic hurricanes and corresponding SSTs by Patrick Michaels and colleagues from the University of Virginia (Geophysical Research Letters, May 2006).

The tropical storms that do reach the Southwest can provide drought relief or cause floods, sometimes both. The remnants of Hurricane Javier (Figure 5) helped break a string of dry years in September 2004, ushering in a wet winter by gently soaking parts of droughtparched Arizona and New Mexico.

Too much of a good thing led to flooding in the autumn of 1983, when four cyclone remnants visited the Southwest. The storm from former Hurricane Octave created the most havoc, causing $500 million in flooding damage to Arizona with its days-long rains.