Growers along the southern-most areas of the West and Southwest should complete harvests and related field work in a timely manner this fall due to the increased chance of above normal winter precipitation caused by El Niño.

University of Arizona agricultural meteorologist Paul Brown says Southern California, Arizona, New Mexico, Texas, plus northern Mexico could experience a “back-loaded” El Niño winter with the most precipitation from January to March.

“I would not procrastinate in getting fall harvests and related work wrapped up,” Brown suggests.

Brown shared his weather outlook and advice in late September during a UA field crop meeting in Marana, and a day later at the Arizona Pecan Growers Association annual meeting in Tucson.

El Niño is one phase of an oscillation in sea surface temperatures in the tropical Pacific Ocean which generally produces more precipitation in the southern-most areas of the country and drier conditions in the nation’s northern section.

La Niña, the opposite condition in the Pacific Ocean, typically pushes winter wet weather into the Pacific Northwest and eastward across the northern U.S.; leaving the southern areas high and dry.

The current El Niño forecast is a sign that growers in the southern-most areas should be prepared.
“In the West and Southwest, wet field conditions from November to January can delay field work since evaporation rates are slow,” Brown said. “In un-harvested cotton, wet conditions can degrade lint and reduce growers’ profit potential.”

The National Weather Service and the U.S. Climate Center issued the El Niño watch in September which indicated conditions were ripe for the development of an El Niño weather pattern.

Brown said Pacific Ocean water temperatures between North Africa and Indonesia were several degrees warmer than usual early this fall which suggests an El Niño winter.

El Niño is not caused by global-warming concerns. Current forecasts call for the strength of this winter’s El Niño to reach a moderate level.

“It is almost a coin toss whether the El Niño will actually bring below normal, normal, or above normal precipitation when the strength remains weak or moderate,” Brown said. “There is the potential for this El Niño to be a less-than-stellar winter precipitation event.”

The El Niño-La Niña cycle generally lasts from two to six years. The last two winters have been La Niña dry periods.

“The resulting lower snowpack melt for the West is one reason why water supply issues have become more tenuous along the Colorado and Rio Grande rivers,” Brown said.

Trending toward wet side

An El Niño occurrence does not fully guarantee a wet winter. An El Niño can be a failure, but the probability increases for wetter-than-normal weather.

About a dozen moderate El Niño events have occurred in the U.S. since 1950.

“Three of the last four moderate El Niño’s have been whoppers,” Brown noted.

The 2004-2005 El Niño pattern was so strong that Arizona cotton growers could not meet the mandatory plow down date which forced some to file for extensions.

Brown says the moderate 2006-2007 El Niño winter was a “bit of a dud but was back loaded.” Heavy precipitation in January delayed the pecan harvests in Arizona and New Mexico.

Brown believes this winter’s El Niño could resemble the back-loaded 2006-2007 event. “If I was a cotton farmer, I would not procrastinate in bringing in the crop given the recent El Niño history. This fall and winter are trending toward the wet side,” Brown said.

Growers also should expect cooler winter temperatures if El Niño conditions develop this winter. More precipitation leads to more cloudy days and higher evaporation rates which result in cooler temperatures.

Brown also addressed the prolonged drought issue in the West and Southwest. While part of the drought is tied to dry La Niña events, Brown also pointed to “Pacific Decadal Oscillation (PDO).”

This phenomenon refers to a longer-term fluctuation in the water temperature in the Pacific Ocean. Warm and cool phases relate to temperatures off the northern Pacific Coast of the U.S.

When PDO is in a cool phase, it suppresses precipitation in the U.S., particularly in the West which results in drought. A warm phase increases precipitation.

Climate experts know less about PDO than El Niño-La Niña weather patterns.

“Some climate experts believe PDO could be partially responsible for our longer trends of lower precipitation,” Brown said. “Wet years can occur during the dry cycles, but overall the trend is on the dry side. This may have implications on why we’re in this prolonged drought.”

If this is accurate, Brown says the West and Southwest are about halfway through the current PDO cycle.