Awaiting the El Niño of 2015-16 - December 23, 2015
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

As you have likely heard, weather experts are predicting a wetter than normal winter this year due to the presence of strong El Niño conditions. El Niño is one of three states of the El Niño-Southern Oscillation (ENSO), a natural fluctuation in oceanic sea surface temperatures and surface air pressure between the east and west tropical Pacific Ocean. El Niño refers to above average sea surface temperatures, La Niña refers to below average sea surface temperatures, and when sea surface temperatures are average, it is referred to it as neutral.

More specifically, an El Niño event occurs when easterly trade winds weaken, allowing warmer surface water from the western tropical Pacific Ocean to flow eastward. El Niño events cause the winter path of the jet streams to move over Arizona, usually delivering more winter rain and snow in the southwestern US.

Scientific understanding of the global impact of El Niño events has improved considerably over the last few decades. In particular, we can often detect the development of an El Niño event months in advance, long before it will ever have an impact on seasonal weather conditions. However, there is still plenty of variability when comparing El Niño events, and there is no guarantee that any given El Niño event will lead to wetter than average conditions in Arizona and the southwestern US.

However, experts say we are now in rare territory with respect to the current and forecasted strength of this El Niño event. Current forecast models use the Oceanic Nino Index (ONI): which describes temperature anomalies (warm or cool) in the central and eastern Pacific Ocean along the equator. Models indicate that ONI values should reach approximately 2 degrees C above normal over the upcoming winter season, making it one of the strongest El Niño events on record and on par with past big events like 1982-83 and 1997-98. When the ONI gets into this range, it has almost always translated into above-average precipitation.

Looking back at historical ONI data, three “strong” events (1973, 1983, 1998) as well as the five “moderate” events (1958, 1966, 1987, 1992, 2010) all saw above average precipitation, which means that as long as this event stays strong, we will likely see a wetter than average winter. If this occurs, does it mean the drought will be over? The answer is definitely “no”. We have been in a drought for several years and one year of above average precipitation is not likely to refill all reservoirs to capacity. In addition, natural groundwater recharge is often poorly understood and variable depending on subsurface geologic features. In areas like Sedona, the watershed has lots of exposed bedrock, and precipitation moves quickly into creeks and the Verde River where it flows to the Phoenix area before it can be captured behind a dam. Increased snowpack is desirable and can have positive effects on flow from springs and seeps. Regardless, we should continue to be conservation-minded with respect to water supplies and any positive impact El Niño has on local precipitation.

While we often discuss drought conditions, flooding is also a distinct possibility given the current El Niño predictions. Yavapai County Flood Control has information on flooding, potential impacts, floodplain maps, and flood insurance. They also have weather stations and stream gauges which are linked via radio transmission to provide real-time weather and flood information. This data is also used by the National Weather Service to issue flood warnings.

As we move into spring, keep in mind that rain storms that fall on snowpack can increase the severity of floods. Just keep in mind that floods can and do occur. It is important to be prepared. I have included several useful references with the online edition of this Backyard Gardener (see URL below).

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Additional Resources

What is the El Niño–Southern Oscillation (ENSO)?
Climate Assessment for the Southwest (CLIMAS)

El Niño is here…what exactly does that mean for Arizona and New Mexico?
Climate Assessment for the Southwest (CLIMAS)

Yavapai County Flood Control
Yavapai County Government

Real-Time Weather and Streamflow Data from Yavapai County Flood Control
Yavapai County Government

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: December 16, 2015
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