General winter pattern across North America during moderate to strong El Nino episodes
Will El Niño Spell Drought Relief?
Historical Averages Say Yes, But Recent Trends Less Certain

By late July, the drought across the southern third of Texas was being compared with those from the 1950s. The drought, fed by periods of intense heat since May across the Rio Grande Valley, had farmers and residents alike worried about if, and when, the next substantial rains would come. The arrival of El Niño, while potentially favorable for reducing the numbers of hurricanes, could also reduce the potential for such storms to drop much needed rain over the region as the peak of the 2009 season arrives in August and September. Could this same El Niño spell relief at a later time? Read on!

El Niño: Curse or Savior?
Historically, the impact of El Niño on late summer temperature and rainfall across the Rio Grande Valley has been generally dry and hot weather. In 2009, this would worsen the ongoing drought. August and September are two of the wettest months in Deep South Texas; much of the rain is accounted for by tropical cyclones, waves, or remnant moisture. Reduced tropical energy in the Atlantic basin knocks down the potential for substantial rains. However, it only takes one tropical cyclone or slow moving wave to produce welcome rainfall. Even with El Niño underway, one or more waves could spread rain–producing moisture across the Valley later this summer.

By mid autumn, the development of a lower latitude jet (above image) quickly opens the door for tropical and subtropical Pacific energy to spread across the southern tier of the U.S. Historically in South Texas and the Florida peninsula, this has opened the door to frequent and sometimes copious rainfall, along with average to below average temperatures, extending into mid winter, and sometimes spring, especially near the coast. Figure 1 shows temperature departure from average during moderate to strong El Niños; Figure 2 shows precipitation departure from average.

Temperature departures from the 1971 to 2000 30 year average during El Nino events, since 1950, nationwide (click to enlarge)
Figure 1. Temperature departures during moderate or stronger El Niño years compared with the 1971 to 2000 longterm average. Does not include 2006/07. Values in degrees Fahrenheit.

Graphs of average and median rainfall values for Brownsville, Harlingen, and McAllen tell the tale of hope for welcome rains beyond the summer of 2009. The data are shown for three month running periods; for example, "OND" covers October through December, "NDJ" covers November through January, etc. To review, average rainfall sums each three month period for the entire record, roughly divided by the number of years of record, for all years and for El Niño years only. Median rainfall represents the "middle" value of the entire data set. Assessment of the median compared with the average better accounts for the frequency of high or low values in the data. In all cases, the mean and median rainfall during El Niño compared with all years of record are significantly higher during the mid autumn through mid winter. From mid winter through late spring, the correlation is indeterminate.

Rainfall departures from the 1971 to 2000 30 year average during El Nino events, since 1950, nationwide (click to enlarge)
Figure 2. Rainfall departures during moderate or stronger El Niño years compared with the 1971 to 2000 longterm average. Does not include 2006/07. Values are in inches.

El Niño: Curse or Savior? (continued)
While the overall late autumn through mid winter monthly three month rainfall values show above average conditions during El Niño, predicting which individual months may have abundant rains and which might be near or even below average is impossible during the previous July or August. For example, during the strong El Niños of 1972/73, 1982/83, and 1997/98, there is wide variability of monthly rainfall between November and February. Using Brownsville, November rains ranged from 0.87 inches in 1997, to 3.11 inches in 1982. February rains, which might better show the impact of a long duration, strong El Niño, were higher, but also showed a range, from 1.72 inches (a bit above average) in 1998 to 4.74 inches in 1973.

More than El Niño is at work when assessing rain potential for the Rio Grande Valley. First, our position relative to the storm track, even when the enhanced subtropical jet stream is considered, is not as favorable as locations farther north, including South and Central Texas and stretching across the Gulf to the central Florida peninsula. Second, the influence of the Sierra Madre Oriental in northeast Mexico can often disrupt the necessary low level flow for rainfall enhancement. And, localized convection can dump torrential rains in some locations while leaving others dry. For example, in October, 1997, a strong El Niño was underway. More than 13 inches of rain fell in Brownsville that month, with no tropical cyclone present. However, the bulk of the rain – 9.36 inches – fell in association with the season’s first cold front. For the next 110 days, through the end of January, 1998, only 2.76 inches fell!

Be Wary of Trends
So, this means that Deep South Texas can expect considerable drought relief between November 2009 and February 2010, right? The short answer is maybe. While long term historical data point to welcome rains, the recent trends, or data collected during the past fifteen years or so, are not so certain. For example, note the difference between the composite anomaly for South and Southeast Texas (green colors, wetter than average) and the trend (light brown, slightly drier than average) on this December to February chart.

While some El Niños have had prodigious rainfall periods (such as the end of the 2006/07 version), others have been a mixed bag for Deep South Texas (including the strong 1982/83 and 1997/98 versions). Evolution of the current El Niño, combined with other atmospheric patterns and teleconnections, will ultimately determine the evolution of the ongoing drought. For everything El Niño, surf to the following locations:

 

Departure from average of sea surface temperatures in the tropical Pacific 2009
Departure from normal of Sea Surface Temperature in the Tropical Pacific. Red, Orange, and Yellow colors denote warming; blue colors denote cooling. When warming occurs toward the right side of the map, El Niño conditions exist; cooling in this area signifies La Niña conditions.
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