James A. Reynolds
NWSO El Paso, TX (Santa Teresa, NM)
As of this writing in October, 1997, the currently evolving El Niño-Southern Oscillation (ENSO) event is the strongest since the significant event of 1982-83 and prolific media attention in regard to this phenomenon is helping to generate great public interest in the topic. Indeed, a clear relationship has already been made between certain ENSO events and increased precipitation patterns over North America by Ropelewski and Halpert (1986).
Work done by Andrade and Sellers (1988), and even more recently by Bradshaw (1993), specifically quantified the relationship between ENSO events and their effect on precipitation in New Mexico, in which state lies the largest portion of the El Paso County Warning Area (CWA). No work of this nature has been completed for the West Texas portion of the El Paso CWA, however. The purpose of this paper is to quantify the relationship between certain ENSO events and precipitation received at El Paso, TX.
ENSO events between 1899-1983 were ranked in order of intensity by Rasmusson (1984). A Category 1 event was considered very weak while a Category 4 event was considered strong. Only those ENSO events ranked as Category 3 (moderate) or Category 4 (strong) were used in this study. Those included 24 events of the 36 originally identified by Rasmusson (table 1).
Andrade and Sellers (1988) found that stronger ENSO events have a direct positive effect (increase) on precipitation received in the Southwest United States, especially during the autumn season at the onset of an ENSO event, and also during the following spring. In an effort to evaluate this research for far West Texas, El Paso yearly and seasonal precipitation totals were calculated separately for both ENSO and non-ENSO years of this study, which were then compared to average yearly and seasonal precipitation totals between 1899-1984. Winter season was classified as December to February, spring as March to May, summer as June to August, and autumn as September to November. Seasonal comparisons were made from the autumn of an ENSO onset year to the spring of the year following an ENSO event for a time span of 21 months. This was done for two reasons:
1) To determine what effect moderate to strong ENSO events might have on autumn and spring precipitation patterns during an ENSO event, and;
2) To determine if there was a corresponding enhancement of precipitation during the autumn and spring seasons in the year following an ENSO event.
Similar to the findings by Andrade and Sellers (1988), El Paso seasonal precipitation totals indicate a direct positive effect on increased precipitation amount, especially during the autumn of an ENSO onset year and the following spring (table 1). In both seasons, 15 out of the 24 ENSO years (63%) received greater than normal precipitation. Precipitation received either in autumn or spring exceeded 300% of normal four times , but interestingly, precipitation for these seasons averaged only 130% of normal for all of the events. When only the 12 Category 4 (strong) events are considered, greater than normal precipitation was received during autumn in seven out of the 12 cases (58%) and in eight of the 12 cases (67%) during the following spring. Considering only the Category 4 events, the average percent of normal precipitation for the autumn season of ENSO onset rose slightly to 132%, while the following spring reached 147%.
Winter season precipitation totals exceeded normal in only nine of the 24 cases (38%) while 11 of the 24 cases (46%) had above normal summer precipitation (table 2). The strong ENSO event of 1914-15 yielded winter precipitation of 5.54 in., which was over 400% of normal, however, seasonal precipitation totals for winter and summer rarely exceeded 200% of normal. This was more common during autumn and spring seasons.
No clear relationship was seen in terms of corresponding enhanced precipitation during the autumn and spring seasons following a full ENSO event. In some cases, ENSO events lasted for more than a year, with seasons at the end of an ENSO event overlapping with those of the next event, thereby masking any possible connection to subsequent enhanced precipitation. When successive ENSO years were not accounted for, greater than normal precipitation was received during the autumn and spring seasons following a full ENSO event in only one of 13 years.
Precipitation at El Paso and Percent of Normal during Category 3 and 4 ENSO Events
|Onset Year||Category||Autumn Precipitation (in.)||Percent of Normal||Following Spring Precipitation (in.)||Percent of Normal|
A direct positive effect is seen in El Paso seasonal precipitation totals during moderate to strong ENSO events, especially during the autumn of an ENSO onset year and the following spring.
Average precipitation for these seasons , however, averaged only 130% of normal for all of the ENSO years included in this study, and rarely exceeded 200% of normal. Winter and summer seasonal precipitation totals did not show an obvious enhancement in correlation to ENSO events, nor did yearly totals. Yearly totals during ENSO events were greater than normal in only 13 out of the 24 event years (54%). Precipitation averaged 9.55 inches per year for ENSO years only, while non-ENSO years yielded 7.99 inches per year. When considering precipitation totals for individual ENSO years versus the average derived from non-ENSO years, greater than normal precipitation was once again received in only 13 of the 24 ENSO years. The data from this study indicate that precipitation at El Paso is affected only to a certain degree by moderate to strong ENSO events.
No relationship was found in regard to a precipitation enhancement relationship during the autumn and spring seasons following a full ENSO event. Only one year in 13 cases indicated greater than normal precipitation during this time. (Successive ENSO events were discounted.)
|Seasonal Precipitation (in.) for Category 3 and 4 ENSO Events from Autumn of Onset Year to Following Autumn|
|Seasonal Averages 1899-1984||2.56||1.34||.88||3.63||2.56|
Thanks to Val J. MacBlain, NWSO El Paso, and Dan Smith SSD SRH, for providing suggestions and insights that led to the production and improvement of this paper. Thanks to Charlie Liles, NWSFO Albuquerque for help in locating various topical references.
Andrade, E. R., and W. D. Sellers. 1988. El Niño and its Effects on Precipitation in Arizona and Western New Mexico. Journal of Climatology, 8, 403-410.
Bradshaw, T. 1993. Relationships Between El Niño and New Mexico Seasonal Snowfall Patterns. NWSFO Albuquerque Local Study.
Rasmusson, E.M., 1984. El Niño: The Ocean/Atmosphere Connection. Oceanus, 27, 5-12.
Ropelewski, C.F., and M.S. Halpert, 1986. North American Precipitation and Temperature Patterns Associated with the El Niño/Southern Oscillation (ENSO). Mon. Wea. Rev., 114, 2352-2362.