James A. Reynolds
NWSO El Paso, TX (Santa Teresa, NM)
This paper was written to serve as a follow-up to the Technical Attachment that I wrote last fall (Reynolds, 1997), which outlined a direct positive effect (increase) in El Paso, Texas yearly and seasonal precipitation during moderate and strong El Ni?o events. As the current El Ni?o slowly weakens during this spring of 1998, interest is now beginning to turn towards the potential effects of La Niña on various locations. While La Niña events do not always proceed El Ni?o events, it has been shown by Kiladis and Diaz (1989) that during the year preceding the development of an event in the Southern Oscillation (either El Ni?o or La Niña), climatic anomalies tend to be opposite to those during the following year. Thus, it is appropriate at this time to investigate the occurrence and effect of La Niña so as to understand the influence on this region of the two different climatological anomalies.
Given the nature of cooler than normal waters in the eastern Pacific during La Niña events, climate effects related to this phenomena are directly opposite those of El Ni?o, as described on the NOAA-PMEL-TAO webpage (1998). The purpose of this paper is to show that there is a direct negative effect (decrease) on El Paso, Texas precipitation during La Niña events.
As with the El Ni?o study, a determination first had to be made in regard to what constitutes a La Niña, or cold event, year. Different sources list different years for La Niña events. Ultimately, the Japan Meteorological Agency (JMA) Index for ENSO and non-ENSO years was incorporated into this study because it lists the well known La Niña events (Legler, 1998). The index is a 5-month running mean of spatially averaged sea surface temperature anomalies over the tropical Pacific. If the Index values equal or exceed -0.5C for 6 consecutive months, including October, November, and December, then the anomaly year from October through the following September is considered a La Niña.
Events classified as La Niña on the JMA Index date back to 1869 and end with the last well documented event of 1988. El Paso precipitation records are readily available back through 1879. The first La Niña event on the JMA Index after 1879 is 1886. Thus, the years of this study range from 1886 to 1988, and it is from these 103 years that normals were calculated. Of the 103 years in this study, 26 are La Niña years. El Paso monthly and seasonal precipitation totals were calculated for all of the La Niña years of this study, which were then compared to monthly and seasonal precipitation totals for all of the years between 1886-1988 to determine the departure from normal precipitation during La Niña years. Also, average seasonal precipitation totals for La Niña years were compared to average seasonal precipitation totals for the non-La Niña years of this study. The two sets of calculations were to performed to show that La Niña does indeed have a direct negative effect (decrease) on the El Paso, Texas precipitation record. The autumn season was classified as September to November, winter as December to February, and spring as March to May.
When considering La Niña and non-La Niña years together, 20 of the 26 autumns and springs during La Niña years (77%) had below normal precipitation, as did 17 of the 26 winter seasons (65%) (table 1). On average, precipitation received during any one of the three seasons was 61%-76% of normal when normals computed with all the years inclusive of this study were used.
|Autumn, Winter and Spring Season Precipitation (in.) and Percent of Normal During La Niña Events|
|Onset Year||Autumn Precip||Percent of Normal||Winter Precip||Percent of Normal||Spring Precip||Percent of Normal|
The spring season showed the most variability in precipitation during La Niña years. This would not be unexpected since the average precipitation was the least for spring and the distribution of the observations would more likely approach either a gamma or a strongly positive skewed normal distribution than the other seasons. Precipitation received during the spring of 1893 approached nearly 300 percent of normal while the spring of 1910 received none. Beyond this, precipitation received in any one of the seasons during La Niña years exceeded 175% of normal only one other time.
The comparison between average seasonal precipitation totals for La Niña years and average seasonal precipitation totals for the non-La Niña years of this study further demonstrates that La Niña has distinct effects on El Paso, Texas precipitation. For the three seasons, average precipitation during La Niña years ranged from a high of 64% to a low of 53% of the average seasonal precipitation received during non-La Niña years (table 2).
|Average Seasonal Precipitation (in.) for La Niña Years vs. Non-La Niña Years|
|La Niña Years||1.78||.87||.54|
|Non-La Niña Years||2.75||1.50||1.01|
|Seasonal Difference in percent from La Niña to Non-La Niña Years||64||58||53|
In accordance with information found on the NOAA-PMEL-TAO webpage, there is a direct negative effect (decrease) in El Paso seasonal precipitation totals during most La Niña years.
The most positive statement that can be made from this study is that precipitation received in any season during La Niña years will most likely be below normal with the autumn and spring season more likely than the winter season to have below normal precipitation.
Conversely, in very rare cases, autumn, winter, or spring seasonal precipitation can exceed 175% of normal during La Niña years.
Thanks to Val J. MacBlain, NWSO El Paso, and Dan Smith of Scientific Services Division, NWS Southern Region Headquarters, for providing suggestions that led to the improvement of this paper. Thanks to Roland Schweitzer and Catherine Smith, Climate Diagnostics Center - Boulder, Colorado, and Deirdre Kann and Kerry Jones, NWSFO Albuquerque, for help in locating various topical references.
Kiladis, G.N. and H.F. Diaz, 1989: Global Climatic Anomalies Associated with Extremes in the Southern Oscillation, Journal of Climate, 2, 1069-1090.
Legler, D.M., cited 1998: [Available on-line from http://www.coaps.fsu.edu/~legler/jma-index.html]
NOAA-PMEL-TAO, cited 1998: El Ni?o Theme Page. [Available on-line from http://www.pmel.noaa.gov/toga-tao/el-nino/]
Reynolds, J., 1997: El Paso, Texas Precipitation and its Relationship to the El Ni?o/Southern Oscillation (ENSO). Southern Region Technical Attachment 97-47.