A Review of an ENSO article by Monteverdi (SFSU)
and ENSO Update
NWSO Midland, Texas
Last year's vigorous El Niño and the present La Niña state of the tropical Pacific have gathered much attention by not only our country's top scientists, but also by many in the media and in the public. The ongoing La Niña event in the tropical Pacific has brought about a different set of synoptic parameters that have dominated the weather in West Texas this past winter into the present spring season. Questions to address at this time include: How did La Niña affect this past winter in West Texas? How will La Niña affect the springtime weather in West Texas? What does the John Monteverdi article about La Niña mean for West Texas this spring?
A quick look at some climate statistics for Midland, TX reveals that the forecast of warmer and drier conditions than normal for West Texas this past winter of 1998-1999 verified quite well. Specifically, meteorological winter (the months of December, January, and February) had high temperatures above normal nearly 70% of the time while the drought from last year continued into the new year with only 58% of the average precipitation occurring in these winter months.
The effects of the present La Niña on the weather for the spring season in West Texas and a brief synopsis and review of the John Monteverdi La Niña study will conclude the ENSO section of the spring edition of the Dust Devil Dispatch. Late this past winter John Monteverdi, a professor in the Department of Geosciences at San Francisco State University, published a paper describing the different synoptic conditions for severe weather in the plains states between an El Niño and La Niña year. The general consensus before the study was that Great Plains tornado events are more likely in El Niño years and less likely in La Niña years. Monteverdi compares general synoptic variables for the months of May and June in both El Niño and La Niña years. These variables include 500 mb height, 500 mb height anomalies, 500 mb omega, 500 mb omega anomalies, surface pressure, surface pressure anomalies, surface wind fields and surface wind field anomalies. Basically a comparison of these variables was made between an El Niño and a La Niña year. Monteverdi's comparisons reveal the following characteristics during a La Niña year in May and June: a deeper trough in the west with a mean ridge axis in the southern Great Plains (southwest flow aloft), upper divergence over southwest U.S. and southern Plains region, synoptic scale upward motion for the south-central Plains and southwest desert areas, deeper moisture return through Texas and strong low level shear, a higher frequency of surface cyclogenesis and cyclone passage over the Great Basin and western Plains. He concludes that these composite patterns for La Niña events would suggest much more favorable environments for severe storm formation compared to an El Niño year.
Despite the fact that the forecast of the Climate Prediction Center/NCEP is for the continuation of a moderate La Niña through the summer and for drier and warmer conditions for the southern tier of the U.S., I agree with the Monteverdi study that severe events in the southern plains will be more likely this spring storm season than last year or for a typical El Niño spring season. Last spring a powerful subtropical jet stream dominated the southern tier of the U.S. propelling storm systems quickly through the area, hence not allowing for a return of lower level moisture from the Gulf of Mexico into West Texas. So far this spring, the synoptic setup has been much different with a less active subtropical jet stream and more frequent powerful western troughs of low pressure which has not only allowed more active waves to move across the area, but has also permitted lower level moisture return into the area which is critical for storm formation. From a climactic standpoint so far this spring (March through the present) Midland has had high temperatures above normal only 49% of the time while receiving 140% of the normal precipitation amounts. I expect this more active trend to continue through the spring season with some episodes of lively weather separated by hot and dry periods. Although a more active spring scenario is likely and the accompanying rainfall well appreciated, the ongoing drought will most likely not be broken. Expect the overall temperature trends to be above normal, but not to the extent of the past winter season. The overall teleconnections or influences of an ENSO episode decrease as the summer season approaches and increase once again in the late fall season into the winter. Before that, though, the ever present La Niña in the equatorial Pacific is expected to play a role in providing a more active spring storm season to West Texas.