SAN ANGELO, Texas (October 25)- If you were hoping for a very cold winter with more precipitation (snow), models are showing the opposite. Fall continues to progress with days of above-average temperatures and we could be facing the same outcome for Winter 2021. According to the National Weather Service San Angelo, Texas could see drier and warmer weather going into this winter season. All these factors point out to La Niña, the cold phase of the El Niño Southern Oscillation (ENSO).
What is ENSO
ENSO refers to the change in sea-surface temperatures, convection rainfall, surface air pressure, and atmospheric circulation over the equatorial Pacific Ocean. ENSO has three phases warming, cooling, and neutral. Both El Niño(warm phase) and La Niña (cold phase) are two extremes that affect daily weather across the world. You can also experience a Neutral phase of ENSO, where you are not experiencing either La Niña or El Niño conditions.
Between 2000-2021, we have seen all ENSO phases. In 2013, we were in a full neutral phase, 2000 in a full El Niño phase, and in 2015 we were in a full La Niña phase. La Niña is represented in blue in Figure 1a, you can see that in 2019-2021 we were in months that experienced a Neutral phase and La Niña phase. The last time we saw El Niño conditions develop was in 2018.
What is La Niña
La Niña is the cooling of the ocean sea surface temperatures (SST’s) in the central and east-central Pacific that usually occurs every 3- 5 years. However, we have seen “double-dipping” year of La Niña which is not typically common.
Also known as the cool phase of ENSO, La Niña is usually depicted as the annual cooling of ocean waters off of the west coast of Peru and Ecuador. This cooling is activated due to the process of upwelling.
Upwelling is the rising of deep cold waters to the surface. This process is usually due to increasing easterly and westerly trade winds over the Eastern Pacific. There are more characteristics that activate this cold episode of ENSO.
Characteristics of La Nina
In Figure 2a, notice the warmer sea-surface temperatures (SSTs) are developing over the western equatorial Pacific and northern Australia. Notice there are cooler SST’s over the eastern equatorial Pacific, just off the coast of Peru and South America. This is one of the main characteristics of La Niña.
In figure 2b, SST Anomalies are reflected for La Niña conditions. “Cold tongue” develops due to upwelling, which brings cooler waters to the surface of the eastern Pacific Ocean. Due to increasing trade winds off of the equatorial Pacific Ocean, upwelling increases. SST’s ranged between 1o-2oC during La Niña which is shown in the figure above. This leads to wetter conditions for the western equatorial Pacific and drier conditions for the eastern equatorial Pacific.
The graph above illustrates the variations of SST Anomalies for a 3-month average. The graph shows the transitional season beginning in Summer 2021 and ending in Winter 2020. La Niña usually shows more negative (colder) SST values that occur at values less than or equal to -0.5 degrees Celsius. The Oceanic Nino Index (ONI) measures the deviation of sea surface temperatures by more than 0.5 degrees Celsius. Figure 3b shows values beginning with -0.5 degrees Celsius and dropping lower to -1.5 degrees Celsius during Winter 2021. This means La Niña is expected to continue through Winter 2021 and return to the neutral phase by the early Spring 2022. Neutral phase SST anomalies are between 0-0.5 degrees Celsius.
La Niña across the world
With the increase of warmer sea-surface temperatures across the western equatorial Pacific, we see wetter conditions for the Philippines and Indonesia. On the other hand, conditions become drier along the coast of Ecuador and Peru.
Effects of La Niña on the United States
The set-up for La Niña targets the United States in a big way. The first transition comes with the increase in meridional flow across the United States.
The Pacific jet then increases over the Northern Pacific, which shifts the jet stream further north over the southwestern United States and Canada. Then, a blocking high-pressure develops over the North Pacific. This locks in the coldest air to the north of the country.
La Niña’s setup brings snow and rain to the northwest and northeast parts of the United States but keeps places like the south and southeast dry.
Effects for Texas
With the setup of La Niña in full effect, Texas is forecasted to see above-average temperatures and drier weather. This will hold up true for what currently going on.
Temperatures have been warmer than our average temperature of 78 degrees for the Fall season. We have seen temperatures between the lower and middle 90s and even broke our record high for today with a high of 95 degrees.
We could see less precipitation this year due to the La Niña setup, which may be disappointing to some of you. However, last year we were experiencing La Niña conditions and saw several snowstorms including February’s Snowmageddon, which brought 10 inches of snow to San Angelo.
Last winter, we were in a La Niña phase and many of us would remember it forever. We saw a fair share of winter storms across Texas that included snow and ice that affected the whole state. February’s big storm was not the only one. We started early with an early snowstorm on October 27, 2020, where we saw 1.7 inches of snow in San Angelo. After that, there was the December 31, 2020 snow event that brought 3-5 inches across the Concho Valley.
The year 2021 began and on January 10, San Angelo experienced 3.8 inches of snow with 6 inches for some of our counties in the Concho Valley. Lastly, Snowmageddon occurred on February 14, shocking the whole state of Texas.
The snow event brought 10 inches of snow to San Angelo, along with icy roads and freezing conditions across Texas. Many people were without water and power for days and they were several flight cancellations and delays. Road conditions were unbearable with temperatures plunging into the tens making it one event for the record books.
In conclusion, La Niña’s conditions do not nullify the possibilities. However, preparing for them early will make more of the difference in the end.
All source material was gathered from the NOAA National Weather Service Climatology Data, National Weather Service San Angelo NOW Data, and NOAA Climate Prediction Center