Just last week, the Climate Prediction Center, a branch of NOAA’s NWS, declared that the atmosphere has officially entered a La Nina. So, what does this mean for our weather and how will it affect the upcoming months? La Nina is just one phase of a three-phase oscillation called El Nino Southern Oscillation (ENSO); the other two are El Nino and ENSO neutral. ENSO is an oceanic and atmospheric coupling that is a result of sea surface temperature changes from normal (also called an anomaly) and surface pressure changes. The images below show the sea surface temperature anomaly signatures and corresponding surface pressure signatures of all ENSO phases:
El Nino: defined by warmer sea surface temperature anomalies in the equatorial Pacific (greater than 0.5 Celsius above normal) and lower pressures resulting in weaker or even reversed trade winds. This forces the thermocline to get pushed deeper into the ocean and warmer water from the West Pacific moves East. This is illustrated by the bright orange colors (above average sea surface temperatures) in the black circle on the map below (credit to NESDIS):
The resulting El Nino atmospheric circulation with lower than normal pressures and reduced trade winds are shown below (credit Fundamentals of Physical Geography):
La Nina: defined by cooler sea surface temperature anomalies in the equatorial Pacific (greater than 0.5 Celsius below normal) and higher than normal pressures causing enhanced trade winds. This pushes the thermocline to the surface and allows cooler water to upwell. This is illustrated by the blue colors (below average sea surface temperatures) in the black circle in the image below (credit to NESDIS):
The resulting La Nina atmospheric circulation with higher than normal pressures and enhanced trade winds are shown below (credit Fundamentals of Physical Geography):
ENSO Neutral: defined by sea surface temperature anomalies in the equatorial Pacific near normal (between 0.5 and -0.5C). This is shown by the mix of light yellow and light blue indicating small changes in sea surface temperatures compared to normal (credit NESDIS).
The current sea surface temperature anomaly configuration strongly supports a La Nina. Notice the large tongue of blue (in the image below), which indicates sea surface temperature anomalies that are colder than normal. This upwelling of colder water is caused by stronger than normal easterly trade winds as a result of the higher pressures in the equatorial Pacific blowing toward lower pressures over the Maritime Continent in the Western Equatorial Pacific (credit NESDIS).
This graphic below shows very strong trades (blue and purple colors) that will continue to upwell colder waters over the entire equatorial Pacific (credit Mike Ventrice).
One other main factor that can go into enhancing ENSO effects and drive intraseasonal oscillations such as the AO (Arctic Oscillation), PNA (Pacific North American Oscillation), NAO (North Atlantic Oscillation), and EPO (Eastern Pacific Oscillation), is the Pacific Decadal Oscillation (PDO). The PDO has two main phases: positive and negative. The phases are defined by a horseshoe shaped ring of colder sea surface temperature anomalies in the Northern Pacific (-PDO) or warmer sea surface temperature anomalies in the Northern Pacific (+PDO). As seen below, the sea surface temperature map in the North Pacific has a lot of warmer sea surface temperature anomalies as indicated by orange colors on this map. The PDO index published by the CPC is -0.89. The current PDO numerical index is weak to moderately negative, but the cooler anomalies indicated by the cooler sea surface temperatures in the subtropics near Hawaii and signs of emerging cool anomalies off the California coast would go further to enhance the La Nina (outlined by the black circles below). The aforementioned intraseasonal oscillations help drive our day to day pattern in the fall, winter, and spring with specific phases being favored during La Nina combined with a -PDO.
The classic -PDO sea surface temperature configuration is shown below:
So, what should we expect moving forward into fall and winter? Classic La Nina climatology is a warmer pattern with an active polar jet stream. While climatology is an average, I suspect we will see some colder shots of air (as intraseasonal changes occur over the course of days to weeks), but the fall and winter are expected to largely average above normal temperature-wise.
La Nina’s tend to favor:
· +AO: this goes back to the strong, low amplitude jet stream that will favor a progressive pattern meaning less likely prolonged periods of colder weather.
· +NAO: often works in conjunction with AO leading to less Eastern troughing and less prolonged cold periods.
· +EPO: the -PDO helps feed a lower amplitude and stronger jet stream with less troughing over the Eastern U.S.
· -PNA: also influenced largely by the -PDO, this would mean more ridging centered in the Eastern half of the country, meaning warmer than average winter.
It is looking almost certain that a moderate La Nina is in the works with a -PDO working in conjunction for a warmer than normal fall and winter with an active polar jet stream. How small varying intraseasonal changes in the AO/NAO/PNA/EPO cannot be pinpointed in this range and we will have further updates as changes occur throughout the fall and winter.