26 January 2023
(WGHP) – This year marks the third consecutive La Nina winter in the United States. However, the most recent data released by the Climate Prediction Center shows that a pattern change may be in our future come springtime.
Let’s take a look at how La Nina is impacting our winter weather and what a pattern change would mean for the remainder of the year.
What is La Nina?
La Nina is essentially the cooling of sea surface temperatures in the eastern tropical Pacific Ocean, along the west coast of South America.
The cooler surface temperatures in the Pacific are the result of unusually strong eastward-moving trade winds and ocean currents which bring the cooler water to the surface in a process known as upwelling.
How does La Nina impact weather patterns?
The process can affect patterns of rainfall, atmospheric pressure, and global atmospheric circulation. Atmospheric circulation is the large-scale movement of air that together with ocean currents, distributes heat energy on the surface of the Earth.
La Nina is known to bring higher-than-normal pressure over the central and eastern Pacific. As a result, drier-than-normal conditions are observed in much of the southern United States.
A typical La Nina winter set-up of the United States.
La Nina impacts include continued or worsening drought conditions, as well as below or above average temperature and precipitation fluctuations across the United States. La Nina can also lead to a more severe hurricane season.
For the Piedmont Triad, a La Nina winter typically means over a three month period, December through February, temperatures are expected to be warmer-than-normal with near-normal precipitation.
Are we transitioning out of a La Nina pattern?
The latest update from the Climate Prediction Center (CPC) favors, with 82% odds, an ending to La Nina and a transition to an El Nino Southern Oscillation (ENSO) neutral pattern from the beginning of February and through May 2023.
What does an El Nino Southern Oscillation (ENSO) neutral phase mean?
In an ENSO-neutral phase, tropical Pacific sea surface temperatures are generally close to normal. However, there are some instances when the ocean can look like it is in a La Nina or El Nino state but the atmosphere is not playing along or vice versa.
In the winter, a typical ENSO-neutral phase means cold air in the northeast with warm, wet weather in the southern United States.
The transition to an ENSO-neutral phase means La Nina would no longer have an influence on our weather heading into the spring months.
Sometimes, like we’ve done in the past three years, La Nina conditions can lead to an ENSO-neutral phase and then return to La Nina conditions. However, there are signs that we may swing in the other direction by the end of the year.
The most recent update from the Climate Prediction Center points to the beginning of El Nino conditions from July through September of 2023. El Nino is considered the “warm” phase of the ENSO climate pattern.
What is El Nino? How does it impact weather patterns?
In an El Nino phase, trade winds weaken which pushes warm water eastward, toward the west coast of the Americas. This results in above-average sea surface temperatures and increased rainfall in the central and eastern tropical Pacific Ocean.
Typical El Nino Winter set-up in the United States.
The eastward shift of the warmer waters causes the Pacific jet stream to strengthen and move south of its neutral position. With this shift, there’s a moisture-rich storm track through the Gulf of Mexico and off the North Carolina coast.
South-central Texas and the southeast U.S. will be mostly cool and wet because the cold, moist air from the Pacific Ocean enters the southern states, enhancing clouds and rainfall which cools temperatures due to lack of direct sunlight.
In turn, much of the northern United States and Canada will be warm because the polar jet stream swings further east over the northeastern United States. But in the Gulf Coast and Southeast, these periods are wetter than usual and have an increased flood potential.
The strong subtropical jet stream also tends to disrupt hurricane development in the Atlantic and Gulf of Mexico. Because of that, fewer hurricanes are likely during an El Nino year in the tropical Atlantic.
What are the chances of an El Nino pattern developing?
Based on the graph below, the probabilities are equal for La Nina and an ENSO-neutral phase for January through March but a neutral phase is favored by February through April.
From the three-month period of March, April, May, through July, August, September, the probability of El Nino conditions will likely continue to increase. While the odds for El Nino are not overwhelming, they are favored in comparison to a neutral or La Nina phase heading into fall 2023.
Hurricane season peaks in August and September. If an El Nino phase is in place when NOAA is expecting, we could see a below-normal hurricane season in the Atlantic.
Similar to La Nina, El Nino has a bigger impact on our weather during the winter months compared to other times of the year.
It’s far too early to know whether a potential El Nino pattern would carry into next winter. But, if we did end up with an El Nino 2023-2024 winter we could see a colder and wetter pattern for the Piedmont Triad.