Another “La Niña” is Likely This Winter. What That Means for Central AL | CBS 42 Weather

Weather
October 02 2021 06:00 pm

In the latest forecast data available from NOAA, sea surface temperatures in the equatorial Pacific are expected to get cooler over the next 2 to 3 months. This means another “La Niña” event is likely to take shape this winter, bringing with it some changes in our weather here at home in Central Alabama…

What exactly does “La Niña” mean?

A La Niña means a negative phase of the El Niño Southern Oscillation, or ENSO for short. The Southern Oscillation is a seasonal shift in the strength of the trade winds over the Central Pacific Ocean, which in turn causes a seasonal shift in sea surface temperatures in the area. Periods where the trade winds are weaker causes ocean temperatures to warm up, often developing into an El Niño. Periods where the trade winds are stronger causes ocean temperatures to cool down, often developing into a La Niña.

Where do we look to find out if we are in an El Niño or a La Niña?

The most important area that we look at for changes in ocean temperatures is the “Niño 3.4 region” — an area in the Eastern Pacific Ocean stretching from 5°N across the equator to 5°S, highlighted in the area below.

What our sea-surface temperature “anomalies” are (how far we are from the average temperature) in this region determines what phase of ENSO we are currently in. Above is an example taken from Christmas Day 2017, where we were in a strong La Niña phase of the ENSO.

How does a La Niña affect our weather in Central Alabama?

Historically, the biggest changes to our weather in the Southeast from an El Niño or a La Niña happen during the winter, when both phases of ENSO are typically at their strongest. During an El Niño, the subtropical branch of the jet stream gets stronger over the Southern US and Mexico. This often leads to a greater influx of moisture over the area, and in-general leads to cooler, wetter winters on average here in Alabama & across much of the deep South.

During a La Niña, the polar jet stream and upper-air patterns in general gets wavier, with a stronger blocking high pressure over the Northern Pacific. This in-general leads to milder, drier winters on average here in Alabama & across much of the deep South.

How will this winter compare the winter of 2020-21?

Coincidentally, this past winter was also during a moderate La Niña, with cooler than average ocean temperatures in the Central Pacific. Last winter, in Birmingham, we had 36 out of 90 days with temperatures at or below freezing (≤32°F). Since 1896, the average number of days in winter with freezing temperatures in Birmingham is 35 — thus, last winter in Central AL was fairly “mild” compared to our average, which is what we would expect during a La Niña winter.

Of course, that doesn’t mean we were mild all winter long, and that we were totally free from some big “Arctic chills” — there were a few days this past February where we even saw temperatures in the teens here in Birmingham. This is just an on-average look at our winter. When you look at it through this metric, the winter of 2020-21 was indeed a “mild” one.

As far as precipitation goes, we were also par for the course in Birmingham for a La Niña winter, receiving 10.64″ of total precipitation. That’s just over 4″ less than our average winter precipitation for winter at 14.85″.

So, what does that mean for this coming winter?

If we see a La Niña develop that’s as strong as the ensemble mean in our guidance below, that would mean we can also expect a milder, drier winter on average this year — albeit to maybe a slightly larger degree compared to last year. Notice how that “dip” in the graph goes a bit deeper than the dip of late 2020.

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