Winter is around the corner and forecasts are being fine tuned. Latest modelling shows a shift to the colder, with multiple factors, such as El Niño, hinting at a frostier and snowier setup for the United States, Canada, Russia and Europe.
A building El Niño will undoubtedly be a key driver of the 2023-24 winter across the Northern Hemisphere, but added to this we have such forcings as the Polar Vortex, a new anomaly in the stratosphere; and also the unprecedented stratospheric injection of water vapor following the Hunga Tonga eruption of January 2022.
Based on the latest analysis, there are four large-scale factors that will determine the upcoming winter season:
- an El Niño event (ENSO)
- the Stratospheric Polar Vortex
- Quasi-Biennial Oscillation (QBO)
- Stratospheric Water Vapor
Each of these factors possess different phases: warm/cold, positive/negative, east/west, etc., and different combinations of each can lead to wildly different winter outcomes.
This is where the complexity in weather/climate forecasting resides, the mechanisms are plentiful–we still don’t know all of them–and their interactions are poorly understood (And that’s without mentioning the driver of it all, the Sun, which itself is experiencing a cumulative (two cycles) of historically low output).
Focusing on this winter’s setup, below is a simplified breakdown of what the latest models suggest is in play.
With regards to the El Niño Southern Oscillation (or ENSO), it is generally accepted that we are entering the warm phase (El Niño) after having been stuck in the cold phase (La Niña) for a rare three winters — a ‘triple-dip’.
You can see the emerging El Niño in NOAA’s latest ocean anomaly analysis:
El Niño is known to significantly impact tropical rainfall, pressure patterns, and the complex ocean-atmospheric exchange — and after a little lag, these system/circulation changes spread globally.
Below is an analysis graphic courtesy of NOAA-CPC, showing the long-range ENSO forecast.
Currently, NOAA is expecting a +1.5 degrees anomaly this time around (by mid-winter), which transplates as a moderate to strong El Niño event. Note, Weak = 0.5 to 0.9; Moderate = 1.0 to 1.4; Strong = 1.5 to 1.9; and Very Strong ≥ 2.0.
Typically, an El Niño winter will see a strong and persistent low-pressure area in the North Pacific.
This often drags the polar jet stream north, bringing milder temperatures to the northern United States/western Canada, but it also drops the southerly Pacific jet south, meaning colder conditions and more precipitation to the southern US.
This setup is conducive to more snowfall overall: lower temperatures in the central/southern-half of the United States brings heavier accumulations to the likes of Texas, while temperatures, though slightly warmer than normal, are still cold enough to deliver sweeping snowstorms to the north, to the likes of Montana.
Stratospheric Polar Vortex
In simple terms, the Polar Vortex describes the broad winter circulation over the northern (and southern) hemispheres.
The Vortex extends high into the atmosphere, through the troposphere and into the stratosphere:
The higher (stratospheric) section has a more circular and symmetrical rotation, with fewer obstacles in its flow. Conversely, the lower (troposheric) part of the Polar Vortex is uneven, disrupted, due to the influence of the terrain and strong pressure systems.
The strength of the Polar Vortex varies, with largest impacts coming at the extremes: when either strongest or weakest.
A strong Polar Vortex means a tight and stable circulation (jet stream) which keeps the cold polar air locked within the Arctic Circle, meaning milder conditions for the lower latitudes. A weak Polar Vortex does the opposite, the flow becomes ‘wavy’ and it has a harder time containing the cold air up north, with ‘outbreaks’ routinely spilling south into the likes of the U.S. and Europe.
For a fiercely cold and snowy winter you need a weak Polar Vortex–aka a breakdown of the stratospheric Polar Vortex.
This is usually achieved by increasing the pressure/temperature in the polar Stratosphere (aka “Sudden Stratospheric Warming”). But such a setup requires a lot of energy, and at various levels. This leads us to the next forcing…
Quasi-Biennial Oscillation (QBO)
This atmospheric phenomenon (the QBO) is a changing wind anomaly in the tropical Stratosphere. Its presence has been well known for many decades, and it appears to play a crucial role in the seasonal weather patterns.
Strong stratospheric winds flow along an equatorial belt:
Approximately every 14 months, these winds completely change direction — the tropical stratosphere winds flip from west-east or east-west every year-and-a-bit. Currently, the QBO is in a west-east flow, or what’s known as a ‘negative’ phase.
The below graph shows the equatorial zonal wind anomalies of the past 40 years (at 24 km/15 miles altitude).
Akin to a heartbeat, these wind shifts are regular.
A recent NASA radiosonde analysis from Singapore shows confirms the ‘east’ QBO phase will be dominant this winter:
The QBO has proven itself a a important part of winter weather development, as it can–among other things–impact the Polar Vortex and the jet stream, i.e. the power and direction of the polar jet stream can change with the QBO.
The QBO’s exact influence also depends on outside forcings, such as ENSO. The Oscillation alone doesn’t result in a fixed weather outcome: an east (negative) QBO will impact the Polar Vortex differently depending the ENSO state: El Niño or La Niña.
During an easterly QBO, a signal for a warm anomaly in the polar stratosphere is apparent (SSW events), which in turn results in a weak Polar Vortex/stratospheric winter circulation and increases the chances of ‘outbreaks’ of Arctic air to the lower latitudes.
The combination of El Niño and an east QBO increases the odds for a Polar Vortex collapse: a cold and snowy winter.
The Winter of 2009-10
I remember this winter well. It was a brutally cold and snowy one for much of the Northern Hemisphere, including for me (then in the UK). And lo and behold, the atmosphere was setup in a very similar way to what we’re seeing now.
Firstly, El Niño was active during the 2009-10 winter (though it was a weaker event than that forecast for this coming winter):
Secondly, the QBO had a very clear east (negative) setup that winter:
These conditions resulted in a strong SSW event that season, and a collapse of the Polar Vortex beginning late-January/early-February.
Following a lag (30-days), the surface temperature anomalies tanked across transcontinental Russia, Europe and also the United States (shown below) — cold Arctic air masses were displaced south as the jet stream weakened and effectively buckled.
The temperature anomaly for 2009-10’s winter season as a whole shows well-below average readgins across the United States and Europe, with, conversely, warm anomalies present over northern and eastern Canada, and also Greenland:
Precisely quantifying how much of this cold winter was due to El Niño, the QBO, and the Polar Vortex collapse is impossible to determine. However, the combining of the factors made it possible — a colder-than-normal winter ensued, as was to be expected.
As already mentioned, the 2022-23 winter is setting up in a very similar fashion, though with a slightly stronger El Nino. Early indications have me hanging my hat on a brutally cold and snowy season to come. There is no other conclusion to be reached.
But there is another factor added to the mix this time around: Hunga Tonga’s record-high eruption of Jan 15, 2022–or more specifically, its unprecedented stratospheric injection of water vapor, which has spread from the equator to the poles:
And as contended in one recent study, this will have a “large effect” on the Polar Vortex starting this fall and winter.
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