La Nina is breaking down, and an El Nino event is forecast to emerge in 2023. Returning after several years, an El Nino can completely change the weather patterns for the weather seasons in 2023 and 2024, especially during the Winter season.
Ocean anomalies and especially their changes can significantly influence seasonal weather patterns. Perhaps even more so in Winter, when the pressure systems are strongest. One of the most well-known global weather modifiers is the El Nino event, which is returning in 2023.
First, we will quickly analyze how these ocean anomalies affect global weather. Then we will see how and when El Nino is forecast to emerge and how it can influence the Winter season of 2023/2024 based on historical data.
OCEAN-ATMOSPHERE WEATHER RELATIONS
El Nino and La Nina are just two faces of the ENSO, which stands for “El Niño Southern Oscillation.” This is a region of the equatorial Pacific Ocean that periodically shifts between warm and cold phases. Typically there is a phase change around every 1-3 years.
ENSO significantly influences tropical rainfall, pressure patterns, and the complex energy exchange between the ocean and the atmosphere. As a result, we can observe large-scale pressure changes in the tropics with each developing phase or during its breakdown.
The image below shows the ENSO regions across the tropical Pacific. Regions 3 and 4 cover the east and west tropical Pacific. The main area is a combination of regions 3 and 4, seen in the image as the Nino 3.4 region.
Each ENSO phase influences the pressure and weather in the tropics differently. This affects the overall global circulation over time, changing the weather patterns worldwide.
A new (cold/warm) phase usually develops between late Summer and early Fall. It normally lasts until Spring, but some events can last up to two or three years, as in the current cold phase.
The cold ENSO phase is called La Nina, and the warm phase is called El Nino. Besides the ocean temperatures, one of the main differences between the phases is the pressure patterns they develop, seen below as high (H) and low (L) pressure zones.
During an El Nino, the pressure over the tropical Pacific is lower, with more rainfall and storms in this region.
But during a La Nina, the pressure over the equatorial Pacific is higher and creates stable conditions and fewer storms. These pressure changes translate into global circulation over time, affecting seasonal weather over both Hemispheres.
ATMOSPHERIC WEATHER RESPONSE
The following image below from NOAA Climate shows the typical circulation during a cold ENSO phase, which is still active.
Descending air in the eastern Pacific causes high pressure and stable weather. At the same time, the air is rising in the western Pacific, causing frequent thunderstorms, low pressure, and more rainfall.
This way, ENSO has a strong impact on the tropical rainfall and pressure patterns, affecting the ocean-atmosphere feedback system. Through this ocean-atmosphere system, the ENSO influences the weather globally.
We can observe a global shift in pressure patterns during the emergence of an ENSO phase. But it is usually more influential during the peak of its phase.
But how does ENSO even shift between cold and warm phases? The simplest answer is that it happens because of a complex relationship between pressure, winds, and ocean currents.
Global trade winds usually start or stop a certain ENSO phase by overturning the ocean surface layers and altering the ocean currents. Trade winds are steady and persistent winds, blowing towards (and along) the Equator in both Hemispheres.
But the key here is not just in the winds, as pressure changes drive them. Thus, the ENSO phase directly responds to an atmospheric pressure change called the Southern Oscillation Index (SOI).
The Southern Oscillation Index or SOI represents the difference in air pressure measured at Tahiti (French Polynesia) and Darwin (Australia). The image below shows the location of the two pressure zones important for ENSO.
Positive SOI values mean the pressure over the Tahiti side is higher than over Darwin in Australia. This corresponds to stronger easterly trade winds, supporting La Nina conditions.
But during an El Nino, we see lower pressure in the eastern Pacific, over Tahiti, and higher pressure over Darwin. This produces a negative SOI value and weaker trade winds, which means less ocean surface cooling.
This process is much better seen in the video animation below, showing ocean temperature anomalies from Summer to Fall.
ENSO cooling restarted in August as the trade winds intensified. As a result, cold waves developed across the equatorial Pacific as the winds pushed surface waters to the west.
WINTER SEASON 2022/2023 SO FAR
So while a La Nina is still active, we will quickly look at its influence on the Winter season.
A strong blocking high-pressure system in the North Pacific is the most typical effect of a cold ENSO phase (La Nina). That usually redirects the polar jet stream down over the northern United States. You can see the La Nina winter weather patterns in the image below by NOAA Climate.
The colder air is typically spread over the northwestern United States, the Midwest, and western Canada. Warmer than normal winter weather is found in the southeastern and eastern parts of the United States.
Looking below at the actual Winter analysis so far, you can see a similar pattern. Colder air is focused on western Canada, the northwestern United States, and parts of the Midwest. The rest of the United States is warmer than normal. Europe is also warmer than normal, but that is not a direct La Nina-specific pattern.
That is due to the jet stream changes, which we can see reflected in the pressure anomalies below. A high-pressure anomaly extends from the North Pacific and over the polar regions. This pattern also helped to bring a record cold outbreak in late December, but it was short lasting.
But what about snowfall? The historical data shows that the jet stream from a La Nina also changes the snowfall potential over North America as the pressure systems take a different path.
The colder air is more easily accessible to the northern United States, which increases the snowfall potential when moisture is available. The graphic below by NOAA-Climate shows the average snowfall pattern for La Nina Winters, as expected for this season.
Besides the northwestern United States and the Midwest, we can see more snowfall potential over the northeastern United States and eastern Canada.
Looking at the actual Snowfall anomalies so far, we can see more snowfall across the western and northern United States. Some areas of the northeast are also above normal. But overall, it looks similar to an expected La Nina snowfall pattern. Image by Ben Noll.
But, as you will see, the La Nina is already breaking down and is expected to be replaced by next Winter.
LA NINA STANDS DOWN OVER WINTER
The latest global ocean analysis below reveals ongoing cold ocean anomalies in the tropical Pacific. The active La Nina spans almost the entire equatorial Pacific but is already weakening. Image by NOAA CRW.
Below, you can see the anomaly data from the past years across the ENSO region. You can see the first La Nina event in 2020. The second La Nina occurred in late 2021, lasting through the Winter. A third-year event is currently active but already weakening.
The blue and red lines show the expected progress from the seasonal models. La Nina is expected to phase out rapidly, starting a shift into warm ocean anomalies in Spring.
The current La Nina was 3rd consecutive. No cold ENSO event has gone into the 4th year in the known records. So it is expected that this is the last La Nina phase for some time, increasing the statistical likelihood of an El Nino event.
Looking more closely at the latest analysis of the ENSO regions below, the cold anomalies are breaking down in the eastern regions. So overall, La Nina still looks to be active. But the breakdown of the easterly regions is the first sign that it is ending.
Below we have the latest 7-day ocean temperature anomaly change. You can clearly see the ongoing warming in the eastern and central ENSO regions. This is a continued trend over the past month and is expected to be ongoing into Spring.
But there is more going on beneath the ocean surface as well.
BELOW THE OCEAN SURFACE
The activity below the ocean surface shows a strong subsurface wave of warm anomalies, also known as a Kelvin Wave, expanding from the west. But in the east, you can see the developing breakdown of cold anomalies. This was the subsurface “engine” of the La Nina, now starting to turn off slowly as warm patches appeared.
Looking at the June 2023 ocean forecast from CFS, we can see the warm pool taking over the subsurface waters. This signals that the El Nino conditions may appear on the surface later in the year, creating completely different weather patterns.
But what do the forecasts show for the ocean surface layer?
EL NINO 2023 FORECAST
The ocean temperature forecast from the ECMWF model shows the eastern ENSO regions going warm by Spring. The image below shows the average anomaly forecast for the February-April season.
Below you can see the ensemble forecast for the eastern ENSO region. The La Nina conditions (below -0.5) are forecast to dissipate rapidly. A shift into the warmer territory is forecast during Spring. If sustained over the Summer, it is highly likely to last and influence the next Fall and Winter seasons.
You can see that in the extended seasonal forecast by ECMWF for the main ENSO region (3.4). It shows the cold phase going out over Winter. But a sustained shift into El Nino territory is forecast for the next Winter season.
The extended seasonal forecast is not released each month. However, you can expect a fresh extended forecast in the February update. That will give an even better overview of the potential development of the 2023/2024 weather season.
But you can look at the latest shorter-term seasonal forecast for the main ENSO region below. Compared to the previous month, the latest update shows an even better consolidation in the El Nino territory over the Summer.
The ENSO forecast from the NOAA multi-model forecast is similar. First, it shows the current cold anomalies will weaken over the Winter. Then the forecast suite goes for the shift into the warm phase, but not as convincingly as the other model solutions.
The IRI official probabilistic ENSO forecast also shows the current La Nina lasting over Winter 2022/23. But it is typical for a new phase to emerge in late summer/fall with seasonal pressure changes. Below, you can see a strong probability increase of an El Nino event in 2023.
Looking ahead at the warm season, we can see the ECMWF forecasting a strong tongue of warm anomalies across the equatorial Pacific. These conditions, as forecast, show a full El Nino event starting in Summer.
We can also look at the precipitation forecast, where you can see a strong area of increased precipitation over the ENSO regions. This is because an El Nino event causes lower pressure over the central and eastern tropical Pacific, increasing the number of storms and precipitation.
The North American multi-model ensemble forecast (NMME) also shows the same anomalies developing over Summer. It is somewhat weaker than the ECMWF forecast. But as the anomalies tend to strengthen over Fall, this is a healthy case for an El Nino Winter of 2023/2024.
Another model to forecast an El Nino event during the summer season is the Canadian CanSIPS. It shows a healthy warm anomaly across the ocean surface in the ENSO regions. This raises confidence for the El Nino event starting in 2023, but it is still far out, so caution is advised.
But how does an El Nino change the weather patterns, especially in Winter, if it lasts?
EL NINO WINTER WEATHER
During the El Nino winter season, we usually have a strong and persistent low-pressure area in the North Pacific. That pushes the polar jet stream further to the north, bringing warmer-than-normal temperatures to the northern United States and western Canada.
The southerly Pacific jet stream is amplified, bringing storms with lots of precipitation and cooler weather to the southern United States.
The image below shows the average winter pressure pattern for the past few El Nino winters. You can see a strong low-pressure area in the North Pacific, a high-pressure zone over Canada, and a low-pressure storm track across the southern United States.
Below are temperature anomalies in these winters. You can see the average El Nino winter having colder temperatures in the southern half of the United States and parts of the eastern United States. The northern half of the country is warmer than usual, as is southern Canada.
Precipitation-wise, an average El Nino winter can bring more precipitation to the southern half of the United States, especially in the Southeast. However, drier winter conditions prevail in the northwestern United States and around the Great Lakes, opposite a La Nina’s influence.
Of course, an El Nino also changes the snowfall patterns, as seen in the image below. There is less snowfall in the northern United States during the El Nino winter seasons. But more snow than normal is seen in the central and southern United States during an El Nino. And also over parts of the east.
That is mainly due to low-pressure systems trailing across the southern United States. With cooler air available, more moisture increases the chances of snowfall in the southern half. But a lot depends on the availability of the cold air to the north.
After passing Canada and the United States, the jet stream moves into the North Atlantic, where it can take different paths toward Europe.
But the ENSO effects are much less direct in Europe than in North America. That is why we focus more on North America to track direct (and more predictable) weather changes.
But still, looking at the historical connections of ENSO to Europe, we produced two graphics below. First is the Winter pressure anomaly signal. It shows a high-pressure tendency over most of Europe, with the subtropical ridge expanding in an El Nino.
The following temperature is, of course, warmer than normal over much of the continent. But the signal has a weak strength, as the ENSO influence loses most of any direct influence this far out. So while there is a signal in the average, it cannot be used for any direct weather pattern forecasts over Europe.
As El Nino changes the weather globally, hardly a corner of the world does not feel its effect. But the weather changes in parts further away are less predictable. That is because local weather systems play a specific role, as well as other global weather drivers.
But what does the long-range forecast data show for the upcoming months?
SEASONAL (LONG-RANGE) WEATHER FORECAST
We have already looked at the ECMWF forecast above for the oceans. Next, we will combine its atmospheric forecast with the CFS model forecast from the NOAA/CPC.
Looking at the pressure anomaly forecast for the January-March period, you can see a strong high-pressure system in the North Pacific, typical for Winter, with a still active La Nina. A low-pressure system is over western Canada, keeping the jet stream over the northern United States.
The temperature forecast for North America from the CFS reflects this jet stream pattern. You can see much warmer than normal Winter temperatures over the southern and eastern United States and eastern Canada. Neutral to colder temperatures are expected under the lower pressure and the jet stream to the northwestern United States and the upper Midwest.
Looking at the precipitation forecast below, we see much more precipitation over California and more precipitation in the eastern United States. But some of the CFS forecasts reflect a more medium-range development, overpowering some of the longer-term signals.
Also, looking at snowfall, most of the United States has below-average snow accumulation in the ECMWF forecast below. However, you can see more snow in the northwestern United States, upper Midwest, and southwestern Canada.
The temperature forecast for Europe also shows mostly warmer-than-normal conditions in the first three months of the year. But this is just a three-month average, so come colder days and events can still occur in this time.
The snowfall forecast for Europe also shows lower-than-normal amounts in this period. That is expected in such a warmer pattern.
NOAA OFFICIAL JANUARY-MARCH 2023 FORECAST
We can also track snowfall potential on normal temperature and precipitation Winter forecasts. The highest snowfall potential is usually in regions with colder temperatures and more precipitation.
This is can be seen in the NOAA’s latest official Winter 2022/2023 temperature forecast for the United States. It shows colder temperature probabilities for the northern parts of the United States. The country’s southern half has a higher probability of warmer than normal weather, especially in the east.
But take note of the trough of “equal” temperatures probability extending down low into the south-central states. That can be interpreted as a potential route of winter cold air outbreaks down from the Midwest to the south, creating snow events.
The official precipitation forecast is also quite similar to the latest model forecasts. We see an equal-to-higher probability for more precipitation (and snowfall) over the northwest. But generally, an area of more precipitation extends into the Great Lakes and the eastern United States.
The southern United States is forecast to have a drier winter, looking more like a typical La Nina season. The exception here is likely California, with the recent strong precipitation events due to the effects of atmospheric rivers.
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