The La Nina phase has reached its peak. Cold anomalies are now expected to start weakening in early 2022, but the weather influence will last well into the Spring season. A transition to a neutral phase is expected by mid-year, with an El Nino expected to finally emerge in Fall, ready for the cold season of 2022/23.
But what are these La Nina and El Nino events, and why do they change the seasonal weather across the entire world? As you will quickly find out, they are actually both just a part of a large old system that connects the ocean and the atmosphere called the ENSO.
We will then look at the latest changing conditions across the ENSO regions, and what the latest forecast shows for ENSO in the coming months. You will see just how strong of an influence these oceanic anomalies can force on the weather signals in the latest forecast for the remainder of the Winter Season in 2022 and into the next Spring.
ENSO GLOBAL WEATHER IMPORTANCE
Keeping it simple, ENSO is short for “El Niño Southern Oscillation”. This is a region of the equatorial Pacific ocean. It is always changing between warm and cold phases, with phase changes taking from 1-3 years.
ENSO has a major impact on the tropical convection patterns (storms) and the complex exchange between the ocean and the atmosphere. We can observe large-scale pressure changes in the tropics with each developing phase. After some delay, these changes affect the circulation over the rest of the world.
The image below shows the ENSO regions in the tropical Pacific. Regions 3 and 4 cover the east and west and together cover a large part of the tropical Pacific. Mainly all analysis and forecast focus on a combination of regions 3 and 4, seen on the image as the Nino 3.4 region. Region 3 covers the eastern tropical Pacific, while region 4 covers the central and western parts.
Each ENSO phase has a different effect on the pressure and weather in the tropics. This translates to the overall global circulation with time, influencing the weather worldwide differently. A specific phase (cold/warm) usually develops between late summer and fall and typically lasts till next summer. But some events can last even up to two years.
The cold ENSO phase is called La Nina and the warm phase is called El Nino. The name La Nina literally translates to “the girl”, and the name El Nino translates to “the boy”, indicating an opposite dynamics between the two phases.
We determine the ENSO phases by looking at the sea surface temperature anomalies (warmer/colder) in the ENSO 3.4 region, which you saw in the previous image above.
The image below from NOAA Climate shows the typical circulation during an enhanced warm ENSO phase (El Nino). Air descends in the western Pacific and over South America, causing stable and dry weather. At the same time, the air is rising in the central Pacific, causing frequent thunderstorms, low pressure, and a lot of rainfall.
This way, ENSO can have a major impact on the tropical rainfall and pressure patterns and impacts the ocean-atmosphere feedback system. Through this ocean-atmosphere system, the ENSO influence is then distributed globally. We usually observe a global shift in pressure patterns during the emergence of an ENSO phase but is more influential during the peak of the phase.
But can de ENSO shift between cold and warm phases just like that? There is no simple answer, but we can say that it is a result of the complex dynamics between pressure patterns and winds. The tropical trade winds usually initiate or stop a certain phase, by mixing the ocean surface layers and altering the ocean currents and thus temperature.
What are the trade winds? The trade winds are steady, and persistent winds, which blow towards (and along) the Equator in both Hemispheres. The image below from Weather.gov nicely shows a simplified schematic map of the global prevailing winds. Tropical trade winds are in yellow and red, covering both hemispheres.
We produced a map that shows the prevailing near-surface winds, based on the actual data in the past decades. We can nicely observe the easterly trade winds in the Atlantic Ocean and in the Pacific Ocean, which help to drive the ENSO region warming and cooling.
When these easterly winds get stronger, they can actually start changing the ocean surface currents, and they can start pushing the water from east to west. This moves warm surface waters towards the west, bringing deeper (colder) waters closer to the surface to replace it.
This process is much better seen on the video animation below, which shows the ocean temperature anomalies from Summer to late Fall. You can see new cooling starting in July, as the cold “waveforms” develop across the equatorial Pacific. They form as the surface water is being pushed west by the trade winds, bringing replaced by deeper colder water.
The image below shows the latest ocean surface currents and temperature in the ENSO regions. We can still see strong easterly currents, which are pushing the water towards the west, cooling the ocean surface. The easterly regions are usually always colder than the western due to stronger trade winds. This time last year (top of the image), the cooling and trade winds were stronger in the central and western regions, while the eastern regions are cooler this year.
But the secret is not just in the winds themselves, as they are typically being driven by pressure changes. The ENSO phase directly responds to a unique atmospheric pressure variability, called the Southern Oscillation Index.
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 that are important for ENSO.
Positive SOI values mean that 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 and over Tahiti, and higher over Darwin, Australia. This produces a negative SOI value and weaker trade winds, which means less ocean cooling.
The SOI tells us what the pressure pattern looks like in the tropical Pacific, which then affects the trade winds, warming or cooling the ENSO regions.
Below we have the 30-day average SOI index, which nicely the positive values and confirms the presence and further development of the cold ocean phase. The SOI can be used to determine the potential effect or direct influence of the ENSO phase on the climate in the western Pacific and Australia.
We can see on the bars, that the daily SOI has changed from positive to negative in the past few days. That could be something, nothing, or everything. Changes in the SOI typically precede the changes seen later in the ocean, so every change is worthy of monitoring. But for forecasting, we typically use multi-day data, like 15 days or 30 days or even 90 days averaged SOI.
LA NINA – LATEST DEVELOPMENT
Current global ocean analysis reveals the strong area of cold ocean anomalies in the tropical Pacific. This is exactly across the ENSO regions, seen earlier above. The “wave-like” shapes of the cold anomalies indicate strong trade winds, pushing the surface waters from east to west, creating these ocean surface swirls.
Focusing on region 3.4, you can see on the image below how the ocean cooling began in Summer and Fall this year. That was the development of the current cold phase. The forecast part of the graph does show the cooling maxing out in early winter and starting to weaken in early 2022.
An interesting fact in the image above is that the current cold phase was following the previous La Nina that weakened this Spring. Such an event is called a second-year La Nina, where we have two cold phases following each other.
But it is not unusual to have two La Nina events back-to-back. In fact, of the twelve first-year cold events, eight were followed by a second cold phase the next winter, two by a neutral phase, and two by El Nino.
The current cooling has been quite substantial and persistent from the beginning of the meteorological fall season. In the image below, we have the sea surface temperatures in the main ENSO 3.4 region. We can see a steady temperature drop since September, reaching a bottom in late October and now in mid-December.
But we have to remove the normal temperatures, to see the actual cooling, and the cold anomalies. The image below shows the temperature anomaly in the ENSO 3.4 region and reveals a stronger deviation from the norm since mid-September, due to stronger trade winds. Peak anomalies were reached in late October, with another drop-off in December.
Below we have a close-up image of the ENSO regions. You can nicely see the developing cold “waveforms”, as the pressure patterns are creating strong trade winds. They drive the wind-driven ocean surface cooling. Peak cold anomalies are now focused more towards the eastern regions.
If we look at the actual current temperatures, they reveal a “cold tongue” extending from South America, towards the west, directly across the equator. In this region, the surface waters are always colder than the surrounding regions, but during a La Nina event, they can be up to 4°C colder than normal. We can see the typical wind-driven surface cooling pattern.
Looking at the latest high-resolution analysis by depth under the ENSO regions, we still see colder than normal temperatures below the surface. A cold anomaly pool is sitting 100-250 meters below the surface, with colder anomalies closer to the surface in the eastern regions. This shows that the upwelling is strong there, due to strong trade winds.
One way to look at the whole ENSO region temperature strength is by looking at the ocean heat content. This also takes the water at depth into consideration, not just the surface temperatures.
And below we can see the La Nina of last year weakening in Spring. But the cold anomalies have returned and also peaked in mid-October. Lately, the subsurface cold anomalies have weakened, indicating that the La Nina is likely at (or past) its peak.
Based on all the available data, an official La Nina watch is still in effect, released by the NOAA’s Climate Prediction Center:
“La Niña is favored to continue through the Northern Hemisphere winter 2021-22 (~95% chance) and transition to ENSO-neutral during the spring 2022 (~60% chance during April-June). La Niña is anticipated to affect temperature and precipitation across the United States during the upcoming months.”
Tracking the weather effects of these ENSO phases is not all that easy. It is not as simple to just point at a certain weather event and simply call it “ENSO influenced”. Global weather is a very delicate and complicated system, where everything can have an influence on everything. But of course at quite different temporal and spatial scales.
For example, it is quite simple to find evidence for direct La Nina or El Nino-related weather patterns over the North Pacific or the United States in winter or spring.
But over Europe, the direct La Nina influence slowly fades, due to the generic weather pattern variability. The influence is still there, but it is typically highly mitigated/modified by the existing weather patterns in the North Atlantic.
ENSO SEASONAL FORECAST 2022
You now know what ENSO and its warm and cold phases are. So we are going to focus on its evolution through the current winter season and see what the most recent forecasts show for 2022.
Below we have the ocean temperature forecast for the 2022 part of the current winter season, from combined multiple global seasonal models. It shows a fully developed La Nina across the equatorial Pacific Ocean. At this point, it is already past its peak cooling, with the cold anomalies expected to reduce further into spring. The weather effects, however, can last a while longer.
The ENSO analysis and ensemble forecast below from ECMWF shows that La Nina began to quickly develop during late Fall, reaching peak cooling in early Winter. The forecast does show a quick return to neutral conditions in Spring, with a reduction of negative anomalies. If it continues, this could lead to a new warm phase (El Nino) in late 2022.
The ENSO forecast from the United States CFSv2 model shows the current peak cooling in this month. But it also shows a forecast for a slower return to neutral conditions over Spring, compared to the ECMWF. Going into Summer 2022, there is an increasing chance of a warm phase (El Nino) starting to develop.
The CPC/IRI official probabilistic ENSO forecast shows the current La Nina lasting over the winter season into Spring 2022. What is interesting is the transition into summer, where we do see the chances of an El Nino increasing, just like on the model forecasts above. It is typical for a new phase to emerge in the summer/fall season, so seeing increasing El Nino chances during the Summer next year, is the expected first sign of a change.
The image below is a consolidated analysis and forecast image from multiple North American seasonal models. It also shows the La Nina reaching the coldest phase this early winter season. But going ahead into 2022, we see a return to the neutral phase (between 0.5 and -0.5), with an increased chance of an El Nino developing later in 2022.
It also shows neutral conditions for next Summer. But it is unlikely to have a third-year La Nina, so chances are better for an El Nino emerging, compared to having another cold event.
The most recent ECMWF extended seasonal forecast actually shows the developing warm phase (El Nino), starting around Summer 2022. ECMWF releases these extended seasonal forecasts every few months. The next extended forecast cycle is coming in February 2022, which will give a much better picture of what to expect for the next Fall/Winter season.
We now know that an El Nino is likely coming in 2022 and especially during the next winter season. So it’s time we look at how an El Nino actually differs from a La Nina both in the Ocean and in the Atmosphere.
LA NINA OR ELNINO
To fully understand the ENSO, we must understand both its phases. The forecasts are hinting at the emergence of an El Nino in 2022. This is why we will look at the main difference and the typical weather influence of the El Nino versus a cold La Nina phase that we are currently in.
On the image below we have a direct comparison between the ocean temperature anomalies during an El Nino and a La Nina. Both examples show realistic data, from the 10 strongest events for each phase.
Typically, an El Nino develops stronger anomalies, which are focused more on the eastern regions. But while La Nina has weaker average anomalies, they typically peak more towards the central ENSO regions.
Below we have two real analyses of ocean temperature anomalies. First is the strong La Nina from last year, and the second is the strong El Nino event during the 2015/2016 winter season.
We can see a completely opposite picture and two different patterns over the same area of the ocean. Cooler than normal ocean temperatures during a La Nina, and warmer than average temperatures during an El Nino. As we will see later, their influence on the weather is also almost very much the opposite.
Below is the El Nino event during the winter of 2015/2016.
But besides the temperatures, one of the main differences can of course be found in the tropical pressure patterns. During an El Nino, the pressure over the tropical Pacific is lower, with more rainfall and thunderstorms.
But during a La Nina, the pressure over the equatorial Pacific is high, creating stable conditions and less precipitation. This obviously translates into the global circulation over time, affecting the weather seasons on both Hemispheres.
Looking at the actual temperature instead of the anomalies can also help to see a big difference between the two phases. The first image below shows the sea surface temperatures during a La Nina. The second image shows the ocean surface temperatures during the strong 2015/2016 El Nino event.
Obvious again is the “cold stream” of cooler ocean waters during a cold phase, which is driven by the stronger easterly trade winds. The ocean temperature difference between the La Nina and El Nino can be quite large, up to 10 degrees Celsius in some places.
You can briefly see a very weak “cold stream” also in the warm phase below, but it is very suppressed and hardly noticeable. This cold stream is actually normal due to persistent easterly trade winds. But it is much stronger during a La Nina and much weaker during an El Nino, due to pressure changes affecting the intensity of these easterly winds.
Strong trade winds are also important because they affect the food chain across the tropical Pacific regions and on both American continents. Strong trade winds mean that deeper and cooler water rises up to the surface, replacing the water that was “pushed away” by the winds.
This process is known as “upwelling.” Essentially this is a vertical transport process, as shown in the image below, which helps to bring cooler waters and nutrients to the surface.
Water that rises to the surface as a result of upwelling is typically colder and is quite rich in nutrients. These nutrients can “fertilize” the surface waters. This means that these surface waters often have high biological productivity.
We can see this upwelling effect directly when comparing the temperature by depth during a La Nina and an El Nino event. The first image below shows the water temperature by depth during the 2015/2016 El Nino event. We can see much warmer waters in the upper-oceanic layers, and cooler waters staying deeper down, as they cannot rise on their own.
But the subsurface analysis of the current La Nina event shows a much different picture. You can nicely see how the 20°C threshold is much closer to the surface in a La Nina event. This is due to the upwelling effect, as the trade winds cause the deeper colder water to rise closer to the surface.
The layer between the warmer and colder waters is called the “thermocline”. This is a thin layer in the ocean, where the temperatures start to drop significantly. It basically separates the upper warm and deeper cold layers.
During an El Nino, seen in the image below, the thermocline layer is pushed deeper down. That is due to the warmer waters from the west entering the region, warming the upper-ocean layers.
DIFFERENT WEATHER OF THE ENSO PHASES
Here we have to add a very important “disclaimer”, that an ENSO phase is not just a pure “influencer” for our weather. It can also be viewed and interpreted as one of the responses to the overall global state of the atmosphere and ocean circulation.
Just like a warning light on the dashboard of a car, the ocean anomalies can tell us what is going on with the global circulation above and below the surface. But generally, ENSO does exhibit an important influence on the Jet stream in the Pacific Ocean, and consequently also downstream and upstream around the world.
The jet stream is a large and powerful stream of air (wind) at around 8-11km (5-7mi) altitude. It flows west-to-east around the entire Northern hemisphere, affecting the pressure patterns and their evolution, directly shaping our weather at the surface.
Below is an example of the jet stream in winter at the 300mb level (9km/5.6mi altitude). In this example, the jet stream is curved quite far down over the central United States, which brings colder air down from the north/northwest. It is curved upwards over northwestern Europe. Such formation brings colder weather to the central United States and stormy weather to the UK and Ireland.
Comparing the two phases of the ENSO on the next image below, we can see that they typically produce an entirely different jet stream pattern across North America. We will focus on this region as here, the ENSO has a major/direct influence on the position of the jet stream over the continent.
La Nina features a stronger and extended polar jet stream from Alaska and western Canada down into the United States. But the El Nino features a stronger Pacific jet stream and an amplified storm track over the southern United States.
LA NINA WEATHER PATTERN
Looking closer at the La Nina weather signature below, we can see that its main feature is a strong and persistent high-pressure system in the North Pacific. That usually bends the jet stream from the northwest down to the southeast, creating a colder north/warmer south weather pattern over the United States.
Alaska, western Canada, and the northern United States typically get a colder than normal winter, with more precipitation. Southwestern and the southern United States usually experience warmer and a bit drier conditions during a La Nina winter season.
Changing the jet stream also means changing the snowfall potential over the continent. The colder air is more easily accessible to the northern United States, which also means that it has an increased snowfall potential during a La Nina winter. Especially areas like Alaska, western Canada, and the northern United States benefit from the jet stream above to produce weather patterns with more snowfall. The graphic below is by NOAA-Climate.
EL NINO WEATHER PATTERN
On the other hand, during the El Nino winter season, we 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 flow 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.
Looking at the average snowfall for an El Nino winter, we can see a much different picture than with the La Nina. There is less snowfall in the northern United States during the El Nino seasons. The conditions are mostly warmer than normal and drier than normal in the north. But there is more snowfall in the west-central United States and in the parts of the eastern United States, with cooler weather and more moisture.
After passing Canada and the United States, the jet stream moves into the North Atlantic, where it can take different paths towards Europe. A lot depends on the overall circulation pattern and the existing pressure systems in the North Atlantic. This is where ENSO generally perhaps loses its direct influence over Europe, as regional systems in the Atlantic interfere.
But it usually still has an important influence, as it changes the position of the entering jet stream from the west. The incoming jet stream can merge with the systems in the Atlantic, helping to create a whole new weather pattern for Europe.
Typically, the main problem is that the final outcome is far more unpredictable in this zone than over North America, which feels a much more direct and predictable influence.
ENSO – UNITED STATES TORNADO SEASON INFLUENCE
As history shows, La Nina can have an important influence on the Spring tornado season in the United States. And usually not in a good way.
Going straight to the point, we have a very interesting image below from NOAA Climate. It shows a very nice comparison of hailstorms and tornado events during the spring season in the United States, compared between El Nino and La Nina years.
It is quite interesting to see, that in a La Nina spring season, there is a substantially higher frequency of hailstorms and especially tornadoes in the southern and southeastern parts of the United States. It nicely shows the main outlines of the central and eastern parts of the Tornado Alley.
Tornado Alley is a nickname given to an area in the southern plains of the central United States, that consistently experiences a high frequency of tornadoes and other severe weather events each year from late winter to late spring.
But why are there more tornadoes and other severe weather in the southern United States during a La Nina?
It mainly has to do with the weather patterns we have seen earlier above, forced by a La Nina event. As La Nina promotes a high-pressure system in the North Pacific, that usually corresponds to the pressure drop over western Canada and the northwestern United States.
This can act as a source region for frequent cold fronts, which move from western Canada down towards the south-central United States, where they meet warm moist air coming up from the Gulf of Mexico. Together with the shifted jet stream over the United States, this usually produces a very volatile combination with a lot of available energy for the storms to become severe and tornadic.
Below is a graph that shows annual tornado numbers in the United States from 1954 to 2014, which is a very good sample size. Looking over the years, we can see that from the top 5 most active tornado years, 4 were La Nina seasons. At least during the spring, where the ENSO still has its presence in the global circulation, coming out from the winter season.
LA NINA 2022 SEASONAL WEATHER INFLUENCE
We have seen that the main feature of La Nina winters is a strong high-pressure system in the North Pacific. Below we have an image that shows the average winter pressure pattern from multiple La Nina winters. The main feature is of course the high-pressure system in the North Pacific and low pressure over Canada.
That is the typical signature of the cold ENSO phase. Over Europe, we see lower pressure over Scandinavia and higher pressure over northwestern Europe, but that cannot be designated as a typical weather response from the La Nina.
The main takeaway is the strong high-pressure system in the North Pacific and lower pressure over western Canada and the northwestern United States. That sends the polar jet stream down over the northern United States. We have seen that on the previous images above, which is the typical La Nina weather response if there are no other strong external factors.
We will quickly extend our look into the seasonal trends for early 2022, using the ECMWF forecast. The period in question is the January-February-March season.
That means that the first Spring month is also included in the forecast. But since the winter months have typically more powerful patterns, the signal from March typically has a weaker contribution to the average.
At this point, we have to add an important piece of information. Seasonal forecasts like the one below, typically show a good background forcing and large-scale dynamics. But once in the actual weather season, we always have a lot of sub-seasonal variabilities, from different sources.
Below on the image from a climate study, you can see different weather phenomena, and how they rank on size and temporal scales. Large-scale factors like the trade winds and ENSO and climate variation can be nicely accounted for in the seasonal forecasts. But as we go to smaller sizes and shorter time scales, there is influence from frontal systems hurricanes, thunderstorms, etc,…
These smaller scales are harder to compensate for in the seasonal models, so they are typically the main source (if strong enough) for the difference between the long-range forecast and the final actual weather.
But, the long-range models do also include these processes to some degree of predictability, as they do simulate the entire global weather system. The problem is just that the smaller the size of the weather phenomena, the harder it is to forecast it on a seasonal scale.
We have a very nice example this month, as for example, Europe is generally experiencing a colder and snowier first month of winter than expected. Over North America, the temperature pattern was predicted well for December, as the influence of the ENSO is stronger here, exerting its influence.
EARLY 2022 SEASONAL FORECAST
Now going to the latest forecast data, it was issued earlier this month by EMCWF. As we can see below on the pressure pattern forecast, the strong high-pressure system in the North Pacific is still present. That is the fingerprint of the La Nina and is likely to stay present well into the early Spring season. Notice the much lower pressure over most of Canada, curving the jet stream into the northwestern United States.
Lower pressure extends into Greenland, keeping the jet stream more the north, and allowing a high-pressure area to expand over much of Europe.
Looking at the global airmass temperatures, we see the strong cold pool in western Canada. That is forming this month and is set to stay till the end of the winter season. You can notice the warmer temperatures over much of Europe, which would suggest a pattern change to a more westerly flow in early 2022.
Taking a closer look at Europe, the surface temperatures are mostly above normal over the north and northeast. There is however a neutral zone in parts of central Europe, suggesting that colder air intrusions can continue in January and February.
That is similar to what we have seen now in December, but with a reduced frequency. These forecasts only show the prevailing or average picture over the course of 3 months, which can contain a lot of sub-seasonal variabilities.
Over North America, we see the large cold pool over western Canada and Alaska. That comes from the north flow around the strong blocking high over the North Pacific. In this pattern, cold air can then quickly spread into the midwest and the central United States, as we have seen this month already, despite warmer than normal conditions being forecast in the average.
Looking quickly at the global precipitation forecast, we see mostly drier than normal conditions over Europe, under a high-pressure system. Over North America, more precipitation is forecast over Canada, which is mainly snowfall. More precipitation is also forecast over the northwestern and northeastern parts of the United States. The southern United States is forecast to have normal to drier conditions, typical for a La Nina season.
Below we have the official temperature forecast for the United States, released by NOAA, for the early 2022 season. It shows the temperature probability, with colder to equal chances in the northern United States. The southern half of the country and the northeast have a higher probability of warmer than normal weather, which we saw in the long-range forecast above.
Such a pattern however still allows for strong cold air outbreaks into the Midwest and the northeast. A strong cold pool will be present in western Canada, which can be a strong source of colder air. A cold air outbreak can drop into the United States as soon as the pressure pattern swing allows it.
The official precipitation forecast also supports the models above. We see an equal-to-higher probability for more precipitation (and snowfall) in the northwestern and the northeastern United States. The Southern United States is forecast to have a drier than normal start of 2022, thanks to the active cold ENSO phase.
Both the actual weather and the forecast show a clear sign that La Nina is having a strong presence in the atmosphere. It will exert its influence on the winter season in the United States and the entire Northern Hemisphere, continuing also into early Spring. The confidence is growing for a very different winter season between the northern and the southern United States.
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Source: Severe Weather