La Nina has escaped its collapse in the tropical Pacific Ocean. Cold anomalies are stable and extend their influence into the Summer season. The tropics are showing signs of a possible La Nina influence in the cold season 2022/2023.
What are these oceanic phenomena, and how can they affect the global seasonal weather? They are both part of an old system that connects ocean and atmosphere, called ENSO.
We will examine the changing conditions in these ocean areas and the forecast for the upcoming months. These oceanic anomalies can have a significant impact on weather patterns and change the course of weather globally.
ENSO GLOBAL WATHER IMPORTANCE
ENSO is short for “El Niño Southern Oscillation”. This is a part of the equatorial Pacific, which changes between warm and cool phases. The phase change occurs approximately every 1-3 years.
ENSO has a major impact on tropical rainfall patterns (storms), as well as the complex exchange between the atmosphere and the ocean. Each new phase of the tropics brings with it large-scale pressure changes. These changes can affect circulation throughout the rest of the globe after a delay.
Below is an image of the ENSO regions in tropical Pacific. The tropical Pacific’s east and west regions are covered by Regions 3 & 4. Together they cover a large area of the tropical Pacific. The main area is a combination from regions 3 and 4. See the image as the Nino region 3.4.
Each ENSO phase has a different impact on the weather and pressures in the tropics. This has a time-dependent effect on the global circulation, changing weather patterns around the world.
Each phase (cold/warm), usually begins in late summer or early fall and lasts until the next summer. Some events can last up to two years.
The cold ENSO phase is also known. La Nina The warm phase is also known as El Nino. The main difference between the phases is not only in their temperatures but also in the pressure patterns that they are associated with.
During El Nino, the pressure above the tropical Pacific is lower. This results in more rain and storms.
However, during a La Nina, pressure over the equatorial Pacific rises, creating stable conditions and less convection. These pressure changes affect both Hemispheres as they are affected by the global circulation.
El Nino typically develops stronger anomalies and is more focused on the central and east regions. La Nina, on the other hand, has weaker anomalies and tends to peak closer to the central region than the western.
Below is an image from NOAA Climate that shows the typical circulation in a cold ENSO phase (La Nina). Air descends into the eastern Pacific, creating stable and dry conditions. The air is rising in western Pacific at the same time, causing frequent thunderstorms and low pressure.
ENSO can thus have a significant impact on the tropical rain and pressure patterns, and therefore the ocean-atmosphere system feedback system. The ENSO influence is distributed worldwide through this ocean-atmosphere network.
Usually, we observe a shift in pressure patterns when ENSO phases emerge. However, it is more prominent during its peak.
But why does ENSO switch between cold and warmer phases? It is difficult to pinpoint the exact cause, but we can tell that ENSO shifts between cold and warm phases due to the complex relationship between pressure patterns as well as winds.
The tropicalTrade windsBy mixing the ocean surface layers, and changing the ocean currents or temperature, a phase can usually be initiated or ended.
What are the trade wind? The trade winds are steady and persistent winds that blow towards (and along the Equator) in both Hemispheres. The image below is from Weather.gov and shows a simplified map with the global prevailing wind patterns, with tropical trade winds shown in yellow and red.
Based on data from the past four decades, we created a map showing the prevailing near-surface wind patterns. We can see the easterly trade winds in the Atlantic Ocean and in the Pacific Ocean, which help to drive the ENSO region’s warming and cooling.
These easterly winds can cause ocean surface currents to shift to the east, pushing water from the east to the west. This causes warmer surface waters to move towards the west, but also brings deeper (colder), waters up to the surface to replace them.
This animation shows the ocean temperature anomalies between Summer and late Fall 2021.
ENSO cooling begins in July, when cold waves are developing across the equatorial Pacifi. They form when the surface water is being pushed westward by the trade winds and replaced by deeper, colder water.
The image below shows the latest anomalies in trade winds in the tropical regions. We can clearly see stronger than usual easterly trading winds (negative). Strong trade winds prevail across the tropical Pacific and the central ENSO regions.
The key lies not in the winds, as they are often driven by pressure changes. The ENSO phase responds directly to atmospheric pressure changes, known as the Southern Oscillation Index.
The Southern Oscillation Index (or SOI) is the difference between air pressure at Tahiti (French Polynesia), and Darwin (Australia). The image below shows where the pressure zones are located that are important to ENSO.
Positive SOI values indicate that the pressure is higher over Tahiti than over Darwin, Australia. This is due to stronger easterly winds that support La Nina conditions.
An El Nino causes lower pressure over the eastern Pacific and Tahiti, as well as higher pressure over Darwin, Australia, during which time we see less pressure. This results in a lower SOI value and weaker trading winds, which means less oceanic cooling.
The SOI analysis below shows that there are still positive values. This further supports stronger trade wind and ocean cooling within the ENSO regions, sustaining the La Nina towards the Summer.
LA NINA TO STAY – LATEST DATA
Current global ocean analysis shows the presence of cold anomalies in the tropical Pacific. This is in the ENSO regions as well, as mentioned earlier. The La Nina has survived thanks to the strong trade winds and the absence of a westerly blow burst. It will continue to grow into 2022.
Below, we will be focusing on region 3.4. You can see the first La Nina of the 20/21 seasons in the image. The last year’s Fall and Summer saw new ocean cooling. This was the genesis of the current cold stage. Peak cooling was less than the first La Nina. The second-year event is typically weaker than that of the first.
A January change in the tropical winds has slowed down the cooling process and started the La Nina event’s collapse.
This process can be seen on the graph below. It shows temperature anomalies in ENSO 3. Since late January, there has been a steady increase in temperature. However, stronger trade winds and a dearth of westerlies have prompted a cooling trend since late March.
Below is the ocean analysis starting in February. The eastern tropical Pacific region had stronger cold anomalies. The cold anomalies were weakening in the central and western ENSO areas.
However, the latest anomaly analysis of ENSO regions shows that cold anomalies are returning to the central and western tropical Pacific. Peak cold anomalies now focus more on the eastern regions because of stronger upwelling effects and re-intensification of the trade winds.
Below is a 15-day ocean temperature anomaly variation. The cooling effect is evident in the ENSO areas east. We also have more cooling than heating in the central region. This was quite unexpected considering the forecasts only a few weeks or months ago.
But, perhaps there is a stronger (invisible), process below the ocean’s surface. The image below shows the early February temperature anomalies at depth across the tropical Pacific Ocean. A warm Kelvin Wave was seen below the ocean’s surface, which eliminated the La Nina.
The Kelvin Wave was visible at 80-200m depth. It was pushed in the west by the currents. It caused the breakdown and weakening of cold anomalies in central and western ENSO areas.
However, the ENSO regions have shown us that temperatures below the surface are returning to normal. This was possible due to the weakening and strengthening of the easterly Trade Winds.
The ocean heat content can be used to determine the ENSO region’s temperature strength. This includes the water temperatures at depth as well as the surface.
Below you can see the cold anomalies that developed in late Summer and peak in mid-October. The Kelvin Wave below the surface has caused subsurface cold anomalies to weaken intermittently. As you can see in the images of depth, a new cold wave has begun to take over.
The next image below demonstrates the whole process. It shows the high-level heat anomalies in the tropical Pacific regions. The colling began in late last summer and continued into mid-winter.
In January, the warm Kelvin Wave can be seen pushing in. It is being replaced now by a new, colder wave below the surface. This is due to trade winds and the upwelling process.
We know what La Nina is at the moment and how it got here. So it’s time we look at how it is expected to evolve further into the year, and how it will impact our seasonal weather.
ENSO LONG RANGE FORECAST
Now you are familiar with ENSO and its phases. So, we will focus on its evolution during the current season and look at the most recent forecasts for 2022.
Below you will find the ECMWF ocean temperature forecast for summer 2022. It now shows a continued cold anomaly in the equatorial Pacific Ocean. While some cold anomalies can also be seen to the west and south of the Equator, this is still a compact cold event.
The ENSO analysis from ECMWF and the ensemble forecast below show that La Nina continued to develop last Fall. The forecast does show a prolonged cold phase through the Summer. Fall is more uncertain, but there are more members who prefer the negative phase.
Similar ENSO forecasts from the United States CFSv2 are available. It shows that the current cooling will continue well into 2022. It continues to sustain the cold anomalies throughout Summer and into the 2022/2023 cold seasons. This model was also the first to hint at a possible 3-year cold episode.
According to the official CPC/IRI probabilistic ENSO forecast, the current La Nina will last through the summer and into 2022’s early cold season. It is not unusual for a new phase of the La Nina to emerge in late fall/early winter due to seasonal pressure changes. It appears that the La Nina will likely continue into next Winter.
Below is a forecast image and analysis from several North American seasonal models. It also shows that the La Nina is very much sustaining through the summer season. Looking deeper into the year, we can see the 3rd La Nina event in Fall and Winter 2022/2023.
Trends favor a cold phase, La Nina, which will be sustaining or restrengthening during the second half of 2022. This would create a unique set of weather patterns for the Fall and Winter seasons 2022/2023.
What is the impact of the La Nina event on seasonal weather? And what can we expect for the warm season this year?
LA NINA WEATHER YEARS
Based on all data available, an official La Nina advisory, as explained by the NOAA’s Climate Prediction Center:
“La Niña is favored to continue through the Northern Hemisphere summer (59% chance during June-August 2022), with a 50-55% chance through the Fall. This month, the forecaster consensus predicts Niño-3.4 index values to weaken into the summer but remain below the threshold of La Niña.”
Below is an image that shows the average winter temperature pattern over multiple La Nina seasons. The main feature of the image is the strong high-pressure system over North Pacific and low pressure above Canada.
If we look closely at the La Nina cold-season weather signature below, you can see its main feature: a persistent high pressure system in the North Pacific. That usually shifts the jet stream from the northwestern United States down into the east, creating a “colder north/warmer south” weather pattern over the United States.
Alaska, western Canada, the northern United States, and Alaska typically experience a colder winter with more precipitation than normal. The La Nina winter season brings warmer and drier conditions to the Southern United States and the Southwest.
What can we expect from La Nina’s influence during the warm seasons?
The image below shows the correlation between the summer jet stream and the cold ENSO phase. The jet stream is stronger over the northwestern United States and the subtropical jet stream is weaker over the south in a La Nina Summer.
The North Pacific blockage high-pressure system has been the most common effect of cold ENSO phases in the past. As you can see, the North Pacific high pressure tendency during a La Nina is also present during summer. The high-pressure pattern extends west/central United States from the North Pacific.
However, we can also see stronger high-pressure signals over the northeastern United States. These signal are centered towards the east into North Atlantic.
Because of the strong La Nina influence in the warm season, we are focusing our attention on the Pacific/North American region. Contrary to the cold season, the direct weather pattern effect on European sectors is much smaller than in the warm season.
Below is a special graphic that illustrates the spring temperature impact of a La Nina Phase for the United States. The pattern of cold north/warm south continues into the spring season, as can be seen.
The spring precipitation pattern is similar to that for a cold ENSO. We get more precipitation from the north, northwest, as well as the eastern United States. Conditions are more dry in the far south and southwestern United States.
Below are the Summer temperature anomalies and precipitation anomalies in the United States for Summers following a Spring La Nina.
The western half of the country can see temperatures that are higher than usual. Notable is the fact that the signal for the southeastern United States is colder than normal.
Precipitation-wise we have a drier signal to a La Nina Summer in the northern, central, and western United States. More precipitation is expected in the Ohio Valley and northeastern United States.
These patterns are based upon historical data from similar events in the past. We will now examine the actual long-range forecast and see what model calculations reveal about the La Nina effect.
LA NINA 2022: SEASONAL WEATHER INSLUENCE
We will examine the ECMWF’s forecast for Summer 2022 and the seasonal trends. This is the period of the meteorological Summer Season, which covers the June-July–August period. It is also the peak of the warm seasons.
The majority of the time, we use the ECMWFIt is often called the best-reliable model in the long-range category. However, much can change depending on the season or year. But generally, the ECMWF model is at the top as far as “skill” goes.
Below is the forecast pressure pattern from ECMWF. We can see that there is a La Nina high pressure system in the North Pacific. It extends to the west/northern United States.
As we can see in the La Nina signal graph earlier, the northeastern United States has a stronger high-pressure region. This will have an impact on the weather in the eastern United States of America and eastern Canada.
Another high-pressure region is located over western Europe. It contrasts with the low-pressure area in northern Europe.
The La Nina pattern can also be seen in global temperature distribution. Warm pooling is seen over North America. This is due to the warmer airmass caused by the high pressure anomaly. Warm anomalies also extend across large parts of southern and eastern Canada.
Europe is characterized by warmer than average conditions in the south-central half. Northern Europe is neutral, though it is likely to be a cooler low-pressure region.
Europe looks much more like normal. We see much warmer than average weather across most of the continent. Northern Europe is the exception, and it will be subject to a low pressure system.
The warm anomalies can be seen over North America. However, the southeastern United States does have a neutral area similar to the La Nina summer pattern that we saw earlier.
Also, warmer anomalies are expected to occur in the northeastern United States and central Canada. This region is currently under the influence from the high-pressure system.
Under the low-pressure zone, northern Europe will experience normal to wetter conditions. However, the rest of Europe is likely to experience a drought scenario as it is likely to be much dryer than normal.
The precipitation forecast for North America shows dry conditions, with the exceptions of the central and northern United States. However, parts of the southeastern and southwest United States and eastern Canada have a higher likelihood of experiencing wetter conditions.
This is in line to the Summer La Nina influence, which we have already seen. The east and the southwest United States can have more rain. The northern and central regions are the drier zones.
In this long-range outlook, the south-central United States is likely to experience a hot and dry summer. More storms are forecast for the east and southwest, with higher temperatures and normal to high levels of precipitation.
The official NOAA Summer temperature outlook shows that the majority of the United States is warmer then normal. The core warm anomalies so far are centered on the western half the United States.
The official Summer precipitation forecast is very similar to the model forecast. We see an equal to or higher chance of more precipitation in the east and southwestern United States. However, most of the central and northwestern United States are expected to experience a dry summer season.
The main problem with rain in any La Nina season is the persistence of drought conditions throughout the southern and western United States. Below is the latest drought analysis by NOAA. It shows the current drought conditions in the country.
The majority of the United States’ western half is in drought. The southern and northern United States have the driest conditions. A dry and hot summer, as forecasted for the south-central states and the northwestern United States, will worsen drought conditions.
ATLANTIC HURRICANE SEASON
La Nina cannot be compared to the Atlantic hurricane seasons. Because of the differences in atmospheric conditions, there is a well-known influence from the La Nina on hurricane season.
As the vertical wind shear in the Atlantic is lower and the atmosphere more unstable, there is a tendency for more hurricanes to form in the Atlantic. Due to stronger wind shear, there are less hurricanes in the eastern Pacific.
The ENSO region is showing active cold anomalies, according to the latest ocean temperature forecast for Hurricane season. We see fairly neutral temperatures in the tropical Atlantic Ocean. If there are no warm anomalies in the region, it could indicate that a hyper-active period is less likely.
However, the actual ECMWF forecasts for Accumulated Cyclone Energy are above-normal. This could mean a higher number or a few major hurricanes which can quickly boost ACE.
Accumulated cyclone energie (ACE), is a general metric that represents the energy of a tropical hurricane during its lifetime. To compare different seasons, we can add the total energy from all systems. This does not apply to landfalls.
A hurricane season with low total ACE may see 2-3 hurricane landfalls in the United States. A high-ACE season may have more storms that mainly remain in open waters.
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Summer 2022 extensive long-range forecast
Source: Severe Weather