Teleconnections - Interactions and Impact

[knocker]

Early data shows a Weaker Polar Vortex could develop for Winter 2024/2025

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Early data shows that the Polar Vortex is likely to be weaker than normal in the Winter of 2024/2025. A weaker Polar Vortex usually means a weaker jet stream, which increases the chance of a more dynamic (colder/snowy) Winter pattern over the United States, Canada, and/or Europe.

We can already see how some of the large-scale global factors are developing. Knowing how they change seasonally and how they shape our weather, based on past data, can give us important insight into the future.

Based on the overall state of the major global factors, we can see how they are projected to evolve and what this means for the overall large-scale winter circulation. We will look at each of these factors, but first, we will quickly look at why the Polar Vortex is so important in this story.

https://www.severe-weather.eu/long-range-2/early-data-weak-polar-vortex-usa-winter-2024-2025-fa/?fbclid=IwZXh0bgNhZW0CMTEAAR3iFaAS_I_nxwcL0GBfLK4yzhLnRYbQh2dtNp3xUHdwAXtnqwNaprabIIs_aem_JkAFLkWRIvxIx980gCbeLg

[Met4Cast]

It’s the time of year where we’re all starting to wonder “what will winter hold” so given teleconnections will become ever more relevant in the months ahead, I’ve started a “let’s learn” series on Twitter discussing the various teleconnections & impacts in a (hopefully) simplified way for those who want to learn & apply them to forecasting. I thought i’d copy them here; 

Lets learn No. 1: What is the Madden-Julian Oscillation (MJO)?

You'll likely see me refer to MJO a lot in the coming months, it's a very important intra-seasonal driver of global weather patterns and can often give a clue to the longer-term forecast. 

In essence & without over-complicating things, the MJO is an area of enhanced convection that moves eastwards along the equator in a cyclical cycle of about 30-60 days.

This area of enhanced convection begins in the Indian Ocean and moves eastwards through the Maritime continent, into the west Pacific and onwards through the central Pacific, east Pacific and then into the western tropical Atlantic before "resetting" back to the Indian Ocean.

One way to track the MJO is using space-phase diagrams. The further away from the centre of the diagram, the stronger or more amplified the convection. The stronger the convection, the more impact the cycle can have on global patterns. 

When the MJO resides inside the centre circle, this essentially suggests there is no signal, i.e there is no discernible convection, whilst not always true as these plots can be deceiving, I wont go into the complexities of why here. 

Each phase of the MJO corresponds to the location of that enhanced area of convection. Phase 2-3 are the Indian Ocean, 4-5 the Maritime continent, 6-7 the western Pacific and 8-1 the tropical Atlantic/Africa. 

The location is important as this determines where the MJO begins to influence the jet stream, this has downstream impacts on weather patterns in terms of positioning of high/low pressure patterns.

The MJO has far reaching impacts, it can increase or decrease tropical cyclone/hurricane activity and plays a very important role in the Australian monsoon season.

The MJO cannot be used in isolation and it's not quite as simple as saying "Phase X = X type of weather", but it's one of the most important drivers of weather patterns.

For those looking for colder weather, a general rule of thumb is that we want to see an amplified signal move through the Maritime continent (phase 5-6), into the western Pacific (6-7) and into phase 8-1, this more broadly speaking promotes blocking patterns in the Atlantic/over Greenland during the winter months. 

This isn't the entire story and the progression east of the MJO triggers other events which impact weather patterns too and these also need to be factored into the broader picture to form a diagnostic view of possible future outcomes. (i'll go more into these in the next "Lets learn".

Rule of thumb:

Phase 3-4 = +ve NAO, i.e unsettled, mild
Phase 6-7 = -ve NAO, i.e blocked, colder, drier. 

Edited October 16 by Met4Cast

[Met4Cast]

Lets learn No. 2: Stratospheric Polar Vortex & Why It's Important

The stratospheric polar vortex (sPV) forms during the autumn as the Arctic cools increasing the thermal gradient between the Arctic and the equator. Winds in the stratosphere move in a westerly direction and can be a key driver of weather patterns. 

A strong sPV can help to increase the strength of the jet stream lower down in the troposphere leading to milder, stormier weather, conversely, a weaker polar vortex can lead to a more sluggish jet stream allowing for blocked/colder patterns to develop. 

Strong sPV's can override tropospheric patterns and can really drive the weather across the northern hemisphere.

October & November are often key developmental months for the sPV and many meteorologists/weather enthusiasts watch as this could give early signs of what type of winter we're going to have.

Occasionally, something known as a "Sudden Stratospheric Warming" (SSW) can take place, when this occurs temperatures in the stratosphere can rise by 60-70C in the space of just a few days, this can cause a weakening of the sPV and even cause the westerly winds to reverse to easterly temporarily. 

When this happens, those easterly winds can propagate downwards into the troposphere weakening the usual westerly winds leading to periods of blocking in the mid/high latitudes. The BFTE in 2018 was a result of an SSW although this was one of the most extreme events on record, around 70% of SSW's that result in a full reversal (westerlies reversing to easterlies) produce cold weather outbreaks into the mid latitudes during winter.

SSW's occur on average once every 2 years and can be one of the most dramatic weather events on earth, predicting when these will happen though is quite difficult with lead times generally in the 1-2 week range.

Cold/snow lovers want to see a weaker vortex in general during winter as this increases the chance of blocked weather patterns being able to develop.

This is a very basic overview of the stratospheric polar vortex and why it's talked about so much during the autumn/winter season.

[Met4Cast]

Let's learn No. 3: Atmospheric Angular Momentum (AAM)

The AAM is a very complicated topic & an area of science that is relatively new in terms of research and understanding. I'll try my best to explain the broader outline as simply as I can, but please feel free to ask questions!

The AAM forms part of the Global Synoptic Diagnostic Model (GSDM), a model designed to apply a diagnostic approach to medium range weather forecasting, whilst it can't tell you specific weather on specific days it's instrumental in predicting broad-scale weather patterns in the 2-3 week range & sometimes a little beyond that. 

Today, i'll focus solely on the AAM. Atmospheric angular momentum (AAM) is a measure of how fast the atmosphere rotates around the Earth's axis. 

A faster atmospheric rotation = a positive AAM, a slower atmospheric rotation = negative AAM. It's essentially a measurement of momentum within the atmosphere when averaged across the globe.

Various weather events can either add or remove westerly momentum from the atmosphere and the MJO (previously discussed) is a big player in this, when the MJO progresses eastwards through the Maritime Continent frictional torque generated against earths surface acts to slow down earths rotation, energy must be conserved however so this momentum transfers into the atmosphere and the result is we see an increase in atmospheric momentum. 

The largest player in adding or removing momentum however is mountain torque. During a positive mountain torque event, large mountain ranges are "bumping" into areas of higher pressure, this slows down earths rotation and that transfers momentum into the atmosphere. Mountain torque always proceeds frictional torque.

The opposite to the above is also true, the MJO returning to the Indian Ocean sees a return to negative frictional torque with negative mountain torque following, this does the opposite to a positive event and removes westerly momentum, transferring that momentum back into earths rotation and out of the atmosphere. 

During La Nina, AAM is typically negative due to an increase in the easterly trade winds across the Pacific (less westerly momentum), the opposite is true for El Nino, weaker easterly trade winds tend to produce positive AAM. 

You can however see positive AAM during La Nina, or negative AAM during El Nino, these are often called "mini ENSO cycles" and are more broadly driven by the MJO.

Complicated. How does this tie into weather patterns?

A very rough rule of thumb is that a positive AAM (increased westerlies) tends to produce a more wavey/amplified jet stream which increases areas of blocking high pressure in the mid/high latitudes, a negative AAM does the opposite and produces a flatter, more zonal jet stream. 

The direction of travel is often more important than the total. For example; total AAM could be negative but certain events are adding westerly momentum to that atmosphere & thus AAM tendency (direction of travel) is positive. 

You also need to account for "where" those positive or negative anomalies are heading. Are they moving towards the equator? In which case this tends to enhance sub-tropical highs and we see a stronger Azores high and often milder weather, or are those anomalies moving towards the poles? In which case blocking in the higher latitudes is more likely. I've annotated this AAM tendency chart to try and show this.

You also need to account for phasing of the MJO, does this align with the current direction of travel with the AAM? For example; let's say the MJO is in phase 6, this typically correlates to high latitude blocking, but AAM anomalies are moving towards the equator, this goes against the idea of high latitude blocking. 

Two opposing signals make the medium range forecast more uncertain. Looking at the MJO alone, you'd expect blocked patterns but the AAM is telling a different story, this is why it's important not to take the MJO in isolation. 

When both of these drivers are in alignment however, you can be far more confident in the medium range forecasting space in terms of the broader pattern, often being able to flag the potential for "high impact weather" far earlier, albeit on a broader scale, specific details of when/where/how bad will always be reserved for shorter range forecasting & models. 

I hope this explanation makes at least some sense & explains why AAM plays an important role in medium range forecasting. Any questions please feel free to ask. 

I’ve only done those three so far but i’ll post the next one here too once I’ve written it.

Next time: Frictional Torque & Mountain Torque.

[Mike Poole]

On 16/10/2024 at 21:22, Met4Cast said:

The AAM is a very complicated topic & an area of science that is relatively new in terms of research and understanding. I'll try my best to explain the broader outline as simply as I can, but please feel free to ask questions!

First, thank you @Met4Cast for taking the time to post these, they are really helpful.  

I have a question about AAM.  I understand that positive AAM means a more meridional/wavy jet and negative AAM means a more zonal pattern.  But what I have not understood is how it is known or predicted exactly where (in terms of longitude) the ridges and troughs will occur in our vicinity, particularly for the more meridional setups.  

I have a vague notion that where things are in the Pacific is important, and also that ‘seasonal wavelength changes’ are a thing, but there seems to be a certain amount of witchcraft in this aspect that I have never got my head around.  A scientific explanation on this would be something that I, for one, would find most helpful.

Mike

Edited October 18 by Mike Poole

[Met4Cast]

4 hours ago, Mike Poole said:

I have a question about AAM.  I understand that positive AAM means a more meridional/wavy jet and negative AAM means a more zonal pattern.  But what I have not understood is how it is known or predicted exactly where (in terms of longitude) the ridges and troughs will occur in our vicinity, particularly for the more meridional setups.  

Yeah, that's something I'm yet to fully get my head around too if I'm honest!

From my understanding it depends "where" in the jet the +ve anomalies are being injected, i.e where are the strongest jet streaks in the Pacific and where in the cycle is the GWO. I believe it's similar to how the MJO influences things, i.e a nudge of the jet stream over the west Pacific is going to lead to a different downstream pattern compared with a nudge further east in the Pacific, the same is true for the positive AAM anomalies, which is why you need to factor in the entire budget.

Equatorward propagating AAM anomalies feed into sub-tropical highs inflating them, i.e the Azores, Poleward propagating anomalies weaken sub-tropical highs and amplify highs into the higher latitudes, figuring out "where" though is the tricky part.

Rossby wave propagation is a big part of this but like I said, very much still a part I'm trying to figure out/learn with very little online information!

[Met4Cast]

Really nice example recently of propagating AAM/GSDM processes & their impacts on northern hemispheric patterns & the AO. 

If only it were this clear cut during the winter months! Going forward the sPV will become far more relevant & will need to be factored in and accounted for when producing longer-range analysis. 

Still too early to know what the state of play will be regarding that.