To shear or not to shear – these 5 images show El Niño’s possible effect on Atlantic hurricanes, and other jet stream stuff

Ten years after Katrina* the world is on the brink of a whole new cluster of climatic disasters, including wide-spread coral bleaching, Pacific atol floods, possibly another devasting Brazil drought and another record-breaking hot year, following from the currently developing Super El Niño.

Another devastating Atlantic hurricane will most likely not be a part of disrupting extreme climate events this year. These five images tell you why:

In 2005 Katrina awoke America to the reality of climate change
In 2005 the Atlantic hurricane Katrina awoke America to the reality of climate change. Events in the Arctic that same year were much more troubling (and more strongly related to the trend of global climate warming!) – but went unnoticed to many.

[*) In 2005 'the world' – as we're all living in America – was shaken up to the reality of climate change by the destructive hurricane Katrina, that damaged and flooded the city of New Orleans, and helped promote Al Gore's An Inconvenient Truth a year later [we're no haters – except for a few slip-ups we still think it's a good film, worth watching]. We are not so sure though where exactly to fit Katrina in the actual scientific trend of climate change – that is far from clear in the specific field of Atlantic hurricane development. What should have shaken the world was the Arctic Melting Record of 2005. That was a very clear sign that things are indeed growing horribly out of control. But who lives on that ice sheet but some 25,000 polar bears without voting rights in the various countries that claim their territory for further fossil fuel exploitation – oh shame on you, mankind.]

Climate image 1: the general circulation in 2D

First let’s take a look at this beautiful all-you-need-know image of the northern hemisphere climate system – or general circulation:

Climate system general circulation with Hadley Cell and jet streams
The northern hemisphere ‘general circulation’ – workhorse of the climate system, showing tropical convection, subtropical subsidence, the Hadley Cell, the Polar Cell and both the subtropical and polar jet stream. A classic. We salute this image each day we wake up.

It shows for instance the Hadley Cell that is driven by Earth’s tropical convection engine and the subtropical zone of air subsidence (‘horse latitudes’) that we talked about in our last article, about possible northward Sahara expansion.

Please note above the two ‘jet streams’ – high-velocity, high altitude westerly winds that circle the entire hemisphere. Jet streams, their variable force and specific position, are very important in weather forecasting – but why, why, why do these winds blow from West to East? Doesn’t the Sun set in the West – ‘therefore’ the Earth spin counter-clockwise? It would make more sense for winds (because of the inertia of air molecules) to blow from East to West!

Climate image 2: jet streams the result of a 3D atmosphere

You are very right to think so. And the majority of air molecules indeed do move from East to West. Around the equator – where Earth’s spinning velocity is of course greatest – this leads to dominant easterly winds across the surface, known as trade winds. (These also turn towards the equator – so NE on the northern hemisphere and SE on the southern hemisphere – because of rising air motion (convection) around the equator. The below image shows what we are talking about – the northern hemisphere general circulation in 3D. In fact, easterly winds are dominant across Earth’s surface, as they occur in both the Hadley Cells and the Polar Cells – somewhat hard to believe for those of us who live at the mid-latitudes, where westerlies dominate. In fact, so much air is blown from East to West, that at the borders of the Hadley Cells and Ferrel Cells (mid latitude) and Polar Cells, where air converges, a very strong compensating band of eastward flowing air is needed, to keep the whole system closed: those high-power jet streams.

Earth's climate system is actually 3D! Hence jet streams
Earth’s climate system explained: 3D atmospheric cells lead to 2 seperate high-speed, high-altitude westerly jet streams

We now know that a typical jet stream (the subtropical jet) forms at the front (high in the atmosphere, just below the stratosphere) where the Hadley Cell and Ferrel Cell collide – and another typical jet stream (also just under the troposphere-stratosphere border) where the Ferrel Cell and Polar Cell collide – the Polar jet stream.

But that is about all that is normal and regular.

Sometimes jet streams stop altogether, usually they meander enormously, sometimes they merge, sometimes they blow in the opposite direction (for a short while), sometimes they blow exceptionally fast – and sometimes one of the two is dominant over the other.

Why? No time to explain [we're in the midst of a planetary crisis – work to be done!]. All we can say is that if one of the above atmospheric cells has good reason to be stronger than usual – then also the jet stream it produces at its border(s) is likely to be stronger.

Scenario 1: a stronger Hadley Cell leads to a stronger Subtropical Jet Stream – and to Atlantic hurricanes NOT reaching the news

That can be the case for a typical El Niño situation, where a positive (tropical) temperature anomaly (in Pacific Ocean waters) leads to stronger convection and therefore a more powerful local Hadley Cell. A stronger Hadley Cell means more air convergence where it collides with the Ferrel Cell – and a stronger Subtropical Jet Stream.

Scenario 2: a weaker Polar Cell leads to a weaker Polar Jet Stream – and to ‘Polar Vortex’ cold spells reaching the news

The opposite – when a cell is weaker than normal – has the opposite effect. Recall that paradoxical Arctic warming cold winter hypothesis? When the Arctic is warmer than normal (in winter) there is less compaction of air at the Arctic (ice) surface – so a smaller build-up of the normal high pressure system. This leads to a weaker Polar Cell or Polar Vortex – and in turn a weaker Polar Jet Stream. A weak jet stream means an easily meandering jet stream, so therefore a higher chance of Polar Vortex cold spells breaking out to lower lattitudes.

Climate Image 3: Typical El Niño Pacific jet streams

Shown below is the typical situation corresponding with scenario 2. Warmer-than-usual tropical waters associated with El Niño put the Hadley Cell in higher gear and lead to a stronger, more dominant, and also less meandering Subtropical Jet Stream. This influences North American weather in many ways, discussed in our special El Niño climate model forecast.

El Niño leads to powerful Pacific jet stream
Typical El Niño influence on jet stream behaviour over the Pacific and North America during winter months. The Pacific subtropical jet stream is fuelled by a more powerful Hadley Cell (due to larger convection over warmer tropical ocean waters). This leads to a larger oceanic influence on US weather – usually translating to a milder winter. A powerful subtropical jet stream is a straight one – with a smaller chance of merging with the Polar jet stream. The polar jet stream might therefore be of smaller influence on US weather than in non-El Niño years, especially La Niña years.

But wasn’t this article supposed to be about Atlantic hurricanes? Thank you!

Hurricane formation is complicated meteorology, but two factors are actually quite easy to understand. You need to start with warm (26C+) water, preferably warmer, preferably with relatively cool air on top of it. That is the engine for convection, and needed to create a tropical storm, the starting point for any hurricane. The warmer the water, the stronger the engine – and possibly the stronger the hurricane, or the higher the number of hurricanes produced.

But hurricanes are rather tall weather systems too – and in order to become that closed circulation system, it needs to be fixed over its engine of warm water. Strong lateral winds can be a disturbing factor – but what’s worse (from a developing hurricane’s perspective) is winds from different directions at different altitudes.

Climate image 4: wind shear – a well-placed jet stream can blow the head off of any hurricane, dispersing its energy

This is called wind shear and easily explained in the below’s very basic climate image:

wind shear during El Niño disturbs hurricane formationDuring El Niño a strong subtropical jet stream leads to high-velocity high-altitude westerly winds, decapitating Atlantic hurricanes. This wind shear factor can even be stronger when easterly trade winds blow at a lower altitude.

Do you feel we’re there yet? Indeed, we haven’t looked at actual forecasts! Below is a NOAA image of expected sea surface temperature anomalies in the Pacific and Atlantic Ocean over October 2015.

Climate image 5: something to base an actual hurricane forecast on!

Here at Bitsofscience.org you’ve probably seen plenty of such SST anomaly maps. You’ll probably recognise that typical El Niño warm water anomaly (and a potent one by October!) in the central and eastern tropical Pacific. That could lead to higher convection and a stronger local Hadley Cell, therefore a stronger subtropical jet stream.

El Niño SST forecast 2015
NOAA’s sea surface temperature anomaly forecast El Niño for October 2015. A lot to see in one image.

But that’s not all this map shows. It also shows that not just the tropical East Pacific waters are relatively warm – but that it goes for the entire East Pacific (for instance a 2+ degree positive temperature anomaly off the coast of Canada and also relatively warm Californian coastal waters.

As in meteorology (at least for local forecasting) relative temperatures are far more important than absolute temperatures we should note that these warmer North Pacific waters might actually disturb a clear cut distribution of zones of convection (over warmer waters) and subsidence (over cooler waters). From the above map we conclude that the shown SST anomalies do favour a stronger-than-normal Hadley Cell and therfore stronger-than-normal subtropical jet stream – but that it is not quite clear if the zone of subsidence (where high pressure systems develop) would lie around the latitude of California, or possibly further north (see negative temperature anomaly NW of Hawaii). If the Hadley Cell is stretched further North, the subtropical jet stream might also follow a path further to the North than the one shown in the ‘typical El Niño situation’ shown above.

That might have important hurricane ramifications. A jet stream over the Gulf of Mexico, would blow off the tops of Atlantic hurricanes before they could landfull on US territory. If hower that same strong jet stream would blow further North, from for instance northern California to the Carolinas – it would prevent a Sandy-type hurricane hitting New York, but not protect against southern hurricanes, like Katrina, making landfall around New Orleans.

The above map also clearly shows the Western Hemisphere Warm Pool, a mirrored El Niño effect, with relatively warm water in the Gulf of Mexico and on the Atlantic side of Florida. This too favours a ‘stretched’ Hadley Cell and therefore a subtropical jet stream path somewhat further to the north.

So, should we warn the Gulf coast and the Caribbean, that these regions might in fact not be protected by El Niño’s strengthened jet stream?

Not that fast. The SST anomaly image has one more to show. That mirrored effect of El Niño in the Gulf of Mexico, is actually a sort of ‘Atlantic La Niña’: water in the (north)west is warmer – and in the (south)east – between Brazil and Africa – relatively cool.

This is actually caused by El Niño too! The extra strong convection it causes in the East Pacific sucks in enormous amounts of air. Therefore the West Pacific trade winds are shut down (typical for El Niño, also why the warm water stays put!) – whereas the Atlantic trade winds are actually stronger than normal. Not only does this increase wind shear in for instance the Caribbean Sea (westerlies at high altititude, but strong easterly trade winds around the ocean’s surface – not good for hurricane development!) also the actual cradle of Atlantic hurricanes is simply not very productive, as that is precisely that blue zone around the Atlantic equator – where tropical storms form – if at least the water is warmer than the air above it.

There you go. Atlantic hurricanes should not be our concern for now. Let’s shift our attention to the areas of our planet that are much worse off in this globally climate-record-breaking year – a year in which politicians must decide on a new global climate treaty, in December in Paris, at COP21. For updates of our climate coverage please return to our website, or follow us on Facebook or Twitter.

© Rolf Schuttenhelm | www.bitsofscience.org

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