Butterflies and the Christ Child
Chasing El Nino: unveiling the science behind global weather surprises. Words by Tom Donaldson.
You have probably heard the analogy before: a butterfly in Bombay flutters its wings and a tornado spins up in Texas, a world away. Developed by American mathematician and meteorologist, Edward Lorentz, it is used to illustrate the central idea of chaos theory – that a tiny change in initial conditions can produce dramatically different outcomes in a given system. The analogy isn’t, however, just a poetic metaphor to illustrate a fairly wild idea with applications from quantum mechanics to the economy. It captures the reality of our global weather systems and a fascinating concept called “teleconnections”, the prefix “tele-“ being a Greek word for “far off/distant”. Teleconnections describe how the weather and climate in one part of the globe can ultimately influence the skies in another part of the world. What distant wing beats may surprise us this winter?
This story starts around 400 years ago, when Peruvian fishermen started to notice something peculiar. The sea temperatures, normally a biting cold, would periodically warm up every few years. It often coincided with tremendous rainfalls and floods. For the fishermen, it not only made a trip overboard slightly less unpleasant, but it also had the effect of driving their catch away from the usually nutrient-rich, cool waters. These served as unpleasant Christmas presents, of sorts: this phenomenon also seemed to hit its peak around December. As such, the Spanish-speaking Peruvians named this occurrence, “El Nino de Navidad”, or “the Christ child”, a nod to its particular timing. Today, it is simply known as “El Nino” (the boy), and the boy is currently active.
In the years since, El Nino has been linked to numerous other weather anomalies around the world. Suppressed hurricane activity in the north Atlantic, Australian wildfires (or bushfires, in local parlance), enormous ocean waves in Hawaii, smaller swells along Australia’s east coast, floods on the west American seaboard, mild winters in the southern US, those aforementioned dramatic rainfalls in Peru and Ecuador, drought in eastern Asia, drier conditions in south-central Africa, and even reductions/additions in sea ice around Antarctica have all been tied to the Christ child and his teleconnections. So why does this rather naughty boy create such havoc?
The first thing to understand may be somewhat surprising: the sun is not responsible for heating the air around us. Not directly, anyway. It’s the earth (or in this case, the sea) that heats the air above it. Of course, it’s the sun that heats the land and sea. This has to do with the nature of the sun rays (“insolation” – incoming solar radiation, in scientific speak) and how they interact differently with solids (land/sea) and gases (the air surrounding the earth/our atmosphere). But I digress. It’s enough for us to know that when surfaces warm up, whether they are land or sea, they then heat the air above them. As we might remember from school, or just from looking at hot air balloons, we know that warmer air rises. If this air is above large water bodies, this warm air can then take some of that water with it to form clouds.
The second key to our explanation has to do with the behaviour of air in response to temperature differences, like our heated land/sea mentioned above. Let’s imagine a balmy July summer’s day in Bournemouth. Tens of thousands of your closest friends from London and the Midlands have made the hike south to spend it at the beach. As the hours pass, the land (and thus, the air above it) warm up thanks to this cloudless, sunny day. The warming air rises. Now, we’re about to see that your mates aren’t the only ones who are charging for the coast. As that warm air rises up from the land, more air must replace it. Where can it come from? Well, in July, as the sea temperature is cooler than the land, the air above it hasn’t been rising. Instead, this air is pulled/pushed towards the land to replace the rising air. In other words, it is wind, and in this case, it is called a “sea breeze”.
To tie these ideas together (and I promise, to begin to explain El Nino): the earth’s surface is responsible for the heating of the atmosphere; when the surface is warm, it causes the air above it to rise; if the surface is relatively cooler, the air above it will sink/not rise; when air is rising, barometers will tell us that the atmospheric pressure is lower (because the air isn’t pushing down on us) and when the air is sinking, atmospheric pressure is higher (the air is pushing down on us); air, like everything else subjected to different forces, will be pushed by higher pressure to areas of lower pressure.
Phew! Are you still with me? Good! Thanks to the sun’s direct hit, the areas around the Equator warm up the most. The warmer air rises and cooler air from higher latitudes (i.e. further from the equator) rushes in to replace it. And, because the earth spins, this makes the wind approach at an angle (us nerds know this is due to the Coriolis effect). These winds around the equator are known as the Trade Winds. As these winds are (typically) blowing from east to west, they drag the surface waters of the Pacific Oceans with them. As these waters are warm (it’s around the Equator, after all), that means the warm water usually keeps getting pushed towards Australia, the South Pacific Islands and east Asia by those Trade Winds. And what do we know happens to the air above warm water? The air rises, it carries water vapour with it, clouds form, and those areas receive rain. And as for the eastern Pacific, around the coastlines across central America? That water that is constantly being pulled away by the Trades has to be replaced, and so the cold water from the depths is drawn up to the surface, making for much cooler sea surface temperatures. Cool surfaces = cool air = no rising air = no clouds = no rain.
You may have noticed that I used the words “typically” and “usually” above. This is because El Nino is most definitely not the normal state of affairs. If God were to peer down and see his son at work, he would see those trade winds weaken or even reverse. As those winds were responsible for dragging the warm surface waters across the Pacific, those warm pools are now able to spread back across the Central Pacific towards the Americas. Of course, you remember that what happens on the surface doesn’t stay on the surface: this warmer water will take the clouds and rain with it, leading to the aforementioned droughts in Australia and eastern Asia and floods in the western Americas. The effects often hit a peak around December and then subside, with the cycle recurring roughly every 3 – 7 years. Scientists still aren’t exactly sure what will kick start things, but they reckon that changes in the location of the different pressure systems (i.e. the high and low pressure areas), shifts in the sea surface temperatures, and variability in the location of cloud and rainfall are responsible. Unfortunately, individual butterflies haven’t been identified as the definitive cause. Still, such is the difficulty of studying chaotic systems, initial conditions and climate and weather.
Speaking of pressure systems, though, these are ultimately the things that explain how the young boy can then reach around the world, far from his Pacific birth. If we had typical conditions, the cool waters of the central-eastern Pacific would have a high-pressure area above it, as the cooler air descends. As the air hits the Pacific surface, this air is then forced to spread outwards to the east and west. The air heading east then crosses central America and, as we know that air is forced from high pressure to low pressure, it reaches an area of low pressure in the Caribbean and western Atlantic. And just like James Bond, tax havens and blissful getaways, the other thing we associate with the Caribbean are hurricanes: the offspring of these areas of low pressure. However, when that pool of warm water drifts back to the central-eastern Pacific, and El Nino is born, the different pressure systems shift with it. Now, we have warm (and ascending) air in the central Pacific – a low pressure system. Air is now rushing in at the surface from the west Pacific and the east i.e. the Caribbean. So instead of low- pressure areas being fostered in the Caribbean, and the winding up of hurricanes, we have winds at the Caribbean surface moving away towards the Pacific, producing areas of high pressure behind them. Hurricanes are often suppressed in the Atlantic as a result. As you can see, it is the coupled-up dance of these pressure systems that determine weather far from where the music starts.
What does this all mean for us? It depends (because of course it does – as you’ll have picked up by now, there are no certainties in this space). These El Nino-charged inter-ocean connections are often observed to pull storms further south in the Atlantic, leading to wetter and milder winters for southern Europe, and colder, drier winters for northern Europe. But…depending on just how far those warm waters shift in the Pacific, we may see something else. If the warm water stops at the central Pacific (a “CP El Nino”), the storms springing up off the Eastern USA that usually crash into us, can take off for Norway instead on a more North-Eastern trajectory. If the warm water charges on all the way across to the eastern Pacific (an “EP El Nino”), the storms of the Atlantic tend to plow straight across into the UK and mid-southern Europe. We are currently experiencing an EP El Nino.
Since we’re talking about their neighbourhood too, I’m now going to borrow some American slang: there are other curve-balls in the mix beyond the Christ child because the North Atlantic is also under the influence of other major climate cycles as well. The “Atlantic Multidecadal Oscillation” (the AMO) describes phases of warm and cool ocean temperatures through the North Atlantic over 70 years. It is currently in a warm phase, which typically promotes hurricane formation as well as wetter summers. The “North Atlantic Oscillation” (NAO) refers to air pressure differences between a high-pressure area around the Azores and a low pressure area around Iceland. If those pressure differences are large, weather systems are more likely to be steered into us from the south-west which translates to more storms (wind and rain) but relatively milder temperatures. If the differences are small, we cop more weather from the north, which translates to colder but drier conditions. These oscillations operate on a time scale anywhere from days to seasons to years i.e. the pressure differences can change within days or last for months and longer. We are currently neutral.
You’d be a brave person to bet on how this Winter will unfold and a foolish forecaster to accept the wager. If you focus on the influence of El Nino, you could argue that we’ll be wetter, windblown and could confidently take off for a hurricane-free Caribbean holiday. If your attention is held by the AMO, and exceptionally high sea temperatures in the Atlantic, you’ll cancel your Caribbean holiday. If your money is on the NAO, we’ll have bang-average cold temperatures and spells of rain and wind. What do I see and what would I like? I want the right butterflies to quiver and produce an EP El Nino, maintain the warm AMO, and force a positive NAO: the ingredients for storms to drive swell up the Channel and produce a winter of glorious surf. Will that happen? Only God knows.
