11 Roaring Poleward
The stormiest region on Earth is located where the ocean wraps all the way around the world, in between Antarctica and the southern tips of Australia and Africa. The Southern Hemisphere storm track experiences massive storms day after day throughout the year. It is called the storm track because meteorologists can follow the paths of the fierce weather systems that pass continually over this band, tracking their progress as they move from west to east. The Southern Hemisphere storm track is the fast track: winds average over 10 meters per second in much of this violent band. Gale force and stronger systems pass through week after week, with gusts frequently above 25 m/s (60 miles per hour). The winds churn up some of the world’s highest seas in their wake. No other region of the world even comes close in average wind speed: not Hurricane Alley in the Atlantic or its equivalent in the Pacific, and not the Great Plains of the United States with its sustained gusts. Not even other high latitude oceanic areas, like the energetic North Atlantic Ocean region south of Greenland can compete with the great circumpolar winds of the Southern Hemisphere storm track.
This most blustery region of our planet has experienced a metamorphosis in recent decades. The storms have shifted towards the South Pole, bringing their high winds, rainfall, and clouds with them. Along with their southward shift, the winds have become even stronger. The transition is not natural; recent research has shown that the shift has been driven mostly by the development of the ozone hole over Antarctica. That same trace gas in refrigerators and hairspray cans that opened the ozone hole have also displaced the most powerful weather systems on the planet.
The region between 40 and 50 degrees south latitude was dubbed the “Roaring Forties” by mariners who discovered their strong, persistent winds. Since the prevailing winds usually blow from the west, they were useful for clipper ships looking for the fastest possible route from the Atlantic to Australia. On the return trip from Australia, they would travel eastward past South America to take advantage of the winds again, almost making a complete circle around the world within the Roaring Forties. To get past South America, the sailors had to drop down into the next latitude band, known as the “Furious Fifties,” which has even higher seas and stronger gales. The crossing of the Drake Passage, the narrow opening between Cape Horn and the Antarctic Peninsula, was particularly perilous. The strong storms, swift ocean currents, and icebergs from Antarctica encountered on these voyages made many a sailor wonder if the route was worth the time it saved in transit. The “Screaming Sixties” are a bit less intense than the Furious Fifties or Roaring Forties, but are seldom entered by ships because of their remoteness and the prevalence of sea ice and icebergs.
Halley’s Storm Track
One sailor who traveled into the Southern Hemisphere storm track was Edmond Halley, of comet fame, who at the end of the 17th century led the first ever purely scientific voyage in history. Halley made contributions in a wide variety of areas of science, both theoretical and practical. He was the first to theorize that his eponymous comet had a periodic orbit, and made an accurate prediction of its next appearance. He recognized the world-changing importance of his colleague Isaac Newton’s work on gravity, inspiring Newton to write his most important work, the Principia. Halley eventually paid for the publication of Newton’s masterpiece, and wrote a commendatory poem in Latin for its introduction. Halley also invented and tested a diving bell that allowed divers to remain underwater for hours at a time with recirculating fresh air, described in a manuscript he wrote called “The Art of Living under Water.” In addition to his talent as an astronomer, physicist, and inventor, Halley was not too shabby an atmospheric scientist. He was the first to show how the Sun drives large-scale circulations in the tropics.
Halley asked the government for a research vessel in order to understand the Earth’s magnetic field and its effect on compass direction, which would aid with navigation. All sea voyages prior to this had focused on exploration, commerce, or warfare, but the royalty agreed that the scientific quest was important and gave Halley a ship. He was made captain of the HMS Paramore, a fast, maneuverable boat, which he sailed from London deep into the Southern Hemisphere to collect data. His voyage started out disastrously. His officers did not respect the authority of a scientist as captain, and he eventually had to return to London to get a new crew (and court-martial the old one). It was eventually revealed that the most seditious officer had a personal vendetta against Halley, stemming from an incident from years before. The man had written a book about the calculation of longitude that Halley had rightfully criticized due to a lack of originality, and the man took this personally.
While sailing through Newfoundland, the ship was met with a curious sight: all the local fishing boats seemed to scatter away from them. While entering the harbor, the loud boom of a cannon was heard: one of the fishing boats was firing upon them! More shells continued to fly, ripping through their sails, while Halley deployed his rowboat to try to communicate with the offending captain. It turned out the fishermen thought that the unusually small Paramore was a pirate ship. This was the Golden Age of Piracy after all, and the fishing boats were on edge due to an encounter with true pirates just a few days before. Luckily no one was hurt in the incident.
In the port town of Recife on the coast of Brazil, Halley was again suspected of piracy, this time by an English investigator. The man could not believe that such a bold voyage had a scientific purpose. After harassing Halley’s men, he invited Halley to his house, pretending to want to talk business. He instead arrested the legendary scientist and performed a thorough search of the Paramore. The man embarrassedly released Halley after discovering that precious datasets were the only booty on board his ship.
Despite the hardships, the expedition was a tremendous success scientifically. Even while the ship was being tossed around amidst mountains of waves in the Roaring Forties, Halley continued to take careful measurements, sometimes while the crew bailed out seawater as giant waves crashed over the deck. Heading further southward in late January 1700, the middle of summer in the Southern Hemisphere, temperatures plummeted below freezing. Even long sunny days and northerly winds from the tropics couldn’t break the chill. The unceasing cold was baffling to Halley. He was at the same latitude as London, in the middle of summer, experiencing temperatures that were colder than typical conditions in the dead of winter off the coast of England. The average summertime temperature at 52.5 S, 25 W (Halley’s farthest south location) is 4o C, while the average wintertime temperature at 52.5 N, 14 W (to the west of England and Ireland) is 8o C.
At their farthest south, the crew was excited to see what they thought was a series of flat-topped islands, each covered in snow and surrounded by cliffs. Upon closer inspection, these were large icebergs, which the crew narrowly avoided through some deft maneuvering in foggy conditions. The same small ship that aroused such suspicion in other legs of their voyage allowed for their narrow escape from the perils of the Furious Fifties and their eventual safe passage home.
Bringing the roar
Why are the winds so strong in the Southern Hemisphere storm track? The story of Halley’s voyage in the box above has many of the clues we need to answer this. The scientific name for the massive storms that bring the roar to the forties is “extratropical cyclones,” meaning spinning storms that form outside of the tropics. Extratropical cyclones also occur with great strength in the Northern Hemisphere, where they bring day-to-day weather features like fronts, rainfall and snowfall. These storms are not to be confused with tropical cyclones, also known as hurricanes or typhoons. Hurricanes, which require warm ocean temperatures for their formation and thus occur in summer and fall, are quite distinct beasts from the weather systems of the Roaring Forties.
Extratropical cyclones instead derive their energy from the temperature difference between the warm tropics and cold poles. When storm systems curve in just the right way, they are able to extract energy from the thermal contrast and turn it into the kinetic energy of winds. This process is named cyclogenesis, a word that literally translates to “birth of circles.” When the temperature difference is larger – either from the tropics getting warmer or the polar regions getting colder – there is more energy available for cyclogenesis, and more intense storms occur. Since the Sun heats the equatorial regions with much more direct light, this makes the tropics hotter than the poles and also ensures that there is plenty of energy available for the growth of weather systems.
A unique aspect of the Southern Hemisphere storm track, which is ultimately key to its status as world’s windiest, is that strong gales blow through all year round. Halley faced massive storms even in January, the middle of summer in the Southern Hemisphere. During summer in the Northern Hemisphere, winds slacken substantially. The weakening is due to a reduction of the thermal contrast, as the Arctic warms in summer, especially over land. In the Southern Hemisphere, on the other hand, the continent of Antarctica and the cold ocean around it cause bitterly cold temperatures to exist all year round. So the frigid temperatures that Halley observed in the Furious Fifties are part of the cause of the tempests he encountered on the way there.
The aspect of Antarctica that is key to keeping the surrounding latitudes cold is its white color, as this reflects away large amounts of sunlight. The high elevation of Antarctica keeps it cold directly over the continent, but actually causes the surrounding region to be slightly warmer (Ogura and Abe-Ouchi 2001; Singh et al 2016), because colder air radiates less longwave radiation to space.
The presence of such cold temperatures at high latitudes ensures a large temperature difference, which in turn allows cyclogenesis to occur with great strength over the Southern Hemisphere storm track all year round. While temperatures near the surface are most important, the storm track is affected by weather conditions far away as well, including those high up in the stratosphere.
Tugs from Above
The stratosphere is quite the underdog in terms of its potential to influence weather at the surface of the Earth. Existing over 12 km high over much of the world, visible attributes of day-to-day surface weather such as clouds and rainfall are almost completely absent in this relatively quiescent layer. Since it is so far from Earth’s surface, the air in the stratosphere has very low density. In total, it weighs in at only around a quarter of the size of the weather layer below. Research has shown that storminess is in fact sensitive to the air way up there.
The link between the stratosphere and surface weather systems is perhaps best exhibited through a remarkable natural phenomenon that occurs over the Arctic known as a sudden stratospheric warming. In winter, there is usually a quickly rotating, extremely cold mass of air high above the North Pole called the polar vortex. About every two years the polar vortex breaks down abruptly. There are massive temperature changes in the stratosphere in response to this. At around 20 km above the North Pole, temperatures can increase up to 40o C within just a matter of days. Sudden stratospheric warming events occur when giant-sized waves in the atmosphere propagate into the stratosphere and mix the air, disturbing temperature and circulation patterns.
Following a sudden stratospheric warming the Northern Hemisphere storm tracks shift systematically southward, away from the pole. In response, throughout the U.S. and Europe, there is a greatly increased chance of colder than average temperatures in the following months. This well-established result, seen time and time again after an event has occurred, has proven that the tiny stratosphere can have a strong influence on the much more substantial troposphere and its weather systems below.
Sudden stratospheric warming events helped scientists appreciate the fact that conditions high up affect the storm track as well. While temperatures near the surface primarily control the strength of storms, stratospheric temperatures have a greater influence on the position of storms. This occurs in the Southern Hemisphere as well, and since ozone has affected stratospheric temperatures, the storm track has responded. With colder air in the stratosphere above the South Pole, extratropical cyclones are coaxed towards the poles, resulting in the winds of the Roaring Forties becoming weaker and the Screaming Sixties becoming stronger.
Picture a cowboy spinning a giant lasso from over the South Pole. The loop of his lariat extends out to the Furious Fifties, where it marks the center of the storm track. When the wrangler increases the size of the lasso, the loop widens and storms spin down closer to the equator. He widens his lasso when he gets a hot head, i.e., when there is a warming in the polar stratosphere. When the stratosphere is cold, on the other hand, cooler heads prevail and the wrangler tightens his loop, and the lasso as well as the storms come towards the pole. Variability in storm track location happens randomly from week to week and year to year so a wobbly lasso is an appropriate analogy to picture shorter timescale undulations as well. The stratospheric temperature difference gives weather systems a little tug from above, letting out or taking in the lasso, affecting the location of storms.
As the storms shift, their winds change in strength as well. When the lasso is extended outwards towards the equator, our cowboy rotates it slightly slower, representing a deceleration of the winds. When it moves inward in response to cooling over the polar stratosphere, it rotates more quickly, bringing even more furious winds.
Ozone depletion has caused large changes in the temperature of the stratosphere. The stratosphere is warm in the first place because of the absorption of high-energy UV rays within the ozone layer. When there is less ozone, the UV passes through without depositing its energy and stratospheric temperatures drop. A precipitous decline in stratospheric temperature has been observed over the South Pole, especially prominent in the months just after the ozone hole appears, and it is driven in large part by ozone loss from CFCs.
Because of the drop in stratospheric temperature, our cowboy’s head has been cooling off, and he has tightened his lasso loop; the Southern Hemisphere storm track has shifted dramatically towards the poles. In response, the Roaring Forties no longer have quite the roar that they used to have. The Furious Fifties and Screaming Sixties, on the other hand, have increased in strength substantially, both from the poleward shift of storms and the accompanying strengthening of the storm track. These locations have increased their average summer wind speed by about 20% since the days before the ozone hole existed. The largest changes have been seen in summer, which fits with ozone as the prime cause since this is immediately following the ozone hole and is when stratospheric temperature changes have been largest.
It is remarkable that the strongest feature of the Earth’s climate has undergone such a dramatic transformation, and that industrial activity is to blame. But all of the links in the chain of evidence are rock solid: the ozone loss, the temperature change, and the circulation shift, occurring at the right times and with the right strength to match the theoretical expectations. Our depletion of the ozone layer has made a lasting, unexpected impact on the climate, and as we shall see, the effects reach through the Roaring Forties and beyond.
Patterns of change
In the early 2010s, New Zealand, which spans much of the Roaring Forties, suffered devastating droughts that have crippled the agricultural output of the islands. The drought of 2012-13 was the worst since modern observing equipment has been installed, and cost the country over 1 billion US dollars in lost agricultural yield. The countryside of the North Island became so parched that a noticeably browner hue could even be viewed from space. This drought is part of a larger trend towards drying which has lasted several decades, since around when the ozone hole first opened up.
The islands of New Zealand are much greener than its dry neighbor Australia. The Kiwis receive their rainfall from the northern edge of the massive storms of the Roaring Forties and Furious Fifties. Particularly strong deluges occur when the storms encounter the dramatic mountain ranges of the South Island, but the North Island is rather rainy as well. Since the storm track has shifted southward in summer due to the ozone hole, many of these tempests now miss the Kiwis entirely, and in response, New Zealand has dried substantially. Any climate event as severe as the 2012-13 drought involves some bad luck too, but this kind of event has a much higher probability of occurrence now due to the presence of the ozone hole.
Even though the ozone hole will close in future decades, we expect drought to continue to plague places like New Zealand and Australia. Global warming also shifts the Southern Hemisphere storm track towards the pole, and has made a noticeable impact during the spring, fall, and winter. The forecast for the coming decades is for precipitation to continue to decline. And since heat waves bake the soil, increasingly warmer temperatures parch their pastures even more.
Drought in New Zealand is only one of many ways that the Southern Hemisphere storm track dislocation has affected the Earth’s climate. Since the storm track is so powerful, it determines the climate not just where its winds blow, but far away as well. For instance, the polar vortex of rapidly rotating air miles above the South Pole has become much stronger. When the frigid polar vortex strengthens, it tightens and keeps cold air close to the pole. In response, large parts of Antarctica have cooled, while Southern Africa and South America have experienced much more rapid warming.
The Furious Fifties and Screaming Sixies have become wetter and even windier due to the ozone hole. While most of these latitude bands are unpopulated, one should not assume that they are devoid of natural habitats. The Southern Ocean, which is located around the continent of Antarctica, is home to some of the most vibrant marine ecosystems on the planet. Fountains of upward moving ocean water brings up nourishing nutrients that act as fertilizer for massive growths of plankton that accumulate near the surface. The plankton form the base of the food chain for an ecosystem that also includes creatures like whales, penguins, and seals.
Ocean circulation is driven both by winds, which push water around in the upper ocean, and also by the water cycle, which evaporates fresh water from the salty ocean and rains it back down elsewhere. Rainfall affects ocean circulation because fresh water is less dense than salty water, and density affects how easily seawater can sink and how currents form. Since both the winds and rainfall have been altered due to ozone depletion, ocean circulation in the Southern Hemisphere has changed. The great gyres of the subtropical oceans have strengthened. The upwelling water that brings up nutrients is thought to have shifted polewards and strengthened, although this is difficult to observe. The mighty Antarctic Circumpolar Current, which transports more water than any other current in the world, has shifted along with it. Such changes must affect the rich marine ecosystems of the Southern Ocean, but so far they have been difficult to separate from other influences such as climate change, ocean acidification, overfishing, and natural variability.
The Southern Ocean is notorious for being one of the hardest places to explore on the planet, due to its ice, inhospitable climate, and remoteness. We simply do not have adequate observations of the physical and especially biological systems for us to know the full implications of the shift in the Southern Hemisphere storm track yet. Since the ozone hole will be around for another 40 years, there is unfortunately plenty of time for more impacts to become apparent. Maybe we have yet to discover an important connection, and we will have to add another surprise to the story of the ozone hole.
