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Arctic and Antarctic Hurricanes

June 6th, 2010

extratropical-cyclone.jpgResolution is a research ship designed to study the ocean floor. Its work forms part of the Ocean Drilling Program, an international scientific project that involves examining rock and sediment collected as cores drilled from the seabed. Its drilling rig, the pipes carried on its deck, and the hole through its hull down which the drill pipes pass make it a little unwieldy, but Resolution is big and tough. Built in 1985, it measures 470 feet from stem to stern and it is meant to survive anything the sea can throw at it. In the fall of 1995 Resolution came close to sinking in an Atlantic storm.

The voyage began uneventfully. Resolution set sail from Iceland in late September with 120 people on board, heading for the Greenland Sea, deep inside the Arctic Circle to the east of Greenland. The weather was not good, but Resolution sailed unperturbed through conditions that would have caused difficulties for a smaller vessel. Edwin G. Oonk, the captain, had to maneuver the ship repeatedly to avoid icebergs drifting out from the glaciers of Greenland, to the west.

Then the air pressure began falling sharply. There was a severe storm to the east, another to the south, and icebergs between Resolution and the Greenland coast, where the ship might have sheltered. Captain Oonk decided to sail forward into the weather. His tactic might have worked, but the two storms merged, pressure dropped still further, and the wind speed increased.

For two days the storm raged. The dial on the wind speed indicator had a maximum reading of 115 MPH and that was what it read in the gusts. Meeting waves 70 feet high, like solid walls of water, the ship rose high into the air and plunged into the troughs. At times the main propellers were lifted clear of the water. Lookouts, securely lashed down, had to be stationed in the stern to watch for icebergs, because at times the ship was being carried backwards at more than 4 MPH. After a day and a half of this, Resolution was in danger of sinking. The storm abated at last, and the vessel made its slow way to port for repairs.

This was not a tropical cyclone, but it was as fierce as one. Its winds of more than 115 MPH were classified as storm force 12+ (12 is hurricane force on the Beaufort scale of wind strength). In lower latitudes it would have been rated a category 3 hurricane, strong enough to uproot large trees and wreck mobile homes. In 1954, the Swedish meteorologist Tor Bergeron called storms of these force extratropical hurricanes.

They are very similar in some ways to tropical cyclones and occur in both north and south polar regions. Around Cape Horn, at the southernmost tip of South America, the frequent gales led sailors to name the latitudes the “roaring forties,” “furious fifties,” and “shrieking sixties.” This region of frequent gales extends all the way around the world and is not confined to the area south of Cape Horn. It is associated with Cape Horn only because South America projects into the belt and before the Panama Canal was built ships were compelled to pass through it in order to cross between the Atlantic and Pacific. At Byrd Station, Antarctica, the wind is strong enough to produce severe gales about two-thirds of the time.

Occasionally the weather systems that cause them can bring severe weather to places in lower latitudes. Around Christmas, 1995, for example, one extended south and brought extreme cold and heavy snow to Scotland and northern England, with snowdrifts 30 feet deep in the Shetlands, the northernmost group of Scottish islands. It reduced December temperatures over Britain to 4 degrees F below the monthly average and prevented 1995 from being the warmest year on record, although it was still the third warmest since 1659.

Extratropical hurricanes usually develop from polar lows. These are areas of relatively low atmospheric pressure that form near the edge of the sea ice. There, the air temperature over the ice can fall locally as low as -40 degrees F, while the temperature over the sea is close to freezing (+32 degrees F), so there is a temperature difference of 72 degrees F or more between air over the water and over the ice, with low pressure along the edge of the ice. Warm air and ocean currents bring nearly twice as much warmth to the Arctic Ocean as is absorbed from solar radiation and the sea water never cools below about 29 degrees F, so there is a constant source of warmth. Intense polar lows can develop to hurricane force in 24 hours or less. They then travel eastward in the northern hemisphere, but in two days or less they reach land and dissipate.

There is also a sharp contrast in temperature between tropical air moving toward the pole and polar air moving away from it. These airflows meet at the polar front, where air rises as part of the system of convection cells that forms the basis of the global circulation of the atmosphere, and the rising air produces a belt of generally low air pressure. Winds are from the east on the pole ward side of the polar front and from the west on the side nearest the equator. The temperature difference to either side of the polar front can cause fronts to develop with associated areas of low pressure (depressions). If the polar front is drawn as a line on a map, these fronts appear as waves along the main front, with the depressions at the wave crests. Such depressions form repeatedly along the polar front, often in “families”, as frontal systems in which warmer, less dense air rises over cooler, denser air and the entire system travels eastward. When all the warm air has been raised clear of the surface, the front is said to be occluded. It is at this stage that an intense polar low may develop behind (to the west of the occluded fronts if an extreme temperature difference triggers a local disturbance).

Where there is a large temperature difference in the air over the ice and sea close to the polar front, passing depressions intensify, the pressure within them falling. Air is drawn in from the adjacent region of higher pressure and the converging air rises. As it does so the Coriolis effect starts it rotating. This increases the temperature differences at the surface, because the rotation carries cold air from the poleward side of the polar front into a warmer region and warm air from the opposite side into a cooler one. This is a polar low.

Compared with most depressions, a polar low is small. When it starts to form it may be no more than 600 miles across. Air flows into the low near the surface, rises, and flows away from the low at high altitude. This vertical movement intensifies the flow, as it does with a tropical cyclone, and intensification causes the low to shrink in size until, as a fully developed extratropical hurricane, it may be only 200 miles in diameter. At the center, atmospheric pressure may be less than 970 mb (the average sea-level pressure is 1,016 mb), a pressure comparable to that at the center of a fairly gentle hurricane, as hurricanes go. This pressure will generate sustained winds of about 45 MPH with gusts to 70 MPH, which is below hurricane force, but the pressure may be lower and wind speeds much higher.

Many high-latitude storms develop in this way, from polar lows, but they can also begin as revitalized tropical cyclones. When a tropical cyclone moves out of the tropics it crosses cooler water.

Water evaporates into it more slowly and this makes it weaken. Eventually it will die completely unless it encounters a cold front. When this happens, the warm air of the cyclone rides up the sloping boundary of the cooler air, and this forced lifting triggers a new bout of convection and convective warming as the rising air cools adiabatically and its water vapor condenses. The dying cyclone comes back to life and continues its journey, once more with the power of a hurricane.

No matter how it begins, an extratropical hurricane is much like a tropical one. It is circular in shape, with a clearly defined eye that is relatively free from clouds and in which the air is calm. This is surrounded by spiraling walls of cumulus clouds extending all the way to the tropopause. Above the hurricane, the outflow of air produces a thick tail of high-level cirrus clouds. Seen from space, it has the same “spiral galaxy” shape as a tropical cyclone.

There are differences, however. An extratropical hurricane has an even shorter life than a tropical one. It develops from a polar low to a full hurricane in 12 to 24 hours and, once formed, it moves at up to 35 MPH. This is about twice the speed of a tropical cyclone, and it moves in the opposite direction. Carried with the prevailing winds, tropical cyclones travel from east to west and high-latitude extratropical cyclones travel from west to east. In the northern hemisphere, their speed soon brings them to a large land mass where they weaken and die, so they last no more than 36 to 48 hours. There is much less land at these latitudes in the southern hemisphere, so storms there travel further and last longer. In the absence of land to reduce wind speeds and starve cyclones of the water they need to sustain them, winds between Antarctica and the southern continents are more severe than those at similar latitudes in the northern hemisphere.

When an extratropical hurricane does cross a coast, it brings heavy snow or sleet driven by ferocious winds. The winds are seldom strong enough to cause severe damage to buildings, but they can bring down power lines and uproot trees, and when they deliver drifting snow they can seriously disrupt transportation and communication systems.

This is the fourth article of a five-part series on hurricanes. The others can be found at the links below.

Related Articles:

How Hurricanes Are Named and Tracked
Hurricane Preparation, Survival, and Safety
Hurricanes That Reach Europe
Will Global Warming Cause More Hurricanes?

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