Weather patterns in southern Patagonia

An excerpt from the book Enduring Patagonia by Gregory Crouch, Random House (2002).

You can buy this excellent book here and visit the author's website here

Only thirty percent of the surface of the globe is dry land, and most of that land lies in the Northern Hemisphere. And in the Southern Hemisphere, with the obvious exception of Antarctica, most of the land is found above 30° south latitude. Between 30° south and the shores of Antarctica at about 70° south, the world’s surface is almost entirely water. The only significant land masses below 30° south are the horn of Africa, the southern slice of Australia, and the North Island of New Zealand. Below 40° there is just Tasmania, the South Island of New Zealand, and the long southern finger of South America – Patagonia. Sure, other islands dot this Great South Sea, but they are only pinpricks, and this enormous cold ocean rules the climate of Patagonia. In order to understand why, one must examine the general global weather patterns.

Ten degrees to either side of the equator is the region that sailors call the doldrums, a belt of fickle winds and active thunderstorms that are fed by the northeast trade winds to the north and the southeast trades to the south. As the northeast and southeast trades converge, the atmosphere has nothing to do but rise, and rise it does, causing a semi-permanent belt of low pressure around the Equator. The rising air flows out to the north and south and subsides in the subtropics at about 30° north and south latitudes. This sinking air causes belts of high pressure called the “subtropical highs” to exist at the northern and southern edges of the tropics. Moving away from the equator in both directions, from about 30° to 60° of latitude (the mid-latitudes) is an active belt of prevailing westerly winds. Polewards from the westerlies is a zone of generally low-pressure that rings both poles at about 60°. These two “circumpolar troughs” are where the storm fronts that swarm across the mid-latitudes go to die. In the Northern Hemisphere, the land masses of Eurasia and North America break up the circumpolar trough into two storm cemeteries – the Aleutian Low in the North Pacific and the Icelandic Low at the top of the Atlantic. South, however, in the circumpolar trough that rings Antarctica, there are no such continental obstacles, and 60° south is the only place where a line of latitude runs all the way around the globe without hitting dry land.

The fact that the ocean can flow freely around the globe at 60° south keeps the ocean of the high southern latitudes cold. The continental dams of Eurasia and North America allow ocean currents like the Gulf Stream and the Juroshiro Current off Japan to rush warm tropical waters far north in the Northern Hemisphere. These warm currents help to keep the average annual temperature of the Northern Hemisphere one-degree of Celsius warmer than the Southern Hemisphere. No such warm currents disturb the cold ocean temperatures of the high southern latitudes, and climactically speaking, that one degree makes an immense difference.

In both hemispheres, air wants to move from the subtropical high pressures at 30° toward the circumpolar low pressures at 60°. This air gets a strong kick from the Coriolis Effect as it moves – a kick to the left in the Southern Hemisphere and a push to the right in the Northern. So pressure forces acting in concert with the Coriolis Effect spawn the prevailing westerlies of the mid-latitudes. The larger the temperature difference between 30° and 60° the stronger the pressure forces will be and the stronger the corresponding winds, and since the temperature difference between 30° and 60° tends to be larger in the Southern Hemisphere, that is were the strongest west winds blow.

The overall weather picture is simpler and more regular in the Southern Hemisphere, because there aren’t such huge landmasses to jolt the system. Cold and warm fronts form in the Southern Hemisphere, usually between 30° and 40° south latitude, when warm tropical air collides with cooler mid-latitude air, but because there isn’t much land to cause strong contrasts of temperature and moisture, the fronts themselves aren’t as strong as they are in the north. What start as cold and warm fronts in the Southern Hemisphere quickly lose their individual characteristics and adopt the character of the ocean beneath: cold and wet. These broad storm bands of cool, moist air swept along by the west wind migrate to the east-southeast as they are gradually sucked toward the circumpolar trough around Antarctica. But the circumpolar trough isn’t always the same, it waxes and wanes. It’s prone to wax large and stay large and lap over the southern end of the Americas. When it does, the atmospheric pressure in Patagonia goes down and stays down, a succession of wet storms are sucked over the Andes, and the vile weather in the mountains can last for weeks.

The Patagonian Andes act like a ramp and the air suddenly rises. The abrupt rise further cools the air and condenses its moisture into cloud, cold rain, or snow, and with the passing eons that precipitation has formed the glaciers and ice caps of the Patagonian Andes. And the air can actually accelerate as it plunges down again after its rise up the hump of the Andes, a downslope effect similar to the Alps’ foehn and the Rockies’ chinook, downslope effects that magnify the power of wind.

Good weather does happen in Patagonia, it just doesn’t happen very often. When rare clear and windless conditions do prevail, they’re usually caused by a “blocking anticyclone” or a “blocking high,” so called because they block the normal west to east migration of the storms. When the jet stream of fast moving winds that circles the globe at high altitudes above the mid-latitudes splits into two branches. One jet goes far north and the other goes far south and a high pressure system forms in the fork. This is the blocking high, and it usually has an accompanying “cutoff low” to the north, so the whole weather setup puts the cutoff low in place of the normal high pressure at 30° S and the blocking high in place of the normal low at about 50° south. One half of the jet stream goes north around the high, the other branch goes far to the south, and both branches miss Patagonia. Since the jet stream steers the west winds, and the west winds drive the storms, when the jet stream is diverted no storms assault Patagonia.

A blocking anticyclone isn’t stable for the long run, and it gradually erodes, but a strong blocker and the good weather under it can last three or four days, sometimes even up to a week. Stories of perfect weather that lasts twenty days and more pepper Patagonian lore, but I’ve never seen anything close. The best it has ever done for me is eight days of more or less nice weather, but even that mega-spell was split in half by a storm that lasted thirty hours.

Consider it a legitimate Patagonian good spell if the pause between storms lasts forty-eight hours, but clear, windless Patagonian skies often last only thirty hours, and even more often such fine conditions don’t last eighteen hours.

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