Friday, April 20, 2018

Meridional vs Zonal: cooling vs warming 1975-2000

from Chilling Possibilities, Science News, 1975

The winter of 1780-81 was a particularly bitter one for the American Revolutionary forces. Washington's troops hunkered down, ill-clothed and ill-fed, around their campfires at Morristown, N.J., while a few miles away British troops enjoyed the relative luxury of an occupied New York City. But even the British had their problems, for the win- ter was so cold that parts of New York harbor froze for weeks at a time, blocking movement of their powerful fleet. The ice even got thick enough to allow hauling cannons from Manhattan to Staten Island. The colonists had struggled against devastating winters ever since establishment of the earliest settlements, when one of the few holidays celebrated by the stern Puritans was that of Thanks- giving-for a harvest bountiful enough to ensure survival until spring.
Though they didn't realize it, these hardy pioneers were trying to conquer a New World in the midst of some of the worst weather in over 2,000 years, a cold spell that had begun in the early 15th century and was to continue until around 1850, known to later climatologists as the "Little Ice Age."
By contrast, the weather in the first part of this century has been the warmest and best for world agriculture in over a millennium, and, partly as a result, the world's population has more than doubled. Since 1940, however, the temperature of the Northern Hemisphere has been steadily falling: Having risen about 1.1 degrees C. between 1885 and 1940, according to one estimation, the temperature has already fallen back some 0.6 degrees, and shows no signs of reversal.


 [...]
 During warm periods a "zonal circulation" predominates, in which the prevailing westerly winds of the temperate zones are swept over long distances by a few powerful high and low pressure centers. The result is a more evenly distributed pattern of weather, varying relatively little from month to month or season to season. During cooler climatic periods, how- ever, the high-altitude winds are broken up into irregular cells by weaker and more plentiful pressure centers, causing formation of a "meridional circulation" pattern. These small, weak cells may stagnate over vast areas for many months, bringing unseasonably cold weather on one side and unseasonably warm weather on the other. Droughts and floods become more frequent and may alternate season to season, as they did last year in India. Thus, while the hemisphere as a whole is cooler, individual areas may alternately break temperature and precipitation records at both extremes.



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United Nations Fisheries and Aquaculture Org (circa 2000)


Meridional (C) circulation dominated in 1890-1920 and 1950-1980. The combined, "zonal" (W+E) circulation epochs dominated in 1920-1950 and 1980-1990. Current "latitudinal"(WE) epoch of 1970-1990s is not completed yet, but it is coming into its final stage, and so the "meridional" epoch (C-circulation) is now in its initial stage. (It will be useful for the reader to note here the relation that shows that the "transition" from C to W-E is continuous, and the equation balances to 100%, in the form of a simple graphic without any other variables included).

It was found that "zonal" epochs correspond to the periods of global warming and the meridional ones correspond to the periods of global cooling. (Lamb 1972; Lambeck 1980).

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