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When The Monsoon Failed

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Since teleconnections seem to be occupying minds at the moment I thought a very brief sojourn into history may be of interest. Or perhaps not.

It is of course well known that monsoons develop most dramatically in India and Southeast Asia, where intense heating of the land in summer causes low pressure, bringing warm, moist air from tropical oceans across the heated continents. In turn, the air rises due to mountain barriers and further heating giving widespread torrential rain.

During the latter part of the 19th century the monsoon didn’t occur on a few occasions causing widespread drought and bringing a disastrous famine to millions of people. The question was, why did the monsoon fail? Without going into detail great minds were brought to bear on the subject and theories were plentiful such as snow amounts on The Himalayas (not completely wrong as it happens) but the idea that found most support was our old friend sun spot activity. This held sway for some time until it was pointed out that the monsoon failed at times when there wasn’t any activity. Start again.

Enter Sir Gilbert Walker, a mathematician and physicist. To cut a long story short he basically crunched a lot of numbers. He mobilized pharaonic levies of Indian clerks to manually process worldwide pressure and rainfall data through his esoteric regression equations. The widespread drought and agricultural crisis of 1918 gave renewed urgency to these calculations. This was little more than a huge scientific fishing expedition: No conceptual framework supported the patterns he found; his methods were strictly empirical .Although Walker speculated, as had Hildebrandsson earlier, that polar circulation might be a driving force of global pressure fluctuations, it was little more than a hunch.

Nonetheless Walker’s dogged super-empiricism eventually produced a rich harvest. After twenty years of patiently crunching numbers and expanding his data sets, the (after 1924, retired) director-general of observatories in India was able to present overwhelming evidence (following Hildebrandsson’s pioneering work) for three coherent systems of intercontinental atmospheric oscillation:

In 1924, Walker first used and defined the term Southern Oscillation (SO) as a “seesaw†in atmospheric pressure and rainfall at stations across the Indo-Pacific region, where increased (decreased) pressure in locations surrounding the Indian region (Cairo, north-west India, Darwin, Mauritius, south-eastern Australia and the Cape Colony) tended to be matched by decreased (increased) pressure over the Pacific region (San Francisco, Tokyo, Honolulu, Samoa and South America) and decreased (increased) rainfall over India and Java (including Australia and Abyssinia). The two other “oscillations†involved out-of-phase atmospheric pressure between the regions of the Azores and Iceland, named the North Atlantic Oscillation (NAO), and between Alaska and the Hawaiian Islands, termed the North Pacific Oscillation (NPO):

This was a fundamental breakthrough: the global drought pattern first convincingly identified by Blanford in 1877-80 was now unequivocally related to the action of the great barometric see-saw over the equatorial Pacific Ocean. It soon became apparent that the Southern Oscillation provided the most potential in terms of long-range forecasting [of the three oscillations], in that it displayed marked interannual variability in its leadand lag correlations with climatic conditions in each season over a large part of the earth’s surface.

Fast forward. Forty years later the great Meteorologist Jacob Bjerknes argued in his famous 1969 paper that the SO resulted from a “chain-reaction†exchange of energy between the ocean and atmosphere. This isn’t the place to detail the whole theory (even if I knew it which I don’t) but it did leave unsolved a key dynamic element of the problem. What forces or instigates the nonlinear transition from one state to another?

The key to the problem was the development of dynamic oceanography.and it was left to Klaus Wyrtki at the University of Hawai’i in the mid-1970s to rebuild Bjerknes’s theory upon a more sophisticated foundation of ocean physics. A successful predictive model was produced by Cane and Zebiak in 1986.

Edited by weather ship
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I must admit when one thinks of the amount of work behind all this it's quite mind boggling.

A Walker Formula

Southern Oscillation Index (December-February) = [samoa pressure] + [North-east Australia rainfall (Derby and Halls Creek in Western Australia, 7 stations in north Australia, 20 throughout Queensland)] + 0.7 [Charleston pressure] + 0.7 [New Zealand temperature (Wellington, Dunedin)] + 0.7 [Java rainfall] + 0.7 [Hawaii rainfall (12 stations)] + 0.7 [southAfrica rainfall (15 stations, Johannesburg the most northern)] -[Darwin pressure] + [Manila pressure] -[batavia pressure] -[south-west Canada temperature (Calgary, Edmonton, Prince Albert, Qu'Appelle, Winnipeg)] -[samoa temperature] -0.7 [brisbane temperature] -0.7 [Mauritius temperature] -0.7 [south American rainfall (Rio de Janeiro and 2 stations south of it in Brazil; 3 in Paraguay, Montevideo; 15 in Argentina, of which Bahia Blanca is the southernmost)]


Late Victorian Holocausts-El Nino and the Making of Third World by Mike Davis

Edited by weather ship
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