Saturday, 7 February 2009

Anatomy of a record hot day and dry spell

Although it seemed scorching outside in the middle of today, it was still a genuine surprise to find that Melbourne had broken its previous hottest day by as much as 1˚C and its previous hottest February day by 3.2˚C.

Worse still, it seems likely Melbourne will beat its record forty-day dry spell by Wednesday. No general rain is forecast over Victoria until Saturday. More than that, today was forecast back on Sunday to be only 26˚C, and the record hot Friday before was forecast to be a lot less than it was. If the weather bureau keeps up with this record, Melbourne will have a day of 51˚C next Sunday.

The absence of alarm at these trends is disturbing. It suggests people think these two record hot days are an aberration because the first half of the summer was so cool.

In fact, the record hot weather is a symptom of anthropogenic global warming moving the monsoon southward and strengthening northerly airflows that are extraordinarily hot and dry by the time the cross the Great Divide north of Melbourne. Looking at today’s weather chart, the monsoon that before anthropogenic global warming was centred at around the latitude of Darwin of 12˚S, is now centred normally at the latitude of Tennant Creek or 20˚S. That shift of eight degrees has had the effect of making it much easier for extremely hot airflows to develop over southeastern Australia.

Worse still, Melbourne is in exactly the worst location for these hot northerlies. Being precisely on the southern slopes of the continent’s only significant east to west mountain range means that when powerful monsoons develop Melbourne is precisely on the leeward side of hot winds. In addition, as 2007’s Observed Poleward Expansion of the Hadley Circulation since 1979 demonstrates, atmospheric stability has increased greatly at precisely the latitudes where Melbourne is located. This means much longer times between the passages of cold fronts or other disturbances. Consequently, higher temperatures will be able to develop.

In fact, we con confidently predict that from this year on Melbourne will regularly experience the hottest temperatures in Australia during the summer. It is really likely that by 2020 temperatures in excess of 50˚C will regularly occur in Melbourne.

What one hopes is that, if the public in unwilling to really struggle for immensely reduced greenhouse emissions, government and business will ensure that such conditions areat the very least made more tolerable by improved building and rail design.

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