May 13, 2011

Looking through the archives: Melbourne's climate

Satellite image of pyrocumulus clouds over Victoria & New South Wales,
taken by NASA's Aqua satellite in early February 2009
The climate of Melbourne, Victoria, Australia is one of great variation relative to the rest of the continent, but still moderate as oceanic effects and broad relief limit extremes, especially in terms of freezing temperatures and excessive rainfall. The city is located at the south-central side of the state of Victoria, which is the southeast corner of Australia. With a Latitude/Longitude of 37.5°S 145.0°E, Melbourne falls within what is technically considered the mid-latitude region. In terms of global circulation patterns, the city is affected by not only mid-latitude westerlies, which brings air across the west of the continent to the city, but also by the subtropical High and the Antarctic circumpolar vortex. The subtropical High normally resides in New South Wales for most of a typical year, but during the southern hemisphere summer the ITCZ (Intertropical Convergence Zone) descends into northern Australia, thus pushing the subtropical High south into Victoria and Melbourne. These seasonally changing patterns make Melbourne a diverse region in terms of weather, and fosters extreme events such as bushfires & droughts that are amplified by El Niño southern oscillations.

Melbourne at a Glance

Melbourne lies quite flat on the horizon. A coastal city of 4 million with a secluded port as its access to the Indian Ocean (via Bass Strait), Melbourne is at the confluence of two major rivers that flow into Port Philip (Yarra and Maribyrnong). Geologically, Melbourne is mostly underlain by Silurian marine sediments, and modern alluvium from Yarra. The marine sediments were uplifted from the shallow Bass Strait. This highlights how low the general relief of southern Victoria is. With a sea level decrease of just 70m, a land bridge would form between the city and the island of Tasmania.

In terms of precipitation, this low relief makes Melbourne susceptible to flash flooding during more intense showers/thunderstorms in both La Niña and spring seasons. Poor drainage and infiltration through city streets, combined with a low greenspace ratio, exacerbate flash floods in the city.

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