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Many parts of Australia have suffered a run of severe and, in some cases, unprecedented weather events this summer. One common feature of many of these events – including the Tasmanian heatwave and the devastating Townsville floods – was that they were caused by weather systems that parked themselves in one place for days or weeks on end.
It all began with a blocking high – so-called because it blocks the progress of other nearby weather systems – in the Tasman Sea throughout January and early February.
System - Fronts - Tasmania - Winds - Below-average
This system prevented rain-bearing cold fronts from moving across Tasmania, and led to prolonged hot dry northwesterly winds, below-average rainfall and scorching temperatures.
Meanwhile, to the north, an intense monsoon low sat stationary over northwest Queensland for 10 days. It was fed on its northeastern flank by extremely saturated northwesterly winds from Indonesia, which converged over the greater northeast Queensland area with strong moist trade winds from the Coral Sea, forming a "convergence zone".
Trade - Winds - Flank - Tasman - Queensland
Ironically, these trade winds originated from the northern flank of the blocking high in the Tasman, deluging Queensland while leaving the island state parched.
Convergence zones along the monsoon trough are not uncommon during the wet season, from December to March. But it is extremely rare for a stationary convergence zone to persist for more than a week.
Pattern - Climate - Change - Slowing - Weather
Could this pattern conceivably be linked to global climate change? Are we witnessing a slowing of our weather systems as well as more extreme weather?
There does seem to be a plausible link between human-induced warming, slowing of jet streams, blocking highs, and extreme weather around the world. The recent Tasman Sea blocking high can be added to that list, along with other blocking highs that caused unprecedented wildfires in California and an extreme heatwave in Europe last year.
Trend - Slowing
There is also a trend for the slowing of the...
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