Ridgy – Sub-tropical Ridge
This is Ridgy, or as scientists like to call him, the Sub-tropical Ridge. Ridgy's one of the four major drivers that shapes Victoria's seasonal weather. So let's look at how he does it.
As warm air in the tropics continually rises, moves south, then cools and falls, large areas of high pressure are created.
These high pressures, in this case Ridgy, are great at blocking rain-bearing fronts.
From November till April, Ridgy chases away cold fronts around southern Australia for days or even weeks at a time.
Occasionally cold fronts sneak through and if they connect with moist air from the tropics Victoria gets some summer rain.
As winter sets in, Ridgy heads north and the cold fronts find it much easier to reach Victoria and deliver their rain, until Ridgy returns next November.
Ridgy does this every year but in recent decades he's been getting more effective at chasing away cold fronts from Victoria — explaining a lot of our drier weather.
The Bureau of Meteorology has observed that Ridgy's increasing strength is related to the rising global average temperature but the scientists are continuing to investigate how this climate dog might change his behaviour in the future.
Key things to know about Sub-tropical Ridge (STR)
- moves north and south seasonally over Australia
- can affect the passage of cold fronts (a good source of moisture and potential rainfall) across southern Australia
- typically moves north in winter, allowing fronts to pass over southern Australia
- typically moves south in summer, blocking the passage of fronts
- part of the reason why Victoria experiences rain bearing cold fronts during winter
- strength (or intensity) of the high pressure systems also affects rainfall (higher pressure means less rainfall)
- farmers know that the seasons with stronger or more frequent blocking high pressure systems over southeast Australia don't tend to produce the regular rainfall we would like.