Since my last blog the likelihood of an El Nino has lessened, tho we are still officially on “El-Nino Watch “. The IOD is back to (a slightly Positive) Neutral. Although not ideal it is far from what might have been possible a couple of months back, and will no doubt occur again in the future .
I still anticipate a relatively mild , dry winter in general. Hope I’m pleasantly surprised.
We are now seeing the eternal battle of the necessary low pressure systems and their cold fronts from the south and west begin to impact (and at times break down) the sub-tropical high pressure ridge we spoke about last time (refer SAM & AAO).
Quick re-cap in case, like me, you forgot.
The position of the sub-tropical ridge plays an important part in the way the weather in Australia varies from season to season.
During the warmer half of the year in southern Australia (November to April), the sub-tropical ridge is generally located to the south of the continent. High pressure systems (also called anticyclones), which are associated with stable and dry conditions, generally move eastwards along the ridge.
In Autumn the sub-tropical ridge moves northward and remains over the Australian continent for most of the colder half of the year in southern Australia (May to October). Conditions along the ridge, under the influence of the high pressure systems dry and descending air, tend to be stable and drier.
Blocking Highs ( the Enemy )
Blocking highs are strong high pressure systems which have formed further south than usual and remain near stationary for an extended period of time. These highs essentially "block" the west to east progression of weather systems across southern Australia. Blocking highs are often, although not always, associated with a cut-off low (see below) which may form to the north of the blocking high, the two systems creating a blocking pattern. As frontal systems (see below) approach the blocking high, they slow down, weaken and tend to slip to the south of the high pressure system. Blocking highs can affect all of southern Australia. They can occur at any time of year, and can be in the Australian region from several days to several weeks.
Cut-off lows are low pressure systems which have broken away, or are cut-off, from the main belt of low pressure which lies to the south of Australia. They can be at any level in the atmosphere, and therefore may not show on the surface charts. Cut-off lows bring enhanced rainfall to parts of southern Australia.
A cut-off low may develop when a low pressure system forms on an active cold front. Alternatively, they may form in an unstable easterly flow on the northern flank of a slow-moving or blocking high.
Last week we have just seen an East Coast Low , which is a form of intense cut-off low. These are visible on surface charts.
Australia can be affected by both warm fronts and cold fronts, however cold fronts are more common and have a greater impact. A cold front is formed when cold dense (heavy) air advances equator-wards, causing warm air to be forced up and over its sloping surface. A warm front is formed when warm air of lower density (i.e lighter) moves pole-wards, sliding over the sloping surface formed by a colder air mass. Frontal systems bring rainfall & affect all of southern Australia. They can occur at any time of year, however they have the greatest impact during the winter months. Fronts generally move across southern Australia from west to east, and can be in the Australian region from a couple of days to a week.
Ski Heil !
P.S. If you don’t believe me …..or want to learn more of this like I do - look here !.