The Mean Sea Level Pressure (MSLP) anomaly charts for the month of December have come in, and as you've likely read on the site and you'll surely read in the comments below, it was very poor across Victoria and South Australia, with Western Australia not faring much better.

The term 'anomaly' means the difference from the 30 year climate average, and when observing a high pressure anomaly it indicates that the pressure was higher across that region compared to the long-term average, vice versa for lower pressure.

Mean Sea Level Pressure anomaly (difference from long term average) chart for December 2020

Looking at the MSLP anomaly chart above, two main thing stand out. That being the strong high pressure anomaly south-west of Western Australia, spreading across southern Australia and into the Tasman Sea, and secondly the lower pressure across the north-west of the state and small dip across northern NSW.

With La Niña finally kicking in across the country we saw the subtropical high pressure belt shifting south, allowing the northern monsoon to drift south, feeding in moisture across the northern, eastern and central parts of the country.

December 2020 rainfall totals

This is to be expected under La Niña, as is the retraction of the westerly storm to the poles, known as a positive Southern Annular Mode (+ SAM) event. While this is great for bringing the rain and swell for the East Coast (discussed below) it's far from ideal for the southern states.

With the storm track pushed further away and an effective blocking pattern setting up south-west of Western Australia, we saw windy days across the southern half of the state (WA), intensified by a heat trough (the dip in the high sitting across the South West), with instability and moisture to the north.

The block resulted in little to no swell across Perth and Mandurah, with small to moderate events across the Margaret River region, broken late in the month by a large, long-period though inconsistent W/SW groundswell from over near South Africa.

The blocking setup was poor for South Australia, not because of the south-east wind regime (which we expect as this time of year), but because it prevented mid-latitudes fronts pushing in from Western Australia, bringing westerly swells for the Mid Coast.

Victoria deteriorated through December after offering plenty of fun options across the more exposed beaches and breaks through the start of the month. Onshore winds kick in with no quality swells of power and size.

With the polar frontal activity the South Arm in Tasmania faired relatively well with persistent small swells, though troughy weather brought funky periods of onshore winds.

The chart below shows the average winds through December and you can see the strong south-southeast winds off Western Australia, zonal westerlies aimed away from the continent and easterly trades in the Coral Sea.

Mean (average) winds for the month of December 2020

Moving to the east, with the subtropical high shifting south and the increase in instability across the Coral Sea owing to the Madden Julian Oscillation, a significant, supercharged trade-swell event developed, bringing large, stormy surf to the south-east Queensland and northern NSW regions through the middle of the month, followed up by an inconsistent E'ly groundswell event from Severe Tropical Cyclone Yasa.

This can be seen by the dip in the pressure anomaly chart sitting over northern NSW and south-east Queensland, with lighter winds and cleaner conditions topping off the end of the month with fun amounts of trade-swell.

Sydney and the southern NSW coast offered, cleaner, smaller surf and great waves with generally light, workable winds.

For eastern Tasmania there were plenty of fun surf days with surface troughs deepening into lows while moving off the mainland, initially generating swell from the south, then more beefy swells out of the east-northeast. There was also smaller swell energy emanating from the northern Tasman and Coral Seas.

Looking ahead, the La Niña signal is peaking in the equatorial Pacific Ocean, with it due to return to neutral by late autumn. With this the country will continue to fall under the influence of La Niña through summer and likely early autumn, with active swell generating systems for the East Coast, less so for the southern states owing to dominant though weakening high pressure.