We're just past the halfway mark of summer but I feel it apt to provide an intra-seasonal update, or confirmation for those suffering in the southern states that the run is as bad as they think, and to reaffirm that the East Coast has been the place to be.
By the end of this double-dip Niña episode most will be well versed in how such a Pacific Ocean setup effects the swell/surf around the Australian continent. Let's not concern ourselves with the possibility of a triple threat Niña next summer, that's for another day.
I've brought up the Mean Sea Level Pressure (MSLP) and surface wind anomaly charts from the start of December through until now. These charts show the difference in surface pressure in wind from the long term climate average from the years 1981-2010. For those who need a refresher, circulation in and around high pressure is anti-clockwise, while lows see winds blowing clockwise.
What's immediately obvious is the large high pressure anomaly stretching out across the south of the country, extending well south towards polar latitudes.
It's worth remembering that during summer, the sub-tropical high pressure belt shifts further south across the Southern Ocean regardless of being in a La Niña, neutral or El Niño year. This pushes the westerly storm track further south, resulting in smaller, less frequent groundswells.
La Niña years exacerbate this, with the highs shifting even further south, cutting off the supply of swell further. The chart shows this clearly, with the pressure being higher than the long term average for the the December/January period.
The Southern Annular Mode (SAM) which is measure of how far north or south the westerly storm track is from normal also shows the impacts of La Niña. When the index is positive the westerlies are restricted to the pole and further south, as is evident since mid-October in the image below. When negative we see the storm track lifting north close towards Australia.
Coming back to the MSLP anomaly chart, another feature that stands out is the lower than normal pressure across the Coral Sea. This is linked to the warmer than normal sea surface temperatures in the western Pacific Ocean, brought by stronger than normal easterly trade-winds under a La Niña setup.
With low pressure squeezing high pressure to the south, we see a favourable swell generating setup for the East Coast and it's hardly been below 2ft since the start of summer (mostly much bigger). The warmer than normal sea surface temperatures and instability have also produced a couple of tropical cyclones and larger swell pulses. The first for the start of the year being Tropical Cyclone Seth which tracked close to the south-east Queensland and northern NSW coasts, with Tropical Cyclone Cody sitting further afield between Fiji and New Zealand.
An inland low pressure trough can also be seen across central Queensland, putting a pinch on the northern flank of the high pressure anomaly sitting under Victoria, bringing though generally favourable easterly winds (for the beaches) but alas with no swell to capitalise on.
The setup responsible for Western Australia's excessive heat is also visible with an inland heat trough bringing persistent easterly winds which have drawn in heat from central Australia. Swells have been sporadic and not overly sizey leading to a tiny run of surf for the metro regions, with small workable waves in the Margaret River region.
Finally the wind speed and direction anomalies for the past two months shows a stronger than normal east to south-east wind pattern through the Tasman and southern Coral Seas, and those easterly winds across Victoria, Tasmania and South Australia.
Looking at the month ahead (February) and unfortunately it looks like more of the same in regards to a relentless blocking pattern for the southern states, though slowly weakening into the second half of the month, with no let up in the swell generating systems for the East Coast.
We'll provide an outlook for the coming autumn/winter in the coming months.