- Portland hosts their Upwelling Festival, but no upwelling in sight
- TC potential near the Solomons
- More details on the strange tropical cold eddy
Temperatures in the west continue to be cooler than normal. This has been the case since at least early September. The positive Indian Ocean Dipole event may be linked to this persistence even though the official IOD region is further north.
On the east coast the SST anomalies are fairly neutral on average. We were seeing positive anomalies off NSW a few weeks ago, perhaps due to an early southward push of the EAC, but now the climatology has caught up and it’s all looking normal again.
However, the absolute temperatures are certainly warm. Looking at northwest QLD we can see temperatures inside the reef of 30 deg C while the model is showing it a little cooler on or outside the Great Barrier Reef. And in the Gulf of Carpentaria it is very warm at 31 to 32 deg C.
These very warm ocean temperatures are not helping the severe heatwave that is affecting northeast QLD. The heatwave has persisted all this week and will spread further inland in the coming days. This region has also been affected by over 105 separate bushfires that are leading to evacuation of some towns. The warm SSTs means little relief will arrive until there is a major change in airmass which is not expected until Wednesday next week.
Warm SSTs to the far north east of our region are supporting the development of a tropical low which could impact Australia.
Gradient wind analyses show a pair of low pressure systems straddling the equator due to an equatorial Rossby wave. The southern system may strengthen into a tropical cyclone by the weekend.
Strike maps suggest a 40 to 50 % chance of gale force winds around this system by Saturday night.
Track ensembles from the ECMWF model forecast that the low will pass through the Torres Strait and continue travelling westward towards Darwin next week.
The TC Warning Centre in Brisbane has elevated the likelihood of a cyclone to moderate. See the latest TC Outlook for the Coral Sea on the BOM website.
Bonney Coast No-welling
Every year in the first week of November, the Victorian town of Portland hosts an Upwelling Festival to mark the start of the fishing season. The festival includes a Blessing of the Fleet, a parade through the town and an array of live music. This year it was held on the 8th of November.
As described on the Visit Portland website, the local community is strongly linked to the seasonal ocean phenomenon:
The Bonney Upwelling is the epic natural ocean occurrence that powers a seven month ( November to May ) feeding frenzy of tuna, crayfish, giant crabs, fish, seabirds, seals, krill and blue whale.
Supporting an incredibly rich ecological food web, the upwelling is at its greatest off the coast of Portland and is the cornerstone of our lucrative fishing and tourism industry.
But was the festival held a little too early this year?
Recent SST analyses suggest there is no upwelling in sight. In the six-day average SST image the temperatures are a consistent 16 to 18 deg C along the Bonney Coast. (Note, the town of Portland is just off this image to the southeast).
These temperatures are about 1 deg C above normal for this time of year, as shown in the anomaly image below.
Percentiles against the SSTAARS climatology also suggest the water is on the warm side. Most of the Bonney Coast has SSTs in the 70 to 90th percentile.
Ocean colour images show weak chlorophyll activity in the region. Chlorophyll-a numbers hover around 0.25 to 0.75 mg/m3, whereas the fishing community would prefer numbers upwards of 2 mg/m3. Compare the image below to the one of the upwelling in full-flight shown here.
The Bonney Coast upwelling relies on persistent southeasterly winds driving Ekman transport of the surface water away from the coast. This is commonly the case in summer when a strong high pressure ridge establishes in the Great Australian Bight.
We haven’t seen many (any?) strong slow-moving high pressure systems so far this spring. There has been short-term upwelling on the South Australian coast, but this occurred when the wind was more E to NE’ly and consequently the upwelling was further west near Adelaide and Port Lincoln where the coast is orientated west to east.
The atmospheric forecasts for the week ahead do not show any SE’ly winds of 15 knots or more until Wednesday next week, the 5th of Dec. A whole month after the Portland festival.
What about last year? Clicking through the images on OceanCurrent, it seems that there weren’t any signs of upwelling until the 25th of November in 2017.
Perhaps the community of Portland should push their annual festival back a few weeks…
Or perhaps the festival is deliberately premature. Is the Blessing of the Fleet actually a Prayer for Upwelling?
How is it tracking?
The intriguing cold eddy in the far northwest is maintaining its strength and structure. It is now centred at 13 S 110 E.
An Argo profile seems to have sampled the eddy. See below. Argo profiles on the webviewer tend to update about two weeks behind real time so now they are starting to cover the period since the eddy formed. A profile from the 16th of November shows that although OceanMAPS performs better than the climatology, it does not show the true strength of the temperature gradient at 25 m depth. The observations have rapid cooling with depth between 25 and 40 m, while the model has the strongest cooling below 50 m.
The eddy continues to move southwestward. If it maintains its direction then it will miss Christmas and Cocos Islands.
Ocean color images show slightly elevated Chlorophyll-a concentrations in the vicinity of the eddy. We might expect high biological activity around the edge of the cold eddy like the ring we saw in the Tasman. However the temperature characteristics of this feature are weak at the surface (ie. eddy is not evident in SST field) so this may temper any Chl-a signature.