15th Feb – Can you sail uphill?


SST Anomaly 12th Feb – RAMSSA

SST anomalies remain very warm in the Tasman Sea with the marine heat wave continuing. SSTs are still cooler than normal off Western Australia. This has been the case since at least last August.

More recent developments have been in the Gulf of Carpentaria where SSTs in the southeast of the Gulf have cooled. Early summer saw very warm anomalies in that corner, but now it is showing negative anomalies of about 1 deg C.

SST anomaly for Northern Territory, black circle showing area of heat loss during QLD rains – RAMSSA

As we can imagine, these hot waters fed into the QLD rain last week.  Atotal heat flux image below from the time of the peak rain shows large negative flux in the southeast of the Gulf.

Total Heat Flux for 7th Feb at 18 UTC – OceanMAPS
MSLP and satellite image for 7th Feb at 18 UTC – BOM

Hazards across the Torres Strait

During the northwest monsoon, the water levels build up in the Gulf of Carpentaria and there is a consistent positive SLA of 0.3 to 0.5 m. But on the eastern side of the Torres Peninsula the sea level anomaly is negative at -0.2 m.

Today’s thought experiment: if you cross the Torres Strait from east to west, do you have to sail uphill? Is there a gushing waterfall of 0.7 m height that presents a hazard to shipping?

SLA for Torres Strait, with theoretical boat crossing east to west – OceanMAPS

Firstly, what do we know?

The Torres Strait is only about 15 m deep. Some areas are 6 to 9 m deep, while shipping channels are maintained to 25 m. There are 274 islands scattered through the region.

Most of the energy going through the Torres Strait is tidal (Saint-Cast 2008). A lot of water tries to get over a very shallow sill or through very narrow channels. In the main shipping channels the tidal strength can reach 7 knots. However only 30 % of the total tidal energy available actually gets through and the tides on each side of the Strait are incoherent (Wolanksi et al 1988). That is, they are completely out of phase with each other. To complicate things further, there are diurnal tides to the west and semi-diurnal tides to the east. It is one of the most complex areas for tidal prediction in the world and this post cannot hope to address this.

What we can look at is the longer frequency flow due to seasonal sea level patterns.

If you were to sail across the Strait you would sail over sea surface height (SSH), not an anomaly (SLA). Looking below, we can see that there is still a big difference from east to west. It is a step-jump at the Strait.

Sea surface height for 13th Feb, contours show model bathymetry – OceanMAPS

We know that OceanMAPS is constrained in its representation due to resolution. The model needs a minimum depth of 15 m and only has 6 ‘wet’ grid cells spanning north to south. However, it can show us the large scale dynamics.

The sea surface height difference may be expected to cause large currents pouring eastward through the Strait. The strange thing is that this is not observed. Wolanksi et al’s (1988) frequency analysis showed that low frequency seasonal currents (40 to 90 days) are only of the order of 0.1 m/s.

Why is this the case? Theories point to the non-linear interaction with the large semi-diurnal tides. The other culprit is bottom friction.

In the end so little water gets through that low frequency SLA fluctuations on the west side are not felt on the east side (Wolanksi et al 1988). That is, even the SLAs are completely out of phase with each other.

The transfer of water is also negligible for ocean budget calculations. Many an ocean model for eastern QLD uses the Strait as a natural closed western boundary.

What does OceanMAPS say? In the image below we can see eastward flows of 0.7 to 1.2 m/s. This is a 24 hr average that does not include tides.

Surface current velocity for 13th Feb, contours show model bathymetry – OceanMAPS

These values are much higher than the seasonal value of 0.1 m/s. According to the forecast data, this is a particular pulse of current that should drop off as the week goes on.

Surface velocity with timeseries for selected point – OceanMAPS

There is evidence from calibration studies against tidal residuals that OceanMAPS can overdo seasonal SSH signals at Thursday Island. See this timeseries from Andy Taylor, with the yellow line showing the bias correction for ‘eta’ in OceanMAPS.

Verification plot of Thursday Island tide gauge data against an ‘aggregate sea level’ which includes an OceanMAPS eta component – BOM

However even if the OceanMAPS pulse of 1 m/s is true, it still pales in the face of the semi-diurnal tides at 2 to 4 m/s!

In short, the answer to our thought experiment is yes: you will have to sail uphill and against a seasonal current. In the same way that you would sail uphill if you transit over a warm eddy.

However your greatest concern will not be not the upslope SSH. It will be the ripping high-frequency tidal currents, the narrow channels, the multiple reefs, the monsoon headwinds, and the searing heat. Check the phase of the tide and get a good nautical chart!

Ocean Colour in QLD

A lot of fresh water and sediment continues to run off QLD into the ocean. Now that the cloud has cleared we can get more information on what is happening. The latest ocean colour images on the eReefs portal show an algal bloom developing with high Chl-a concentrations near the coast. Values exceed the colourscale. An algal bloom has started.

Chlorophyll-a image 11th Feb 2019 – BOM

The eReefs portal also shows non-algal particulates, such as sediment. Most of the sediment is in a big plume at the mouth of the Burdekin River. This is a big concern to the reef and sea grass, as discussed last week.

Non-Algae Particulates (NAP) image 11th Feb 2019 – BOM

The third image in the portal shows coloured dissolved organic matter (CDOM) such as tannins. The high CDOM values are widespread and moving closer and closer to the reef.

Unlike suspended sediments, which tend to settle, CDOM is dissolved in the water and travels into the Great Barrier Reef lagoon as far as the fresh water itself. In water-quality monitoring, it can be used as a marker for how far the freshwater plume from a river or coastal source has penetrated into reef waters. – BOM

The CDOM also often moves with runoff pesticides, herbicides and heavy metals. This is the parameter to watch as we see the fresh water spread.

Coloured Dissolved Organic Matter (CDOM) image 11th Feb 2019 – BOM
Burdekin River plume on 12th Feb. Visible image from Himawari8 – BOM


Whales and upwelling in the GAB

Blue whale spotted from a tuna aircraft (Source: ABC/Wade Austin)

Blue whales are being spotted in record numbers in the Great Australian Bight this year. Scientific funding for whale observations dried up years ago, so now it is professional tuna spotters who opportunistically count the whale numbers.

Dr Pete Gill, CEO of Blue Whale Study, is not sure why there are so many this year. One possibility is that the South Australian upwelling is slightly early this year.

Is it early? Ocean Current climatology says no. Both January and February climatologies have the Bonney Coast upwelling in full flight, but there is evidence that this year the upwelling was weak in January and only properly strengthened last week.

February SST climatology – OceanCurrent
6 day composite SST for SA on 8th of Feb – OceanCurrent

Upwelling typically means more krill, a vital whale food source. But these blue whales are “pooing white” instead of orange and so they aren’t feeding on krill. Researchers are confused. If it is not early upwelling, and not krill, then what is driving these huge numbers of whales? If you have any ideas, please leave a comment below.


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