21st Feb – A TC surges into QLD


SST Anomaly for Australia – RAMSSA

There has been no major change with SST anomalies around the country this week.

Off the coast of QLD the SST anomalies are neutral. However, absolute SST values above 26 deg C extend all the way down to Brisbane. This means the waters will be able to support a tropical cyclone.

Depth of 26 deg C isotherm – OceanMAPS

TC Oma’s storm surge

Tropical Cyclone Oma (which is a normal name in Fiji but translates as ‘Grandma’ in Dutch) formed over Vanuatu and is now approaching the southeast QLD coast. TC track ensembles still show large divergence, but most forecasts bring Oma close to Brisbane on Saturday.

TC Oma forecast track issued 21st Feb – BOM

Severe weather warnings of abnormally high tides and dangerous surf have been issued. Forecasters are concerned about coastal inundation and erosion. They are working frantically to determine what the storm surge will be.

Severe Weather Warning Thursday 21st – BOM

There are four main components to coastal sea level during a storm:

1. Tide

Large spring tides, otherwise known as ‘king tides’, occurred on Tuesday and now the high water heights are slowing decreasing. High water peaks on Saturday will be 20 to 30 cm lower than they were on Tuesday.

Mooloolaba tide monitoring – QLD Govt

2. Sea level surge

The strong winds of TC Oma will push water onto the QLD coast via Ekman transport. This will cause a large positive anomaly in sea level of 30 to 40 cm. OceanMAPS is representing this component well.

Sea level anomaly for Saturday the 23rd – OceanMAPS

3. Inverse barometer effect

The rule of thumb is that a reduction of 1 hPa will cause an increase of 1 cm of sea level. On Saturday the forecast MSLP for Brisbane is 1008 hPa which is 5 hPa lower than the average ambient pressure. So we could expect an extra 5 cm of sea level.

4. Wave setup

Perhaps the hardest thing to estimate, wave setup is the extra water level caused by breaking waves. The NSW forecasting handbook says that a good estimate is 15% of the shallow water wave height.

Wave setup is often a parameterised diagnostic and there are many different calculations. The BOM storm surge model uses a parameterisation from CSIRO (O’Grady, McInnes, & Hoeke, 2015). The latest model run estimates a wave setup of 40 to 60 cm on windward shorelines between Byron Bay and the north end of Fraser Island.

Total sea level

Sea level components are usually added linearly. If we combined the components we’ve discussed so far for high water at 00:30 UTC on 23rd January at Mooloolaba, the result is:

Sea level above HAT = -0.35 (tide) + 0.3 (SLA) + 0.05 (Inverse barometer) + 0.5 (wave setup)

Sea level above HAT = 0.5 m

How does this compare to the BOM’s on-demand TC StormSurge model?

TC Storm Surge model screenshot with mouse-over on Mooloolaba for 0030 UTC on 23rd January – BOM – click to expand

The above image shows the 95th percentile in the probabilistic forecast. The pink dots mean HAT is exceeded on the windward coastlines. The mouse-over shows the total sea level at 0.41 m above HAT. The surge component (0.11 m) is much lower than the OceanMAPS value (0.3 m), but otherwise the estimates are similar.

Another way to view the output is via a timeseries.

Probabilistic TC Storm Surge values for Mooloolaba – BOM

Things to remember when looking at the StormSurgeTC output: the atmospheric forcing is synthetic and based around the official TC track forecast issued by the BOM so low category systems are probably not well represented; and the tidal component does not match the tide tables exactly.

[For calibrated sea level forecasts against tide tables but without wave setup see the Sea Level Viewer .]

Using both rough estimates and the StormSurgeTC model, we are expecting water levels of up to 0.4 m above HAT. This is concerning for the low-lying areas of the Gold Coast, Sunshine Coast and Brisbane waterways. Inundation is likely in many areas, and press about urban nuisance flooding has already started. If TC Oma brings a large amount of rain then the problem will be compounded.

[See the latest BOM explainer blog on Coastal Inundation.]

However, the calculated numbers still fall a long way short of the official Severe Weather Warning of up to 1.0 m above HAT. It is likely that local forecasters are being cautious and want the public to avoid going to the beach.

Remember that none of the above includes individual wave runup and overtopping which could cause damage and erosion as significant wave heights are expected to reach 5 to 6 m on Saturday.

Significant wave height for 00 UTC on 23rd January – AUSWAVE-R

Surfers, at least, are happy.

For those of you monitoring the storm, you can
– Track the sea level at Mooloolaba here.
– Track the waves at the Brisbane buoy here.

Cold-core eddy near Vanuatu

In the wake of TC Oma, a curious cold-core eddy has developed to the west of Vanuatu. The sea level anomaly is at least -0.4 m.

Sea level anomaly for 21st Feb – OceanMAPS

The temperature profile suggests that the SST is cooler than normal and the water is well-mixed to 30 m before it hits a sharp thermocline.

Temperature field for 65 m depth and temperature profile inside the eddy on 20th Feb – OceanMAPS

How extreme is this? Looking at the percentiles, we see that the major temperature anomaly is between depths of 65 m and 175 m. It is less than the 1st percentile!

Temperature exceedence outside the 1st percentile at 65 m depth on 20th Feb – OceanMAPS

The feature certainly wasn’t there one week ago.

Temperature field for 65 m depth and temperature profile on 14th Feb – OceanMAPS
Temperature exceedence outside the 1st percentile at 65 m depth on 14th Feb – OceanMAPS

We can take a cross-section through the eddy north to south and see evidence of upwelling. That is, the thermocline has been pulled upwards.

Cross-section from north to south of the eddy along 164 E – OceanMAPS

How did it form? Did Grandma Oma suck out all the heat?

It seems so. Digging back through the track map archive we can see that TC Oma hovered over this spot for almost a week! From the 11th to the 18th of Feb. During this time the TC intensified to Category 3.

Tropical Cyclone Forecast Track Map Number 47 issued at 9:44 am VUT on Monday 18 February 2019 – Vanuatu Met Department

In fact, TC Oma sucked out so much heat that the Tropical Cyclone Heat Potential for that area has gone to zero.

Tropical Cyclone Heat Potential – OceanMAPS

Is looks like Grandma Oma has moved on to more fertile waters in the southwest on her path towards QLD.

26 tonnes of pool salt

In the midst of the fresh water deluge in Townsville two weeks ago many thoughts turned to the outflowing rivers and the impact on the Great Barrier Reef.

But spare a thought for the reef that was onshore. The Reef HQ Aquarium. The aquarium is 4 m deep and open to the sky. Over the course of a few days, it received 1.4 m of rain.

ReefHQ Aquarium (Source: TravelNQ)

The salinity was drastically reduced and all fish and coral inside the aquarium were at risk. In a dramatic story about salinity [has such a thing happened before?] staff worked around the clock to increase the salt levels.

According to the ABC article, the only available salt was pillow-sized bags of pool salt. Over 1100 bags were needed. That’s over 26 tonnes!

Reef HQ staff pouring pool salt into the tanks that feed the Aquarium (Source: ABC / Rosie King)

Reef HQ knew to respond quickly after a close call in 1998 during heavy rain when 0.7 m fell into the tank. During that event the salinity dropped from 34 to 28 psu, and although the fish were unaffected that time, a lot of coral died.

This event had twice that amount of rainfall so immediate action was needed. Many local businesses donated use of their trucks and utes to carry pool salt bags to the Aquarium while 10 staff worked day and night pouring out salt during the downpour.

Desperate times mean desperate deeds. – Ashley Finch, Reef HQ

Usually, the Aquarium relies upon water from the nearby Ross Creek to refresh their water stocks with an appropriate amount of salinity. However, the flood waters are only just starting to recede and the creek is still very fresh.

The image below is the 1 km eReefs model from CSIRO that estimates salinity levels immediately off the coast of Townsville at 26.7 psu. This is in stark contrast with waters in the open ocean at 34 psu.

Surface salinity from 1km eReefs model – CSIRO – click to expand

It looks like Reef HQ will need to keep up their supply of pool salt for some time to come.

Bonus question: how many tonnes of pool salt would be needed to restore the whole GBR region to 34 psu?