9th Aug – Surge and snow

SST update

SST anomaly for 7th August – RAMSSA

The anomalous heat off NSW not only continues but has strengthened. There is now a large area 3 degrees above normal for August.

Concerningly, the seasonal outlook shows the warn anomaly along the NSW coast persisting for the rest of the year

ACCESS-S sea surface temperature anomaly forecast – BOM

Surge in the south

The perfect storm surge predicted a week ago has not eventuated as expected, but it is still packing a big punch.

The large low in the Bight idea has changed into a fast-spinning multi-centred set of lows that are set to consolidate into a deepening system to the east of Tasmania on the weekend.

MSLP forecast for Thursday night and Friday from ECMWF global model – BOM

Media outlets are flooding the Bureau with requests for information on the the hazardous weather. Snow levels are forecast to drop to 500 m on Friday and Saturday. Blizzards, avalanches and tornadoes are all possible. Flights have been cancelled in Sydney as Gale Warnings have been issued. In fact, there are Gale Warnings current for four states today: SA, VIC, TAS and NSW.

ABC Online

The storm surge is large.

Firstly, let’s look at the sea level anomaly. The image below shows a big surge into the Gulfs of South Australia. This is a 24 hour average, so the anomaly at particular times will be even higher.

Sea level anomaly forecast for Thursday 8th and Friday 9th August. 24-hour averages – OceanMAPS

The next factor is the waves. In South Australia the waves were largest last night (Thursday).

Significant wave height for 0800 UTC on 8th August – AUSWAVE-R

The National Storm Surge product predicts that areas on the western parts of Kangaroo Island and southeast of Adelaide could see sea levels reach 0.70 m above HAT. This includes a surge component of 0.5 to 1.0 m and a wave setup component of 0.4 to 0.8 m. In places, the total tidal residual is expected to be 1.2 m.

Luckily we are at neap tides at the moment, or is could be worse.

Storm surge forecast for Victor Harbour in SA, based on model run 7th August – BOM

How are the observations stacking up ?

Port Pirie tide gauge readings – National Tidal Centre
Victor Harbour tide gauge readings – National Tidal Centre

Yesterday Port Pirie measured a tidal residual of 1.3 m while Victor Harbour recorded 1.0 m. That’s pretty good agreement with the forecast.

In Port Phillip Bay, the worst is today (Friday). Westerly winds of 40 to 50 knots have battered the area and pushed water levels up on the eastern side. Wave setup and overtopping are also taking their toll.

Gale-force westerly winds impacting the Bayside bike path – BOM/AndrewWatkins

The Frankston Pier has been badly damaged with bits of it floating away, and the Bayside shoreline is almost impassable.

Parts of Frankston Pier washed away into the Bay – ABC/ChrisRansom

Model guidance suggests that the surge component in the Bay would only be 0.4 m.

Unfortunately, right at the critical time, tide observations in Port Philip Bay are not coming into the Bureau. The link is broken somewhere along the line.

The only observations available are those through Vic Ports at Hovell Pile. We can see that the tide isn’t falling like you would expect, but staying elevated.

Tide measurement at Hovell Pile on 9th August 2019 – Vic Ports

The true residual measurements will have to come when the winds die down and Melbourne clears up the wreckage.

In the meantime if you are in the southeast of the country, stay warm.

An unusual JJVV

Hand-held XBT. Source: Turo

HMAS Arunta recently measured an unusual temperature profile. It came from XBT data that was sent ashore in ‘JJVV’ format.

Here are the strange measurements in question:

For those of you that don’t speak JJVV, it says that the measurement was taken at 31.9 S 115.5 E at 2246 UTC on 17th July 2019. ie. off the coast north of Rottnest Island in the early hours of the morning.

The XBT recorded the following ocean temperatures with depth:
19.5 C at 4 m, 19.5 C at 32 m, 19.3 C at 38 m, 19.5 C at 89 m, … then a gap,… then 19.4 C at 247 m, and 19.8 C at 251 m.

The strange thing is that the temperature seems to increase with depth.

Recent studies onboard the RV Investigator in the Leeuwin Current  region showed evidence of subsurface intrusions. Cold fresher water to the south intruded in shallow layers on warm saltier water from the north. Overall these thin intrusions were density-compensating. (Jessica Benthuysen – Ocean Chart Discussion).

Is this what was happening? XBTs only show temperature, we can’t see the salinity changes.

The JJVV occurred fairly close to the shore. The OceanCurrent snapshot SSTs for that day show cool inshore water from winter cooling. There was also a cold front and cloud passing over the area at the same time.

Left – OceanCurrent 4-hourly SST image from 18th July at 12 UTC with a cross showing XBT location. Right – MSLP analysis just before the time of the XBT.

Perhaps the cool surface water was only recently cooled, and/or was fresher than the warmer Leeuwin Current below.

OceanMAPS analysis doesn’t seem to pick up on this level of detail but does indicate slightly warmer water with depth.

Temperature profile for 31.9 S 115.4 E on 17th July 2019. Note the scale on the x axis which amplifies small differences – OceanMAPS
Salinity profile for 31.9 S 115.4 E on 17th July 2019 – OceanMAPS

Those of you with keen eyes would have noticed that the telltale dots of a glider on the OceanCurrent SST image.  An IMOS glider was sampling the waters very close by.

IMOS glider at Two Rocks. Black and red circles showing area of interest. Black cross showing XBT location – IMOS OceanCurrent

The glider showed a temperature boundary near the shelf edge, sampled between the 14th and 16th. The shallower water is cooler, no doubt due to latent heat loss during winter months. Differences in salinity are harder to see.

Putting on our Poirot detective hats, the first theory that springs to mind is that the cool water at the surface was being compensated by slightly warmer slightly saltier water at depth, perhaps intruding right on the edge of the shelf.

But there could be another suspect in this case.

Depth. The location of the JJVV at 115.48 E is well within shelf. The bathymetry only goes to 50 to 80 m there.

The JJVV had measurements at 271 m.

Ah ha. Instrument error? Maybe all measurements below 200 m were erroneous and there was no temperature increase in depth, simply a well mixed layer to 89 m.

Or did the XBT slide 14 nautical miles to the west and fall into the Perth Canyon?

What do you think? Put on your detective hats and please leave a comment below.

4 thoughts on “9th Aug – Surge and snow”

  1. I don’t think XBTs measure depth, but infer it from time. If the system fails to stop the clock then it doesn’t realise the XBT is sitting on the bottom, where it records essentially constant temperature.

  2. Re the storm surges happening now:
    You mentioned that the damage will be less than it might be, because it is neap tides.
    That is true for SA, but not Bass Strait, where, interestingly, it is springs. So hopefully the model is correct that the surge there will be less than in SA.


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