• A cut-off low generates a coastally-trapped wave and flooding
• Mixed layer becomes shallow in the south
• SSTs heating up in the southeast

SST

The warm anomaly in the southeast has expanded out into the Tasman Sea. This is being enhanced by a warm eddy at 33S 157 E that has developed at the end of the main EAC flow.

Further to the south, warm water is still pushing south and westward along the NSW and VIC coast like we have seen since mid October.

In WA the sea temperatures remain cooler than normal, with a localised cold spot just to the south of Cape Leeuwin at 2 to 2.5 deg C below normal.

 

A cut-off low

A small but intense cut-off low pressure system over southeastern Australia has caused extreme weather in recent days. SA, VIC, NSW and TAS have all been impacted. Dust storms, wild winds and flooding have caused chaos for airports, fire-fighters, and emergency services. A heat wave in QLD contrasts with thundersnow in VIC. For the latest video update from the BOM, see the YouTube channel.

Let’s look at the ocean features associated with this system.

first impacts in south australia

The South Australian coastline was the first to be affected on Thursday the 22nd of November with strong winds, high sea level and large waves.

At Kangaroo Island (Cape Couedic) significant wave heights of 7 m were recorded, with maximum wave heights up to 12 m !

Behind the system, cold air filled in over the continent causing large negative air temperature anomalies. Most of NSW and VIC were 6 to 10 deg C below normal.

a coastally-trapped wave

The aggregate sea level viewer is showing alerts right around the southern coast lines as strong winds push up inshore water levels.

The system will generate a coastally-trapped wave of positive anomalies that will travel anti-clockwise around the coast.

The below image is a Hoevmoller of sea level anomaly along the coastline against time. The red diagonal stripe shows a positive anomaly generated west of Ceduna on the 20th of November, and then travelling to Portland by the 23rd and continuing around the corner to Eden and Sydney on the 24th.

To see an animation of this image click the thumbnail below.

 

flooding at lakes entrance

As the low pressure deepened and move east of the Bass Strait today, elevated sea levels were expected at Lakes Entrance. The aggregate sea level viewer expected up to 0.9 m above datum. Late this morning, the measurements were 0.83 to 0.86 m.

To see what impact the sea level can have on Lakes Entrance region, have a look at Monash University’s Lakes flooding visualisation tool. (Click on the left hand side to simulate higher water levels).

Minor flooding levels were reached at Cunningham Arm.

Ben Hague from BOM’s climate team made the following assessment:

• The modelling of storm surge on the sea level viewer was excellent: if very slightly overdone.
• Breakwater and Bullock Island thresholds/levels are essentially the same, which helps to apply to modelling from the sea level viewer to the Lakes Entrance township and should give us more confidence in communicating risks in future similar scenarios where guidance suggests a storm surge of sufficient magnitude (e.g. approx. 20cm above HAT).
• The briefing email to SES resulted in SES tweeting (and picked up by the Local ABC) advice that storm surge could result in some minor flooding.
• The storm surge alone was enough to cause some impacts without any fluvial influence, this has implications for future warnings/advice/briefing for inundation, especially as sea levels continue to rise and the required anomaly (i.e. surge) required for these events decreases, and for a given event strength impacts become more severe.

Wave heights were also elevated today, with significant waves of 2.5 m and maximum waves up to 5.5m.

sound speed

In the wake behind the low pressure system the surface layer will actually become shallower. A high pressure ridge is expected to establish across Australia’s south and cause very light winds for the next 4 to 6 days. With minimal momentum flux and positive net heat flux, the top of the ocean will warm and become stratified from the layer below.

Using depth of maximum sound speed as a proxy for mixed layer depth (MLD), we see:

• In the first image, immediately after the low has crossed, there is a MLD of 20 to 40 m through much of the Great Australian Bight and to the west of Tasmania.
• In the second image, once the high pressure ridge moves in, the MLD has become much shallower at 0 to 10 m.

 

Below is a map of momentum flux for the 26th. These are the values from Access-G that are used to force OceanMAPS. Notice how it close to zero under the ridge.

Comparing three sound speed profiles across the region – we can see a shallow layer has developed at the surface. Below this is a weak duct from 30 m to 170 m, and the deep sound speed channel below 200 m.