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OCEAN OUTLOOK

A weekly discussion about the latest ocean features around Australia. This is
not an operational product and is not for public release.

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13TH APR – SHIFTS TO SHORE


STORM SURGE AND ANTARCTIC SWELL

This weekend hailed a tropical cyclone dance on the mid-north WA coast, while
massive waves impacted SA, VIC and TAS coasts. Both events honed in on regions
where marine heatwaves have stretched over months.

TROPICAL CYCLONE SEROJA (AND ODETTE)

Tracking and human impact of TC Seroja, early April 2021 (Source:
https://erccportal.jrc.ec.europa.eu/)

A tropical low developed in early April in the Savu Sea. After wreaking havoc in
Indonesia and Timor-Leste (more than 80 deaths and thousands displaced), it
tracked southwest, strengthening into TC Seroja on 4 April.

TCs Seroja and soon-to-be Odette off the coast of WA
(https://www.severe-weather.eu/tropical-weather/fujiwhara-effect-cyclone-seroja-odette-australia-landfall-mk/)

With a second nearby tropical low also intensifying into a cyclone (TC Odette),
fingers around the world typed Fujiwhara effect into Google, and forecasters had
their work cut out trying to predict how the two would dance around each other.

 * 
 * 

Tropical cyclone tracking was complicated by the interaction of Odette and
Seroja (Source: BoM).

After interacting with Odette and then strengthening, the Category 3 Seroja
crossed the coast at speed just south of Kalbarri after destroying Carnarvon’s
One Mile Jetty . The Aqualink buoy offshore of Shark Bay recorded the waves
built up by the storm.

 * 

The National Storm Surge model indicated a surge component of around 2.5m at the
furthest inlet of Shark Bay.

 * 00:00 UTC
 * 06:00 UTC
 * 10:00 UTC

National storm surge model, Shark Bay, 11 April 2021 (Graph below for X) Sea
level rise due to TC Seroja (X marks the spot)

Geraldton’s water level rose well above the highest astronomical tide of 1.134m.

Geraldton sea level (source: Transport WA)

The reported damage so far (especially in Kalbarri) has been significant, in
part because a cyclone across land this far south is relatively unusual, so
buildings are not built to withstand one. While the course of Seroja was
ultimately determined by the interaction with Odette, the Ningaloo Niño (a
months-long marine heatwave in the western waters) played a part in
strengthening both systems, particularly Seroja, as it tracked south over sea
surface temperatures 2-3 degrees above average.

THE BEGINNING OF THE END OF THE HEATWAVE?

Having given some heat to the cyclones, do we now see signs of relief from the
heat? Yes! At least, on the top. While still warm, the surface anomalies are
much reduced, even as far south as Perth.

 * 
 * 

Sea surface temperature anomaly on 6 April and 11 April, after the cyclone had
made landfall

Notably in those charts above, we also have a reduction in the sea surface
temperature south of Tasmania, where waters had similarly been languishing in a
long-lasting heatwave. This reduction in heat is thanks to a cold Antarctic air
mass that swept in from the southwest during the weekend, generating a large and
intense swell with a long fetch.

Forecast hazardous surf chart for Sunday morning (EC Marine)

The Bureau’s Cape Sorell Buoy recorded waves of over 16 metres, with a period of
22 seconds.



While the VIC and SA coasts didn’t see waves quite as big, the highly energetic
swell left its mark. 11 metres recorded at Point Nepean, VIC, and 12 metres
recorded off Kangaroo Island in SA.

 * 
 * 
 * 
 * 

Rocky road: the aftermath of a large swell event, Port Fairy, VIC, Sunday 11
April (source: Meredith Carracher)

Another cold front is now pushing through the bight, which will further
accelerate the seasonal deepening of the mixed layer and possibly bring further
cooling to the Tassie waters. Watch this space!

 * 
 * 
 * 

Autumn deepening of the mixed layer, 8-19 April (OceanMAPS)
Author userPosted on April 13, 2021April 13, 2021Categories UncategorizedTags
MHW, storm surge, wavesLeave a comment on 13th Apr – Shifts to shore


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