24th Jan – The heat spreads


SST Anomaly for Australia – RAMSSA

Amidst all the heat onshore at the moment with air temperature records falling like flies, spare a thought for the ocean which is also warming up.

The Tasman Sea warm anomaly is extending westward into the Great Australian Bight due to persistent high pressure over the area. Sea temperatures are 1.5 to 2.0 deg C warmer than normal across most of the GAB. This is worrying.

SST Anomaly for Great Australian Bight – RAMSSA

Media awareness is also slowly spreading. This week an article in The Conversation highlighted the impacts of marine heatwaves on marine ecosystems. The article also mentions the collaboration between BOM and NIWA in seasonal forecasting using the coupled atmosphere-ocean model called ACCESS-S.

As shown in a recent post ACCESS-S predicts that the marine heatwave should decay in late summer. But we could expect that a lot of the heat will stick around longer than that. At the end of the last MHW the surface temps and anomalies also started cooling down by March, but a lot of the warmth had mixed down to depth. Anomalously warm temps persisted well into May 2018 at about 50 to 100 m deep.

Previous ACCESS-S runs did not show a strong positive anomaly in the Bight. However, the latest run does reflect the westward spread of heat. The images below suggest that the surface anomaly will last until at least February and then start to ease in March and April.

Weekly SST anomaly for 20th January from ACCESS-S – BOM
ACCESS-S forecast of monthly SST anomalies – BOM

Cyclone potential

The monsoon trough is approaching Australia and bringing with it some embedded low pressure systems. At least one of them has already spun into a tropical cyclone. TC Riley is tracking along the northwest coast and is expected to reach Category 3 by Saturday. Meanwhile another low near the Gulf of Carpentaria is expected to develop imminently into TC Silvanna.

Analysis chart for 24th Jan 2019 – BOM

Most of this activity is related to the southward movement of the trough and the synchronous timing of Phase 4 of the MJO.

MJO phase diagram for 24th Jan 2019 – BOM

However, let’s look at the ocean heat potential and see how it is contributing. The below image is the Tropical Cyclone Heat Potential calculated from the latest OceanMAPS model run. The numbers show there is support for TC development (ie. it is above zero) but the numbers aren’t excessive.

Tropical Cyclone Heat Potential – OceanMAPS

To get a sense of how much warm water there is we can also look at the depth of the 26 deg C isotherm. Shown below. Most areas north of 20 S have TC-friendly water temperatures down to 50 to 80 m.Depth of 26 deg C isotherm – OceanMAPS

Is this usual for this time of year? In fact, if we look at percentiles at 75 m depth (the lower end of that isotherm range) we can see that temperatures are cool in the west, at less than the annual 50th percentile. So we could surmise that there is less TCHP than there could be.

Exceedence of 50th percentile temperature at 75 m depth – OceanMAPS – click to expand

The positive Indian Ocean Dipole has weakened back to neutral, but it seems that cooler waters persist at depth. Vertical profiles of the 50th percentile exceedence (not shown) indicate that water temperatures are below the mean value between 48 m and 400 m depth. Most of the heat is confined to the surface and the mixed layer depth is shallow (see plot below).

Although, as TC Riley proves, this shallowness is not enough to dissuade TC formation as the monsoon gets going.

Depth of maximum sound speed, an approximate measure for mixed layer depth – OceanMAPS

Sharks in warm waters

Photos of a shark conservationist diving with one of the biggest Great White Sharks ever seen caused a storm of interest on social media earlier this week.

Ocean Ramsey swam with the 6-metre-long shark off the coast of Oahu in order to raise support for the proposed ban on the killing of sharks and rays in Hawaii. Beside the furore about the ‘stunt’ being a dangerous precedent or showing wildlife harassment, you might be quietly surprised to see Great Whites in the warm waters of Hawaii. SSTs there are currently 24 to 25 deg C. Don’t they usually live in cooler waters?

Great White sharks prefer deep waters of at least 50 m, but more usually 500 to 1000 m deep. Most of the Californian white sharks travel westward to forage, but relatively few venture as far as Hawaii, let alone put themselves in shallow waters near humans.

What about Great Whites in Australia?

Studies by CSIRO have shown that there are two distinct populations of white sharks. One group travels between WA and SA, and the other between VIC and QLD and occasionally New Zealand.

Generally these two populations do not venture past their side of the Bass Strait. – CSIRO

Tracks of white sharks around Australia – CSIRO

But in 2017, one white shark was observed travelling from NSW to WA across the traditional divide. Satellite tracking has also shown that sharks sometimes travel between continents rather than just around the coastlines, for example between South Africa and Australia. Previously held theories are now in doubt.

Sea temperatures between southern QLD and southern VIC can range anywhere between 26 deg C in summer and 12 deg C in winter. How do the sharks tolerate such a wide range of sea temperatures? They are endotherms.

White sharks possess a heat-exchanging circulatory system which allows them to maintain a body temperature up to 14° C above that of the surrounding sea water, enabling them to tolerate a wide range of ocean temperatures. – Shark Smart, WA Government

Such uncanny ability to stay warm doesn’t mean that shark activity is unrelated to sea temperatures. One University of Sydney academic, Chris Neff, says that every fatal shark attack in WA has occurred when the water temperature is between 18 and 20 deg C.

There is evidence that sharks move with the seasons. The Spencer Gulf in South Australia is recognised as a popular area for sharks in autumn, winter and spring, and then according to IMOS satellite tracking they tend to move away from the Gulf in summer as sea temperatures rise. However, shark migration is not as predictable as that of whales and much is still unknown about these creatures.


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