4 Oct – Spring bloom in Chlor-a


No much change in the SST anomalies this week. The North West Shelf is neutral and it is largely cool elsewhere. The exception is NSW where there are large positive anomalies due to a southward push of the EAC.

SST Anomaly – RAMSSA

Looking at this region in a bit more detail, a large warm eddy has consolidated off Sydney and pushed inshore. The temperature at Sydney beaches is now up to 18 deg C (compared to 15.5 deg C only a month ago). Offshore the temp are 20 to 21 deg C.

SST and surface currents for NSW, forecast for 5th and 10th October – OceanMAPS

Ocean Colour

The “Spring Bloom” is here: a maximum in chlorophyll biomass that occurs in late September-early October for south-central Tasman Sea (Longhurst 2007).

Indeed, the Tasman sea contatins

the largest noncoastal surface chlorophyll-a concentrations within the South Pacific Ocean

(Tilburg et al 2002)

A few days ago satellites capture a ring of chlorophyll to the east of Tasmania around a cold-core eddy.

Chlorophyll-a image for 27th September from MODIS, arrow pointing to ring of chlorophyll – IMOS OceanCurrent

The cold-core eddy can be seen in an OceanMAPS cross-section of temperature, below. In the eddy the mixed layer depth is shallower and nutrient rich cool water reaches the surface, while around the outside of the eddy is slightly warmer water.

Temperature cross section west to east along 40.1 S showing cold core eddy – OceanMAPS

The rapid increase in chlorophyll can be seen by comparing the satellite image above to one taken just one month ago. See below. Average chlorophyll values at the time were only around 0.3 mg m-3.

Chlorophyll-a image for 24th August from MODIS – IMOS OceanCurrent

Strong westerly winds during the winter deepen the mixed layer which entrains cool, nutrient rich waters, however at that time of year sunlight levels are low. In spring, the winds ease, the mixed layer becomes shallower, and the available sunlight increases. This leads to a rapid increase in phytoplankton. Typical values during the Spring Bloom are 0.8 mg m-3.  See the image below for the annual seasonal cycles (Tilburg et al 2002): the average mixed layer depth decreases from 150 m to 75 m over the course of September. Meanwhile, chlorophyll-a concentrations jump.

Annual cycle of chlorophyll-a concentration and mixed layer depths, averaged for Tasman Sea. Comparison of MLD with North Pacific and North Atlantic basins – Source: Tilburg et al 2002

The research vessel RV Investigator is just concluding a 28-day study of “ocean productivity” along the south-east Australian Coast. The scientific objectives include an assessment of chlorophyll. The voyage plan notes that

In springtime, large phytoplankton blooms can be seen associated with a mingling of
nutrient-depleted EAC waters with nutrient-rich, but iron depleted Southern Ocean waters in the
vicinity of the subtropical front east of Tasmania

(Marine National Facility, Voyage Plan, 6 Sep 2018).


Bridge at Palu damaged by the tsunami – Source: National Geographic

Last week an earthquake and tsunami struck the island of Sulawesi in Indonesia. The death toll currently stands at 1400 but is expected to increase. The devastation and loss of life has been covered extensively in the press; however, the facts about the tsunami warning system have not always been correctly reported.

Articles have erroneously suggested that:

  • Sensors were inadequate and stuck in test mode (when in fact there were no buoys nearby, even broken ones)
  • A more advanced system would have provided earlier warning (in fact a warning was issued within 9 minutes and deep water buoys – even in the right location – would not necessarily have made it earlier)
  • A measured 6 m wave was reported as insignificant (it was in fact 6 cm that was measured at the gauge)

Area affected by the earthquake and tsunami, showing ‘strike-slip’ earthquake – ABC online


In discussions with the BOM tsunami team, the facts understood so far are:

  • The earthquake struck at 1002 UTC on 28th September. It was a  magnitude 7.5 event and occurred 78 km north of Palu.
  • The earthquake type was a ‘strike-slip’ where the plates slide horizontally along a fault line. This type does not usually cause a tsunami. Experts say that there might have been an underwater landslide to generate the large waves.
  • The official warning issued at 1011 UTC can be found here.
  • The first wave impact at Palu was recorded at 1022 UTC and anecdotal evidence and videos point to waves up to 6 m high, particularly at Donggala at the mouth of the bay.
Infographic showing likely maximum wave heights in Palu Bay. Modelled data without including any landslides – Source: New York Times
  • The nearest sea level sensor at Mamuju was 300 km away to the south and out of direct line to the earthquake site. When it recorded a 6 cm wave at 1027 UTC, BMKG cancelled the tsunami warning. BMKG were unable to contact the town of Palu to get observations (because the power and phone lines were down).
  • The Makassar Strait is in a gap between Indian Ocean Tsunami Warning and Mitigation System (IOTWMS) and the Pacific Tsunami Warning and Mitigation System (PTWS).  These international regions each have multiple warning providers. But in the gap, Indonesia takes sole responsibility. BMKG operates the observation network and tsunami warning system for this area.
Cut-out of Existing Tsunami Warning Systems, with blue circle showing the location of Sulawesi – Source: International Tsunami Information Centre


More details will emerge as this event is studied around the world.


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