Sydney SST: The 21 °C break moved 21 km inshore

What the passes show

Passes read

31

The break

21 °C

Step across it

1.0 °C

Seen on

16 of 31 passes

Movement

21 km inshore

72-hour composite

100% of the water clear

31 passes over the water, in order, clouds left as holes (MetOp-B AVHRR, MetOp-C AVHRR, NOAA-20 VIIRS, NOAA-21 VIIRS).

The 21 °C break: a 1.0 °C step across it, seen on 16 of 31 passes. It moved 21 km inshore since Wednesday.

No chlorophyll edge on this line (1.2× across). At 50 m: 20.8 °C under the warm side, 19.0 °C under the cool side; the break reaches 50 m (1.8 °C across) · 1.9 °C at 100 m. Sea surface 18 cm higher on the warm side, 0.9 kn geostrophic current on the line — a current edge.

The break sits 166 km east of Sydney Heads.

Every number is from the satellite passes. Where the water says to look — not where the fish are.

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Water temperature at the grounds

Sea-surface temperature at each ground in season off Sydney, on the 72-hour satellite composite of Nov 10–Nov 13: the mean of the clear water within 6 km of the ground’s charted position. The last column is how far the break sits from it.

A satellite reads the surface of the water, not the water at depth.

A pass, a break, a composite

A pass is one satellite crossing the water once: it sees what the cloud lets it see and leaves the rest as holes. A break is a line the temperature steps across, and it only counts when more than one pass saw it. The composite fills the holes from the other passes of the same 72 hours.

How to read an SST passWhy fish sit on a breakComposite or single pass

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Satellite SST: NOAA/STAR ACSPO L3U via NASA PO.DAAC (subskin temperature)

Chlorophyll: NASA OB.DAAC VIIRS

Subsurface: Copernicus Marine (model)

Sea height: Copernicus Marine DUACS altimetry

Coastline: Made with Natural Earth

Every data source and licence