OceanCurrent News Archive
Sydney-Hobart yacht race: ocean currents looking very favourable this year
20, 22, 23 December 2014
23 Dec: There is very little new information since yesterday, but what there is suggests that the eddy SE of
Jervis bay is slipping southwards. A third (since 8 Dec) drifting buoy is passing by Montague Island
just outside the 200m isobath, slightly slower than the previous two, suggesting the flow speed has
reduced, and/or that farther offshore is where the strongest southward speed is to be encountered.
Our imagery will continue to update (unless something goes wrong in our absence). Our Google Earth feed
allows you to overlay the
CYCA yacht position data
on our maps of the ocean currents. The screen-grab at right (a view looking towards Tasmania from Sydney)
is what you see if you do this today.
22 Dec: The latest satellite data indicates that the two eddies mentioned in our 20 Dec post have indeed
merged, and now form a very elongated eddy whose northern extreme is about 75km off Jervis Bay. Its southern
extreme, which is about 400km south, is less distinct because the eddy is partially attached to a third eddy
(also anticlockwise-rotating) to the east, centred at about 39S 154E. This is most clearly seen in the
larger-scale sea-surface height map for SE Australia. That map also shows
the points where we have sea level data. These are quite sparse, and non-existent between the eddy centre
and southern NSW. Evidence that our interpolation procedures have correctly estimated the eddy's shape is
provided by the tracks of two drifting buoys which first did loops around the unmerged eddies, and are
now looping around the merged eddies.
What should yachts do to make the most of the current? That's the navigator's job to decide. One thing is
clear: ocean currents change significantly from day to day, so there can be a big difference between the
information available pre-race (which can be several days old) and what the yachts experience. For example,
last year the map for 18 Dec showed favourable currents along
the southern NSW shelf. We now know, however, that by 27 Dec
the flow had stopped or even reversed. To see how that happened, view the animations available via the [DATE INDEX]
link on those pages.
20 Dec: It's possible that the ocean currents will give yachts more of a boost this year than in
any of the last ten years. Indeed, if the current holds steady for the next week, which is possible
but certainly not guaranteed, the Sydney-Hobart yachts could benefit from about 2kt of southward flow for
about a third of the distance.
We had to look as far back as 2004 to find a
year when the currents along the rhumbline appeared to be as favourable. But don't take our word for it. See
for yourself how present conditions compare with
As indicated on the map for today (20 Dec),
our most-recent velocity estimates are valid for 15 December, a full 11 days before race day. The situation
could become different by 26 Dec, especially considering that the favourable currents showing at present are
associated with two anti-clockwise rotating eddies that are very close to each other. These may merge or they
could move with respect to each other, possibly leaving a counter-rotating eddy somewhere along the rhumb line.
The navigators know this, and will be watching the imagery right up until race time.
Estimating the drift of dead whales
12 November 2014
A dead humpback whale was about to come ashore on Perth's Cottesloe Beach yesterday, in what might have
repeat of the cause of last week's $170,000 clean-up exercise on Scarborough Beach. Referring to the
right [zoom in]
[zoom in, SST],
and taking the forecast winds into account, Prof Pattiaratchi advised authorities that towing the whale
offshore of Rottnest Island would result in an even chance that ocean currents would take the carcass
far as we know, this is the first time that IMOS infrastructure has been used for an application of this
Media coverage: au.news.yahoo.com/thewest/video/watch/25487737/concerns-for-whale-carcass
East Australian Current 'reversals' off Brisbane - 3 years in a row in June-July
3 Sep 2014
The Brisbane region is where the flow along the continental slope is most consistently fast and to the
because of the East Australian Current, right? Well, yes, but sometimes the flow along the slope does
northward (e.g. as we discussed with reference to the fate of AHS
fact, it has now done this in three June-July periods in succession, the most
recent one resulting in some striking SST imagery. The imagery shows that the EAC is not
reversed - its just displaced offshore.
Data from the IMOS EAC current meter array, which was deployed to measure the transport of this most
important of Australia's ocean currents at a critical location, are now
and not surprisingly the data are rich with interesting features. One of these is a month-long anomaly
June-July 2012 that was an example of the EAC not flowing along the continental slope. This event was
monitored in real-time by satellite observations of low sea
low surface temperatures and high chlorophyll, which all show that the flow of the EAC was displaced
offshore by a cyclonic eddy. What the current meter data (e.g. from the ADCP at 80m in 2000m) reveal is
7 June the flow velocity off Brisbane suddenly dropped from
normal values to near-zero, where it stayed until 11 July.
that period of near-zero flow along the 2000m isobath, the flow farther offshore at the other instrument
moorings swung from shoreward on 21 June to northward on 2 July (as shown at right) then to seaward on 4 July before turning south on 11 July and accellerating to
speeds in excess of 1m/s by 15 July. The satellite data show that this sequence of events can be
by the movement of the cyclonic eddy that in early June was centered more than 100km NE of the array (at
26.4S 155.4E) but then moved west and subsequently south, passing through the array in early July then
continuing along the continental slope.
Cyclonic eddies also appeared inshore of the EAC in October 2012 (e.g. see 25 Oct although on this occasion the eddy was quite small)
in the next two years (e.g. in 2013: 4 June, 25 June and see also the chlorophyll
image, and in 2014: 15 July as mentioned above).
These reversals are important for a number of reasons. To mention just two: 1) they are responsible for
large amount of cross-shelf exchange (as can be seen in the chlorophyll images as well as the
velocity timeseries) with potentially major consequences for transport of fish larvae and everything
the water - near Brisbane and farther south, and 2) turbines harvesting energy from the flow would be
Where are Australia's fastest ocean currents?
6 Aug 2014
This honour probably goes to one of the passages of the Buccaneer Archipelago. Sunday Strait, for
the main shipping route into King Sound and the town of Derby. Hydrographic chart AUS 733 notes that
of ~10kt (~5m/s) occur there. We are unaware of any instrumental measurements to verify that estimate,
this observation: At 1630WST on 29 July 2014 (close to the time of maximum spring ebb tide), it took two
outboards running at 2700rpm to keep the Giant Tides Tour RIB stationary in Escape Pass as dramatic whirlpools swept by. From this, the skipper estimated the
to be 9kt.
Malaysia Airlines MH370: estimated drift of buoyant wreckage 4 July 2014
Australia coordinated the search of the sea surface for wreckage of Malaysia Airlines flight MH370 from
to 28 April, 2014. The Drift Working Group assembled by the Australian Maritime Safety Authority used
techniques, throughout the search, to estimate the drift of potential wreckage as part of the
the conduct of search operations, which involved as many as 12 aircraft and 12 vessels from a number of
The focus of efforts shifted north-eastward along the '7th arc' from early March when it was thought the
could have flown as far south as 45S, to 21S where the flight recorder's pinger signal was thought, in
mid-April, to have been detected by Ocean Shield.
The Australian Transport Safety Bureau's June 26 report "MH370
- Definition of Underwater Search Areas" shows what is presently believed to be the most likely
region of the crash. The panel at right shows two estimates of where three types of debris might have
distributed on 18 March when the search off Australia commenced. These simulations use the ATSB estimate
the 8 March crash region, and
trajectory modelling based on the Bureau of Meteorology winds and ocean current (left panel), and IMOS
surface current (right panel). The red, black and blue dots represent items with three degrees of
to the wind, drifting downwind at 2.8%, 1% and 0% of the 10m wind velocity, in addition to the two
of the velocity of the water. By 28
March, when the estimated location of the crash site was close to the updated location used
our 90km x 700km rectangle of tracked particles is dispersed into a region of approximately 300km x
That 'drifted region' was searched until 4 April and again between 15 and 28 April (from 5-14 April the
crash site was thought to be farther north, where potential pinger signals were detected). At the end of
search on 28 April, the tracked
particles in these two simulations, combined, occupy a 1500km x 1000km expanse of ocean.
Could debris from the aircraft, if still afloat, wash up on the coast of WA? According to our
this is indeed possible, but very unlikely. A few of the 2700 tracked particles are now (29 June) estimated to be at
edge of the continental shelf. Winter westerly winds could conceivably bring those ashore. The vast bulk
the tracked particles, however, are 900-2500km west of Australia.
Pacific Ocean cool surface 17 March 2014
For several months now, the surface temperature of the Pacific Ocean between New Caledonia and Australia
been a degree or more cooler than usual, as shown at right (click to enlarge). The satellite temperature
is rich in detail but it is far from being the whole story. Firstly, note how the cool patches are all
as rotating clockwise. We infer this rotation from the sea level map which features depressions at those
consistent with a positive anomaly of the depth-integrated density, which could be caused by lower
and/or higher salinity. One hypothesis worth exploring is that reduced rainfall in that region has
surface salinity (by failing to balance evaporation). This reduces the near-surface gradient of density,
allowing the wind to keep the thickness of the mixed-layer deeper than usual, thereby keeping the
lower than usual. This hypothesis may explain the measurements by Argo float 5902084, which sampled a
near-surface salinity anomaly of +0.25 in the
100m, but it does not explain the measurements by nearby float 5904257 which sampled a more-normal vertical profile. Here, the
and isohalines are shifted down by about 100m, thus reducing the depth-integrated density anomaly and
the surface elevation, even though the temperature anomaly at the surface is zero or even slightly
This downward shift could only have been caused by a local accumulation or convergence of EAC waters,
might result from the net transport southwards along the shelf being temporarily reduced. The question
this advective deepening of the surface layer was not accompanied by any increase of surface
Rottnest Channel Swim: Don't expect the usual northward current flow 20,21
Recent satellite and HF radar observations, and forecasts of wind, waves and ocean currents (at right, click to
indicate that conditions for the 2014 Rottnest Swim (22
contribute to relatively fast times.
Modelled ocean currents from the Oceans Institute of the University
Western Australia indicate that the recent and persistent shift to an easterly air stream over
southern West Australia will produce anomalous ocean current conditions:- a weak (<0.2m/s) southward or
southwestward flow (becoming weaker or slightly northward towards the end of the race) rather than the
northward currents that are common at this time of the year.
Wave observations and
forecasts indicate the swimmers will have following seas (generated by the local easterly winds).
The strong northward flow last week and more recently brought cooler
water (21-22C) to the surface north of Rottnest Island, and this appears today
to be coming south and into the race area. The image at right also shows the surface current estimated
the two IMOS HF radar systems near Perth; the northern one in magenta and the southern one in red. These
estimates (which are also shown on mapswim) are least accurate at
edges of the coverage (as evidenced by the disagreement where they overlap) and are subject to many
of interference. The Rottnest channel is at the very edge of the coverage, so the north/south component
the flow is very uncertain.
FFI, or to provide feedback, contact Roger Proctor 0400 153 449 or
Extreme Bonnie Coast Upwelling 12 February, 5 March, 13 June 2014
Winds associated with the combination of low atmospheric pressure over SE Australia and high pressure
Tasmania around 4 February 2014 caused an extremely strong upwelling
stretching from the Eyre Peninsula to Portland, so this season can certainly be characterised as one
started early (see below) and included at least one extreme event. The surface velocities associated
upwelling were measured by both the SA-IMOS HF radars, as shown at right (click to enlarge), confirming
expectations from basic theory and numerical modelling that the flow is essentially alongshore in the
region, with just a small component directed offshore to drive the upwelling. The dynamics is very
and more complex, outside the continental shelf in the deep ocean, where eddies exist and the
current is not constrained to follow the coast.
5 March update: Strong upwelling also occurred around 21 Feb to
13 June update: Now that MODIS imagery is available again, we can see the impact on 8 March on
of the upwelling, at right.
Ocean current analysis for the Sydney to Hobart yacht race 22 December
This year's fleet will negotiate three significant ocean eddies. The first, seen at right south-east of
Bay on 20 Dec (click to enlarge), is unfavourable, but may dissipate or move farther offshore between
Boxing Day. Skippers will want to keep inshore of this, other factors being equal. The 2nd is more
Well-exploited all the way from 36S to about 39S, it could give yachts a significant advantage (1-4h)
that do not find this river of southward flowing tropical water. Finally, an isolated warm-core eddy is
Tasmania's Freycinet Peninsula (42S), potentially giving a small boost to yachts keeping a bit farther
Bonnie Coast upwelling: a strong, early start of the 2013-14 season 21-28
The first upwelling of the 2013-14 season occurred from 11-17
strong south-east winds for several days. This was a strong event, especially for so early in the
the NOAA satellites seeing upwelled water at 12-13° C, the HF radar seeing surface flows of 0.3m/s
and tidegauges seeing coastal sea level depressed by 0.2m. This occurence of an early, strong upwelling
November is in contrast to the lack of strong upwelling until March during the previous
summer. The event is quite well simulated by the Bluelink relocatable model animation of hourly surface t and v which shows
agreement with the HF radar data, and an upwelled plume of 15° C water.
22 Nov Update: A run of the model from 1 Nov
Nov forecasts that strong upwelling will continue with colder water continuing to come to the
25 Nov Update: The latest few images (up to 19Z 24 Nov
vindicate the forecast, showing a plume of water at the surface that is even colder than last week (although
seasonally-changing colourbar does exaggerate this a little). Some frontal features appear to have shifted
8km over 3h, or 0.75m/s, suggesting the flow is more energetic than forecast. The imagery also suggests that
cold water is surfacing north of Kangaroo Island, and west of the Eyre Peninsula, where the HF radar has
recorded flows of about 0.4m/s to the NW. It will be a few days before we see if the geostrophic velocity
agrees with this, and somewhat longer until we see what the IMOS ADCPs and CTDs have recorded. An Argo
on 21 Nov confirms that the offshore
was close to normal at that point, with the upwelled 12° C water encountered at 250m.
26 Nov Update: From Peter Gill: The first pygmy blue
sightings for this upwelling season were reported south-east of Portland a week or two ago. Continued strong
upwelling may draw larger aggregations of blue whales in coming months. Last year's season was certainly
characterised by fewer blue whale sightings and weak upwelling.
28 Nov Update: HF radar surface velocity vectors from the Bonney Coast CODAR units are now
Throttling of both the Leeuwin and East Australian Currents 13 November
By chance, the southard flows of both the Leeuwin Current (LC) and the East Australian Current (EAC) are
presently being strongly impeded by eddies near the continental slope. On the west coast, the opposing
from an anticyclonic (anti-clockwise rotating, warm-core, high sea level) eddy that is diverting the LC
offshore, while on the east coast it is a cyclonic (clockwise, cold-core, low sea level) eddy that is
a similar effect. In both cases, some of the flow continues southward after completing a detour around
but it is clear that there are impacts downstream (incuding the end of the warm spell off NSW discussed
month) when these diversions occur, as well as local impacts inshore of the eddies where there is
less tropical water. Both eddies have been sampled near their centres by Argo profilers. The 70m
displacement (compared with climatology) in the Ningaloo eddy is in good agreement with our
satellite-based projection, as is the 100m upward displacement in the Brisbane eddy.
Unusually warm winter waters off NSW 14 October 2013
waters over the NSW continental shelf and nearby in the Tasman Sea have been much warmer than usual
August. Shelf waters off the Sydney-Woolongong shelf are presently 22°C or more in places as shown
about 4°C warmer than usual. A few Argo floats have sampled
anomalous conditions at key locations, confirming the satellite estimates and showing that the warm anomaly extends down to 600m at places.
usually the case, however, the warmth of the regions directly affected by the East Australian Current is
contrasted by a nearby cold anomaly that
3° between depths of 300 and 500m in the centre of a cyclonic eddy farther offshore. The cold
great at the surface, so its location is less obvious in the thermal imagery. It showed up more in the
data, as a sea level depression exceeding 0.6m in August.
Highlights of the ANMN regional (shelf) ADCP data 2 July 2013
Today's Technical News Item announces an update of our time-series
page showing Acoustic Doppler Current Profiler data. Below are some examples of what these data
about events in the ocean, as a continuation of the list below.
Southern GBR, April 2012: The strongest non-tidal current measured east of Heron Island occurred on
13 April 2012. This was associated with
the high sea level event discussed earlier, adding to the list of locations impacted
by that wave.
Harbour, Feb 2013: Very strong upwelling and northward flow on the continental shelf near Coff's
occured around 2 Feb 2013, as a small, intense cyclonic feature grew and then 'rolled' south down the coast.
Two weeks earlier, the East Australian current was flowing as a
nearly-laminar jet along the continental slope and shelf between Fraser Island (25S) and Newcastle (33S),
where it started flowing around a large warm-core eddy forming off Sydney.
Cooler nearshore surface temperatures indicate that coastal upwelling was occurring from Stradbroke Island
(27.5S) southwards, e.g. near Coffs, but especially south of Port
Macquarie (31.5S). On 29 Jan it is clear from the (patchy) imagery
that the upwelling had greatly increased following strong upwelling-favourable winds associated with a high
pressure system centered on New Zealand and a low over Queensland from 24-28
Jan. The CH070 and CH100 ADCPs recorded the low bottom
temperatures (17C) on and prior to 29 Jan, offshore-directed surface-layer flow to the SE on 28 Jan(i.e.left of the wind as expected), then a strong reversal of
the flow to the north on 2 Feb which the HF radar shows to be
associated with a small cyclonic feature centered over the 1000m isobath. This little eddy remains within
the coverage of the radar until about 5 Feb when it gets swept south.
the physics of the eddy's formation is a fascinating combination of 1) geostrophic uplift of the isopycnals
associated with the (essentially laminar) EAC, 2) ageostrophic uplift to balance the wind-driven surface
Ekman transport, and 3) transfer of kinetic energy and cyclonic vorticity from the laminar flow to an
isolated feature. This is a mechanism of first-order importance to across-shelf transport, so it is
important that we obtain a more detailed understanding of it for many reasons.
shelf, May 2011, 2012 and Sept 2012: Current speeds in excess of 1m/s were recorded by the southern array of ADCPs on 21 May 2011 [map view] and 5 Sept 2012 [map view] at the SAM5 mooring in 95m off the Eyre Peninsula and
on 10 May 2012 [map view] at the SAM3 and SAM7 moorings in 165m and 350m off
Kangaroo Island. The ADCP data agree fairly well with the HF radar and geostrophic estimates. On all 3
occasions, the strong along-shelf flow and very high (isostatically adjusted) coastal sea level were driven
by strong westerly winds in the Great Australian Bight in the previous days. The winds resulted from low
pressure systems on 21 May 2011, 4 Sept
2012, and 8 May 2012 but these systems were clearly of very
different nature, and the details of the history of the wind-stress field is possibly what explains the fact
that the May 2011 and Sept 2012 systems drove strongest currents at the midshelf (SAM5) mooring, while the
May 2012 system drove strongest currents at the outer shelf (SAM3) mooring. Aside: the SAM3 (near-bottom) temperature record
reaches an annual peak shortly after the strong current event, because the South Australian Current is an
extension of the Leeuwin Current, with relatively warm waters downwelling to depth along the shelf break
during winter. The temperature maximum at 350m at the SAM7 mooring in 2012 is not until Sept
(after the flow of the SAC has peaked) at which time SST images
show a sharp front along the shelf break.
Ocean currents too strong for NSW lobster fishers
21,27 June 2013
eddy shed recently [April-June animation AVI format, use Internet
from the East Australian Current is making life difficult for lobster fisher Steve Drake of Kiama
because his marker buoys are being dragged under by strong flow along the shelf break near the 200m
[16 June map]. Mr Drake said the strong southward flow has
since 8 May. The image for 16 May 2013 (at right) shows the flow
Sydney-Jervis Bay in mid-May to be part of the East Australian Current going around an anticyclonic
the manner typical of incipient eddy-shedding. The new eddy became completely detached from the EAC
31 May when the cyclonic eddy discussed here earlier came close to the shelf break off Newcastle (33.5S). The new warm-core
likely to persist for months off the southern NSW coast, possibly reaching Tasmania in some form in
presence of a warm eddy off NSW does not always result in warm coastal water temperatures, because the
southward flow can cause coastal upwelling (due to two processes; the bottom Ekman layer and thermal
if the eddy, for whatever reason, comes very close to the coast there will be no cold water to upwell.
appears to have been the case off Sydney from mid-May to now, to the delight of beach-goers.
IMOS ANMN mooring data will confirm or refute this hypothesis, when available.
Mr Drake's problems did not start in early May. From the 12th to 27th of April, his buoys were dragged
by strong northward flow. This is the first time he has seen this persist for more than a few
The image for 23 April shows that the northward flow was
with the southern cyclonic eddy also mentioned here earlier. That cold-core eddy
sampled by IMOS glider mission sg516 [SST image for 9 May with glider
track from Sydney 23 March to Narooma 28 May] so analysis of those data will be valuable for
understanding the dynamics and consequences of this extraordinary event.
MODIS snapshots of the Leeuwin Current off south-west WA, 2002-2012 9,24 May,
The 22 June
2012 Technical News item listed some of the best MODIS Chlorophyll-a images for the initial
batch of MODIS imagery, and compared these with the recent re-processing using SeaDAS6.4. Here, we list
the clearest or interesting images of the Leeuwin Current in south-west WA, for 2002-2011. The peculiar
seasonality of the Leeuwin current and the associated cycle of surface chlorophyll, appearing where you
least expect it, is clear:
- 18 Aug 2002 Winter: low-moderate (for here) surface
- 17 Dec 2002 Early summer: clear waters everywhere.
- 30 Apr 2003 Leeuwin season: warm core eddies off
with higher surface chl, 28 May more developed, 19 Jun more again.
- 2 Sep 2003 decline of surface chl.
- 4 May 2004, 29
May Next Leeuwin season: cycle repeats.
- 12 Sep 2004 declining surface chl, 20 Nov
- 16 Jan 2005 northward shelf currents drive narrow
high-chl water offshore.
- 19 Apr 2005, 27
Jun Leeuwin season: warm core eddies off shelfbreak with higher surface chl,
- 14 Nov 2005 decline of surface chl.
- 20 Feb 2006 pre-Leeuwin field of complex eddies
- 16 Mar 2006 30
Mar 11 May 27
May 5 Jun Leeuwin season.
- 6 Oct 2006 post-Leeuwin decline of surface chl.
- 23 Dec 2006 Mid summer clear waters.
- 20 Jan 2007 A pulse of northward (wind-driven) shelf
small pre-Leeuwin eddies.
- 5 March 2007 clear waters everywhere.
- 24 March 2007 chl increasing on shelf.
- 21 Jun 2008 Strong Leeuwin with higher-chlor eddies
by clear ocean waters.
- 8 Oct 2008 Filaments of chlor-a water.
- 6 Mar 2009 Post-summer clear waters.
- 5 Apr 2009 Three complex eddies.
- 16 Apr 2009 Rare absence (for April) of normal
- 26 Sep 2009 Post-winter conditions: many eddies but
horizontal gradients of chlor-a.
- 23 Nov 2009 As above, but water clearing as summer
- 16 Jan 2010 Mid summer, clear waters, Leeuwin not
- 24 Feb 2010 As above, but with filaments off Abrolhos
- 18, 19, 24, 27
and 5 May 2010 Zoom-in on Perth region showing
an eddy (radar just installed).
- 2 Aug 2010 Winter conditions again: higher min
- 22 Sep 2010 As above.
- 8 Oct 2010 As above, water clearing.
- 31 Oct 2010 As above, anticyclonic eddies retaining
levels of chlor-a.
- 13, 15,
16 Mar 2011 a thin filament of high-chlor-a water is
(or pushed?) offshore from the Abrolhos Islands.
- 11 May 2011 Strong early-Leeuwin conditions:
(warm core) eddies growing, with high surface chlor-a densities,
- 22 Apr 2011 ... then pinching off from the Leeuwin.
Lists for other regions will be made soon. Input is welcome.
8 July update: Sorry, no
that yet, but we have now added 'gallery' index pages to all the images with least cloud. E.g. see
[gallery] link at the SNSW date index.
Fourteen loops and counting: why do cyclonic eddies trap drifters? 3 May
There is presently a satellite-tracked drifter apparently trapped in a cold-core eddy off NSW. On 2 May 2013 it was at 33S 155.5E, doing ~30km-diameter
loops in a 120km-diameter cyclonic eddy. The 'parent' cold-core eddy of the present, smaller eddy has
for a long time but between 13 March and 16 March it became elongated to the NNW. The drifter did
cyclonic loop at the northern limit of this feature (32S 155E) on 20
March, then proceeded to do many more as the eddy meandered about (the images around 24 April are the clearest). The MODIS image for 10 April shows that the eddy had low (near-surface)
in contrast to the other cyclonic eddy positioned a similar distance offshore, off Jervis Bay at 35S.
back through time reveals that the difference between the eddies' chlorophyll concentations is explained
their different origins; the southern one having been formed over the continental shelf near
around 13 March. The tendency of drifters to remain in eddies
noted before but we cannot recall an instance of a drifter doing so many loops in a cyclonic eddy in
before. Does it matter? Yes, because it tells us something about the exchange of water between the eddy
surroundings. It also suggests there may be some convergence at the surface and therefore downwelling at
centre of the eddy, and also that the eddy is certainly not a wavelike feature, with a sea level anomaly
essentially un-coupled from any particular mass of water.
Recent highlights of the ANMN regional (shelf) ADCP data 17 April
Our Technical News Item announces a new page showing time-series
of Acoustic Doppler Current Profiler data. Below are some highlights (listed clockwise from Qld) of what
data reveal about recent events in the ocean.
Townsville, Feb 2011: What caused the depth of the Palm Passage and Myrmidon reef ADCPs to suddenly
increase by 20m, and the current velocity to suddenly spike, on 2 Feb 2011? Hint
Mackay, July 2011: The 2010-2012 summary
the east shows that the Elusive Reef (GBRELR, 21S 153E) ADCP is the one that measured the
steadiest, strongest flow. May-July 2011 stands out as a rare period of low flow speed. The
page-per-deployment plot for GBRELR-1103 shows no reason to
suspect an instrument malfunction. The flow speed reduces during the period from 23 April to 8 May
(starting at 200m then finally reaching the surface) and remains low until 6 July 2011 when it
increases again. The reason for this appears to be that a cyclonic eddy came very close to the ADCP,
diverting the East Australian Current away from it, as can be seen by comparing our gridded sea
map for 7 June (a day in the middle of the event when Cryosat
is dense near the mooring, so the eddy is well observed) with the map for 12 April when comparable Cryosat coverage shows normal
before the event, and 16 July, when the eddy has moved north,
from the mooring.
Coffs, May 2012: Why is the flow suddenly northward, and bottom temperature raised, on
at the Coffs Harbour 70m ADCP? SST and HF radar make it clear.
Coffs, Aug-Sep 2012: The EAC disappearing act of Aug 2012 was thought
associated with a sudden wind-driven northward current pulse on 10 August and associated vertical
mixing, occurring at a time of minimal flow of EAC waters along shelf. The CH070 and CH100 ADCPs certainly recorded
northward current pulse but no temperature rise at the bottom due to vertical mixing is evident.
The southward resurgence of the EAC along the shelf was recorded by the ADCPs, HF radar, a drifter
also SST and altimetry. Most of the EAC was still flowing around the cyclonic eddy off the
shelf in late Sep 2012 [altimetry for 16 Sep] SST for 20 Sep. Over the following weeks, the warm water
resumed its usual southward path over the continental shelf. The advancing 'nose' of the
flow had a complex horizontal structure    and  Sep zoom-ins. We cannot, of course see the spatial
structure at depth but it is clearly very different to the surface flow, as evidenced by the
or near-zero flow recorded by the 100m
ADCP at depth from 14-28 Sep.
Coffs-Sydney, April 2012: On 20 April 2012 we speculated about the
of a large Coastal Trapped Wave that we could see (in tidegauge data) travelling up the NSW coast.
mooring data clearly show now that the anomalous northward pulse was indeed about 0.5m/s as
off Sydney around 10 April, as shown in the depth-time plots of SYD100 , SYD140 and CH070 ADCP data. These data are
shown now in the SNSW map view.
Sydney, Jan 2011: On 22 Jan 2011 we wrote about an extreme warm-core
that was forming off Sydney. ANMN mooring data [depth-time plot of SYD140
[SYD100] clearly show the
on coastal currents of the eddy coming up onto the inner shelf, as also seen in the [SST map for 16 Jan 2011].
Adelaide, June 2011:The South Australian Current in
swing, June 2011, as evidenced by ADCP, HF radar, a surface drifter, altimetry and SST.
the June 2011 animation to see how well these observing
systems agree and/or complement each other.
Perth, Jan-Feb 2012 The switch from strong southward
12 Jan to northward flow on 19 Jan at the Two Rocks 200m ADCP was due to the
passage over the mooring line of the southern edge of a growing Leeuwin Current warm-core eddy. This
eddy was south of the line by 30 Jan (after a period of
alternating hot and cold winds which affected both the SST and inner-shelf currents) so the currents
became more westward. The flow at the ADCPs switched back to southward on 22 Feb. This was after the cyclonic feature seen in SST and radar on 16 Feb had moved a little west, but not
because of that. The beautiful SST image for 28 Feb shows
possible influence of the cold, northward Capes Current.
Perth, July 2012 A cyclonic eddy was off Perth in early
2012 forcing the strengthening Leeuwin up onto the upper slope. The Two Rocks 500m ADCP measured
strong southward flow in the upper 200m from 28 Jun to 16 July. The SST image for 1 Jul shows that the edge of the Leeuwin was just seaward
the mooring, while the animation for the month shows how
passage of perturbations of the front were recorded consistently by both the ADCPs and the radar.
Port Hedland, March 2012: Severe Tropical Cyclone Lua was off the shelf on 16 Mar. The Pilbara line of ADCPs measured the resulting
subsurface flow speed to be quite weak, just 0.5m/s at 50m depth and 0.6m/s at 20m, over the 200m and 100m isobaths [map view], presumably because the cyclone did not track along
shelf, but passed directly across it.
Port Hedland, April 2012: High sea level at Darwin on 5
April signalled the start of a large-scale south-westward flow along the north west shelf
had stronger current speed than the response to LC Lua and much longer-lived and complex in the
dimension [animation of April].
Port Hedland, July 2012: Perhaps the most intriguing flows recorded by the Pilbara ADCPs (during their
6 month deployment) occurred during July 2012 after cold air off the land had cooled the inner-shelf waters, causing plumes
dense water to flow seaward. Many sub-mesoscale cyclonic eddies can be seen in the animation, e.g.
on 25 July, including one with a drifter caught in it. Flows of
~0.5m/s were measured by the ADCPs, providing valuable information to help us understand these small
features which are attracting oceanographers' attention for a number of reasons.
Broome, March 2012: The inertial period at the latitude of the Kimberley 400m ADCP is 45h,
the interval between the times when the flow velocity measured by the KIM400 ADCP at 50-70m was
to the north-east, with magnitude of about 0.4m/s, on 18, 20 and 22 March.
The June animation of those vectors shows the clockwise
of the velocity caused by the impulsive forcing by Severe Tropical Cyclone Lua on 16 Mar. (Note that our sea level analysis is not intended to
represent such short-term events, when the assumption of geostrophy is violated, rendering the
velocities meaningless, so ignore the altimetric vectors on those maps).
Darwin, May 2012: The strongest south-westward flow through the Timor Passage, as recorded
ITFTIS 500m ADCP in the upper
occurred around 13-16 May, quite likely because of strong
easterly winds in the tropics associated the high pressure
passing over the southern states. The strongest counterflow occurred between 150 and 300m,
from 12-26 Feb, for which we can offer no explanation at present. The SST animation is unhelpful as usual in that region during
Strong northeastward currents on the SE Tasmanian shelf generate a lee eddy behind Tasman Island:- IMOS
glider confirms a model prediction 3-12 April 2013
An IMOS Slocum glider sampling the Storm Bay line out from Hobart encountered extremely strong currents
the NE recently, sweeping it up the Tasmanian coast towards Maria Island as shown at right. The glider's
also included a few unplanned loops. One of these was east of the southern tip of the Tasman Peninsula,
mid-shelf between Tasman Island and the Hippolyte Rocks. A CSIRO experimental near-real-time model simulation has an anti-cyclonic
about the same location as the glider's loop, that appears to be a topographic lee eddy resulting from
strong (nearly 1m/s) alongshelf flow streaming past the SE tip of the Tasman Peninsula. [more
about the model, and real-time output] [animation won't play?]
12 April update This [animation of model SST
'model drifters'] is a remarkably realistic-looking simulation of the situation, with some of the
drifters executing anti-clockwise loops on the shelf where the glider did its loops. Interestingly, the
predicts that if the glider had continued north in the cold current as far as the Freycinet Peninsula
it might have sampled a very strange thing: a cold eddy rotating in the 'wrong' direction - anticlockwise.
explanation is that this cold water is also relatively fresh (about 35.1 at 60m according to the glider, in
agreement with the model), so it is less dense, and
therefore rotates anti-cyclonically. So an alternative explanation for the looped path of the glider is that
encountered one of the fresh eddies that the model predicts to be shed inshore of the sharp shelf-edge
The lower salinity (relative to the EAC-influenced water offshore) of this fresher water on the Tasmanian
is not just from Tasmanian rivers, it is characteristic of waters south of the Sub-Tropical Front. For
see nearby recent Argo profiles west and east of Tasmania showing sub-35 near-surface
First major upwelling of the season for the Bonney Coast 6,8 March, 15 April
The Bonney Coast (Portland, Vic, to Robe, SA) is possibly Australia's best-known upwelling hotspot with
well-documented upwelling season from December to March. The upwelling this year, however, has until
been quite weak. The sea level and pressure maps for 1 Mar, SST image for 3 Mar and
image for 5 Mar document the season's first really significant upwelling, with the usual
present: a high atmospheric pressure system west of Tasmania caused strong SE winds along the Bonney
which set the sea level down and thermocline up. Cold water surfaced where the shelf is narrowest. The
of nutrients to the photic zone caused the phytoplankton to grow.
15 April Update: HF radar data
SAG, covering the shelf west of Kangaroo Island) have now been added to the SST maps, quantifying the speed
timing of the northwestward velocity that was already evident from visual inspection of the SST imagery. The
flow speed was about 0.5m/s on 1 - 2 March .
Ocean impact of STC Rusty monitored by IMOS gliders 28 Feb 2013 (updated 5 March)
Severe Tropical Cyclone Rusty made landfall 100km east of Pt Hedland on 27 Feb 2013. An IMOS glider
recorded, for the first time in Australian waters, the impact on various water properties under cyclonic
conditions. Possibly the most notable of these impacts is that turbidity exceeded the intrument's range
way to 30m (the bottom), as shown in Fig. 10 of a WAIMOS
The impact of the cyclone is also clear in MODIS estimates of the chlorophyll-a
concentration on 3 March, which show a 250km-long plume extending seaward from the coast, south-west of the (clockwise-directed) winds.
Tide gauges at Broome, Port Hedland and Cape
Lambert have also recorded the impact on sea level but this does not appear to have been large,
relative to the large tidal range of the Pilbara coast. The elevated sea level, however, is expected to
travel southwards along the Australian continental margin as a Coastal Trapped
Wave, raising sea level by about 0.4m above the tidal prediction at Perth on 1-3
March , according to the Bureau
Meteorology ocean model forecast, as shown at right.
Strong upwelling in North-Eastern Bass Strait
25 Feb 2013 (updated 6 March)
The far eastern coast of Victoria
has one of Australia's upwelling 'hotspots'. An example of intense localised upwelling occurred in the
weeks, starting on
13 Feb. The signal is clear off Marlo (longitude 148.5°E)
by 15 Feb. According to satellite imagery, SST reached a minimum
16°C on 17 Feb and the cold plume extended 80 km to the
Observers at nearby Beware Reef measured surface temperatures as low as 15°C. The cold upwelling
phytoplankton bloom that was clearly evident in a MODIS chl-a image
Feb and even more evident to the Beware Reef diver pictured, who noted only a few m visual range
20m where it was quite dark, and only 12°C. The upwelling event appears to be due to the combined
1) the local wind which was upwelling-favourable from 10 Feb, and 2) dynamic uplift from the along-shelf
associated with a warm-core EAC eddy. The event has been simulated quite well in near-real time using
Bluelink-developed nested-model system but the dynamics have not yet been fully explored. [model estimates of SST and currents]
credit: Friends of Beware Reef.
The Sydney-Hobart yacht race:- pre-race notes on the ocean currents
20,24 Dec 2012
Firstly, a reminder of a change to the way we are showing the strength of the surface current for
such as the one for Sydney-Hobart. We have opted to double the number,
east-west direction, of current vectors but halve the scale factor. So a 1kt current now shows as a
arrow, as indicated by the key. Bear this in mind if you compare this year's maps with previous years'
Now, the state of the East Australian current and its eddies: The 'speed hump' in Bass Strait that
navigators with a difficult decision last year is not there this year. Instead, the rhumbline appears to
fairly free of any strong currents as far as we can tell from the latest satellite information. To the
of the rhumbline there is some favourable current clearly evident in the latest images so the warm,
anticlockwise-rotating eddy is a feature worth watching in the next few days. There appears to have been
northward flow on the continental shelf of north-east Tasmania (as evidenced by the plume of cold water
streaming north from Eddystone Pt) in the last few days but this feature is unlikely to persist until
day. The situation between Sydney and Cape Howe is very complex this year. There are two small cyclonic
(clockwise rotating) eddies off the continental shelf, so the flow is adverse in places seaward of the
isobath. Over the shelf, however, there was evidently a favourable flow of about 1kt on 15 Dec, as can
seen from the southward drift of a satellite-tracked buoy. The
thermal imagery for 19 Dec suggests there was a narrow warm stream
flowing south past Jervis Bay over the outer continental shelf. It is unclear from the altimetry what
speed of the flow was. Navigators should watch the imagery for any evidence of this flow persisting til
time, which is quite possible but certainly not guaranteed. As usual, we wish all competitors an
but safe race and apologize in advance if the imagery on our website stops updating at any point due to
computer failure while our IT staff enjoy their holidays.
An even-higher sea level event in the Great Australian Bight 29 August 2012
Sea level in the GAB was about 1.8m above MSL at high tide on 23 August 2012. The low-pass filtered,
barometrically-adjusted coastal sea level anomaly we used in our sea level mapping briefly reached 0.8m,
the June value discussed below to become the highest (sub-tidal) sea level anomaly recorded in southern
Australia this winter. It was caused by a deep low-pressure system that passed over Tasmania on 25 August. The wind had a strong on-shore component at times, and
of this can be seen in the 24 August data from the HF radar,
senses the total near-surface current velocity. But why is the along-shore component of the radar
clearly less than the estimate derived (by geostrophy) from the sea level maps? The answer to this
includes several considerations. One, demonstrated so clearly off Perth in June, is that strong onshore
raise coastal sea level ageostrophically, i.e., the sea level slope sets up to balance the force of the
Geostrophy, in contrast, is when the sea level slope is in equilibrium with the Coriolis force acting on
along-shore component of flow. This balance takes longer to establish than the set-up due to wind, which
we low-pass filter the coastal sea level before inferring the alongshore flow from the map of sea level.
be that in this case this approach did not remove enough of the sea level slope due to the onshore
wind, yielding an erroneously-high estimate of the flow speed. Another possibility is that the altimeter
estimates of low (-0.2m off Eyre Peninsula) sea level at the edge of the shelf were erroneous due to
correction for the effects that large waves have on those measurements.
The Day the East Australian Current vanished 13 August 2012
Now you see it [8 Aug 2012], now you don't:
[14 Aug 2012].
What made the thin streak of warm EAC water south of Coffs Harbour suddenly disappear? Two things
between 8 and 14 Aug, as shown in the larger-scale map for 11 Aug 2012.
that the cold core eddy at 30S 155E, over the continental slope off Coffs Harbour, was restricting the
flow of the East Australian Current at that point, so most of the flow went offshore around a big warm
centered off Byron Bay. The consequence of this is that the thin streak of warm water south of Coffs was
probably quite shallow. The other thing happening was that an atmospheric low pressure system was off
centered at 34S 159E on 11 Aug. Strong southerly winds drive coastal currents northward and raise
level, as shown in the tidegauge data, but that would not change the surface temperature away from the
The cold southerlies cool the warm surface layer by extracting heat, but possibly more importantly, also
increasing the surface turbulence, mixing warm shallow layers with the water beneath. In summary, we
the winds being so strong, and the flow volume so reduced, are what made the surface exression of the
NSW disappear so fast. 22 August update: a fresh nose of warm
waters can now be seen flowing south again between Coffs and the eddy offshore.
April 2013 update:
CH070 and CH100 ADCPs certainly recorded the
northward current pulse but no temperature rise at the bottom due to vertical mixing is evident.
Extremely high sea level at Perth and in the Great Australian Bight 18 June
On 10, 11 and 13 June 2012, the Bureau of Meteorology National Tidal
gauge at Hillarys, Perth, recorded non-tidal sea level anomalies of up to 0.8m above mean sea level. The
Department of Transport tide gauge at Fremantle Boat Harbour recorded a similar surge but also a
oscillation that contributed to the highest sea level ever recorded at Fremantle: 1.35m above mean sea
Fortunately, these surges came at neap tide (click images to expand - Hillarys data courtesy of BoM, Fremantle data and image
WA DoT). The total sea level might have
additional 0.3m otherwise (e.g., if it had occurred during the high spring tide a few days earlier). Low
atmospheric pressure was a contributor to the high sea level but strong onshore (westerly) winds
the low pressure systems passing south of the mainland was the main cause. A consequence of these strong
being onshore rather than alongshore is that they would not (in contrast to the geostrophic sea level-based estimates shown on our map) have generated particularly strong along-shore currents.
will be confirmed when the radar data processing resumes. Along the southern coast of Australia, where
blew along the shelf rather than across it, they generated a 0.75m non-tidal sea level anomaly on 14 June 2012 at Thevenard, where such events are more common. The
now passed through Portland, Vic (non-tidal anomaly 0.3m) and will possibly also be measurable soon east
P.S.: An animation of the CSIRO Bluelink model
the event, as a 3D perspective (viewed from the SE) of the sea level anomaly. The fit to the time-history of the (independent) Fremantle sea level observations is remarkable.
A radar view of an intense cold eddy off Perth 9 June 2012
Since 29 May 2012 , a satellite-tracked surface drifter has
remained within a small eddy of cold water that we can see off Perth using satellites and the IMOS HF
drifter has made several clockwise orbits of the eddy center but the radius of these orbits is so small
that the frequency of the position fixes are sometimes inadequate. The clockwise rotation of these very
eddies has been seen before in SST imagery and by drifters but this recent example is the first we have
real-time using HF radar. The tight radius of the eddy is well-resolved by the radar and the
the radar data with the SST imagery is striking. We have
radar vectors according to whether the surface velocity field is divergent, neutral (|div(v)|< 5E-6/s) or
convergent. Trapping of drifters in features has often been taken to signal the existence of a convergent
surface layer, and therefore downwelling, but the recent sequence of events has not yet yielded a simple
interpretation. The cold eddy is the remains of an elongated cold feature sandwiched on 27 May between the warm eddy offshore, and warm water over the
The eddy is now just a 'hole' in the surrounding region of warm water. We are interested in these features
because many items such as fish and lobster larvae might also become aggregated in convergent eddies.
P.S.: Animations of radar and SST (AVI format), radar and SST (fli format), radar only (AVI format), radar only (fli format).
Warm beach temperatures at Sydney 11 May 2012
Sydney beachgoers are wondering why the ocean is so warm at the moment [Sydney Morning Herald]. The answer is not that the whole Tasman Sea is
fact, the only places that are warmer than usual for this time of year are quite close to Sydney. Much of
ocean between Sydney and New Zealand is significantly colder (by a degree or two) than usual [SST anomaly]. So why is it so warm near Sydney? The satellite
for 25 April shows the East Australian Current streaming down the
continental shelf, with warm (23°C) water right up against the coast in places, as far as Woolongong
it went offshore, looping around a 100km-diameter anticyclonic eddy before rejoining the parent flow. The
appearance of cooler eddies south of headlands is a clear sign of rapid southward flow along the coast.
Animation [fli format] [avi
format] of the available images for following days shows how the along-shore flow of the EAC was
interrupted as a cold-core, cyclonic eddy moved in from the deep ocean to the continental slope and shelf.
By [8 May], the EAC had changed course at Port Macquarie, with all
the EAC water flowing off the shelf into the deep ocean, isolating the anticyclonic eddy centered off Sydney
from the rest of the EAC. This is the usual course of the EAC eddy-shedding process. It is not every time,
however, that this process results in the inner shelf being flooded by warm EAC water. Strong flows of EAC
normally remain a bit farther offshore, so NSW beach temperatures are usually a few degrees lower than mid-
outer shelf temperatures. A common reason for sudden cooling is north-easterly winds, which, due to the
of the Earth, drive surface waters to the left, away from the coast, drawing the colder subsurface waters to
surface. Southerly or southwesterly winds have the opposite effect: they push the warm water up against the
coast. The southerlies of late April - early May are surely a factor in the present case.
High sea level on Vic, NSW and Qld coasts, 8-15 April 2012 20 April 2012
A wave of high sea level propagated northwards along the entire east Australian coast from Bass Strait on 8 April through NSW on 10
April, SE Qld on 13 April to Cape
on 15 April. It was the result of winds associated with a high pressure system that developed in the
Great Australian Bight. This system developed farther south than normal for April, at a latitude more
for summer-time systems (so it did not cause strong westerlies in the GAB). Unlike a summer system, however,
then tracked onto the SE mainland and up towards Qld. The result was that strong westerly winds off Vic
strong southerlies off NSW and strong south-easterlies off Qld. There is a class of low-frequency (period of
days or longer), large-scale (100's km wavelength) ocean wave known as a Topographic Rossby wave, or Coastal
Trapped Wave that propagates along continental shelves with the coast on the left in the southern hemisphere
(i.e. northwards on the east coast) at a few hundred km per day depending on the depth profile of the shelf.
They are generated by the along-shore component of the wind and reach largest amplitudes when the
of the strong winds matches the speed of the wave. This recent event appears to be a classic example of the
local response to the wind being compounded by a Coastal Trapped Wave. The result was that sea level was
0.5m higher than expected for a day or two in places, something that only happens once or twice, if at all,
year. A northward shift of the prevailing current, of possibly 0.5m/s for a day or two, might have also
over the continental shelf but this is yet to be confirmed. Downwelling of the thermocline over the
slope may also have occurred, especially near topographic irregularities such as Fraser Island.
Update: ANMN mooring data clearly show that the anomalous northward pulse was indeed about 0.5m/s
anticipated, off Sydney around 10 April, as shown in the depth-time plots of SYD100 , SYD140 and CH070 ADCP data. These data are also
now in the map view.
2 July 2013 Update: The northward pulse
also recorded off Heron Island, SGBR, on 13
2012 [map view].
Cold water at Byron Bay 1,2 March 2012
Many of the recent SST images for the NSW-Qld coast are badly affected by cloud but it is clearly
(e.g. 28 Feb, 1 Mar)
cold water is upwelling from the deep and surfacing at the coast. This process occurs regularly on the NSW
but not often as strongly as at present, or as far north as Byron Bay, where a wave buoy has recorded a
temperature drop of 4° (to 21°) . This temperature drop coincides with the arrival at the edge of
continental shelf of a cold-core cyclonic eddy, that happens to have an Argo profiler in it [21 Feb
confirming that the temperature structure is displaced upwards by nearly 200m. This would drop the
at the edge of the shelf from 20° to 15°, while also interupting the southward flow along the shelf
of the East Australian Current [24 Feb altimetry map].
Tasmanian beaches: as warm as Bondi? 11 Jan 2012
Ocean surface temperatures are presently 18 to 19°C off eastern
Tasmania, which is about 3° above normal. The same (or
lower) temperature was recorded at Bondi Beach on Boxing Day 2011, but there it is about 3°
normal. The explanation for the levelling of the beach temperatures is probably as follows: the relatively
Sydney temperature was the result of the combined influence of upwelling-favourable
north-easterly winds and the departure from the shelf, just north of
Sydney, of the East Australian Current (as noted below). The warm Tasmanian temperatures are due in
measure to the presence off Tasmania of the large warm-core eddy that has come slowly south from NSW, even
though the core temperature of that eddy is not high at the surface.
Update: The southward flow of
water continued through February, with the greatest positive anomaly occurring in the week of 25 Feb when weather conditions compounded the effect of the local
current, as shown in this short animation.
Sydney to Hobart yachtsmen: mind the speedhump in Bass Strait 20 December 2011
If our most recent current maps are any indication of what competitors will encounter in this year's
Sydney-to-Hobart race, one of the key decisions might be whether to stick to the rhumbline and endure a weak
adverse current in Bass Strait, or whether to try and pick up a stronger (possibly 2kt) tail current by
wide of Cape Howe. The more eastern route might also position yachts to pick up some tail current off
The strong flow off southern NSW is the inside edge of a warm-core eddy shed from the East Australian
September. The eddy presently off Tasmania is the remnant of the very strong eddy formed off Sydney in March. The EAC is well east of Sydney at present, so strong currents are
evidently affecting the region from Sydney to Ulladulla. The flows may change over the next few days,
so navigators are advised, as always, to watch for updates.
The Leeuwin Current and East Australian Current are both still extremely strong 9 December
Both of Australia's major boundary currents were stronger than usual in 2011. Off Perth, the strong
Leeuwin Current caused water temperatures to be more than 4 degrees warmer than usual from February to April. The
flow continued into the Great Australian Bight where it created
several very energetic warm core eddies, with flow speeds reaching 0.6m/s at times, according to
satellite-tracked drifting buoys. Temperatures are a few degrees higher than usual at present off SW Australia, and the Leeuwin is rounding Cape Leeuwin as a powerful current, billowing seaward at several
instead of relaxing as it often does at this time of year. From Brisbane to Sydney, the strong East
Current early in the year produced the most energetic warm-core eddy measured by satellite altimetry off SE
Australia. In March the eddy was off southern NSW. Its waters are now
south of Tasmania.
Extreme eddies off Sydney 22 January 2011
Two extremely large ocean eddies are off Sydney at the moment, one warm, the other cold. The warm eddy
composed of a large mass of East Australian Current water that has been flowing very fast south from Qld for
some time now, and is about to pinch off from the parent current to form an isolated eddy. The pinching-off
process is associated with the westward movement of a huge and very energetic cold-core eddy, which we
from space as a 150km-diameter area of low (-0.6m) sealevel [map for 17
As this comes ever closer to the continental shelf between Newcastle and Port Macquarie, very cold water
upwell at the coast, from Port Stephens to Sydney, especially if there are winds from the NE to assist the
process. If the warm eddy now off Sydney shifts southward then the present low coastal temperatures south of
Jervis Bay will be replaced by high ones. This whole chain of events has been seen many times before, but
ever this strongly, from memory. Results of a quantitative analysis will be announced when they are
18 March 2013 Update: ANMN mooring data [depth-time
plot of SYD140 ADCP] [SYD100]
clearly show the impact on coastal currents of the eddy coming up onto the inner shelf, as also seen in the
[SST map for 16 Jan 2011].
Rolex Sydney Hobart 2010: 27 December 2010
The main body of the East Australian Current was too far offshore to affect the early segment of the
[Google Earth screen-grabs]. As the yachts passed the NSW/Vic border, however,
had the choice of northward flowing cold water over the continental shelf, or southward flowing warm water
outside the 200m isobath. Most of the leading boats chose the warm water but from halfway across Bass
this is not flowing towards Hobart, but veering east. Cold water on the Tasmanian continental shelf is
northwards but probably not very quickly.
Delayed impact of Ului on Barrier Reef July 13 2010
The cyclonic eddy generated by STC "Ului" has now reached the northern GBR.
update on STC "Ului" May 21, 2010
The cyclonic eddy generated by Ului in March has now [16 May], after
months, travelled half the distance from its origin to the coast. Strong northward currents north of Cairns
be expected as the eddy comes closer, as happened (even more so) in the aftermath to TC Andrew in [April 1997].
update on Sydney May 18 2010
Beach water temperatures are still extremely high but the East Australian Current has now started to
mostly away from the continental shelf at the latitude of Sydney, rather than towards the coast (as it has
late April), triggering the beginning of the end of the high temperatures. The explanation is faily complex:
Between Sydney and Newcastle a clockwise-rotating, cold-core eddy has now become well established inshore of
EAC. Until the last few days, the EAC was flowing south around this cold-core eddy, feeding into a large
EAC water that is warmer than is normally seen off southern NSW at this time of year: [difference from average for 12 May].
The cold-core eddy has now grown so large that very little of the EAC is continuing south: most appears to
flowing offshore at the critical junction point at 34.5S, 152.5E: [imagery
16 May]. To complete this eddy-shedding process, the cold-core eddy off Newcastle is likely to merge
with the larger cold-core eddy to the SE, thereby completely separating the two masses of warm EAC water:
parent body to the NE, and the newly-shed body to the south.
Update: The eddy shedding is now
[23MByte animation of 6-20 May]
Beachtime for Sydney April 29 2010
Swimmers at Sydney's beaches enjoyed water temperatures several degrees warmer than usual for a few
courtesy of the East Australian Current coming very close to the coast at Sydney after flowing around a
cold feature off Newcastle.
[satellite imagery for 29 April]
Severe Tropical Cyclone "Ului" March 25 2010
Ului remained near 13S 159E from [15-18 March], allowing for much
transfer of momentum to one region of the ocean. The low atmospheric pressure raised sea level but the
cyclonic rotation reduced it.
Once the cyclone moved on, the cyclonic rotation and low sea level of the ocean remained:
The SST imagery is not good but the reduction of temperature at the centre of the eddy can be seen.
Centaur- success! December 20, 2009
The Centaur has been found, proving that the survivors did indeed drift slowly north-eastwards (19nm
34h is just 0.6kt) before being found where they were. It appears, therefore, that an eddy similar to the
of Cape Morton on [8 Dec 2009] must have been off Point Lookout on 14
1943. Had such an eddy not been there, and the flow more similar to how it was on [15 Oct 2009], survivors could have been swept as far south as
Head, resulting in fewer being found if they became stretched over a larger area, or taken farther offshore.
Centaur update. December 16, 2009
Imagery off Brisbane has been very poor since the start of the search. [latest
Eddy off Brisbane - good news for Centaur search
December 8 2009
The upwelled water off Fraser Island has become a cold-core eddy, around which the East Australian
has been deviating. The flow is seen by animating a few high-quality SST images that weather conditions
possible on [8 Dec]. The (fairly rare) flow deviation is a welcome
development for David Mearns who is searching for AHS Centaur.
This upwelling has been caused by three factors: 1) the recent northerly winds, 2) a warm-core eddy
driving very strong southward flow (with an onshore bottom Ekman layer) near the tip of Fraser Island,
and 3) a
westward-moving cold-core eddy centred at 26S 155E on 12
causing uplift of the isotherms (see Argo profiles on 8 and 27 November).
Cold-core eddy off Ningaloo: June-August 2007
Cold core eddies do frequently form off Ningaloo Reef, but few are as strong as this one: [animation of SST June and August]
Cold-core eddy off Sydney
March 21 2007
An especially strong cold core eddy off Sydney: [fli animation of SST March
14-15] [.AVI format] [News