Sydney-Hobart yacht race: ocean currents looking very favourable this year

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 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 and 2013.

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

A dead humpback whale was about to come ashore on Perth's Cottesloe Beach yesterday, in what might have been a repeat of the cause of last week's $170,000 clean-up exercise on Scarborough Beach. Referring to the image at right [zoom in] [zoom in, SST], and taking the forecast winds into account, Prof Pattiaratchi advised authorities that towing the whale 15km offshore of Rottnest Island would result in an even chance that ocean currents would take the carcass away. As far as we know, this is the first time that IMOS infrastructure has been used for an application of this nature. 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

The Brisbane region is where the flow along the continental slope is most consistently fast and to the south, because of the East Australian Current, right? Well, yes, but sometimes the flow along the slope does turn northward (e.g. as we discussed with reference to the fate of AHS Centaur). In 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 actually 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 available and not surprisingly the data are rich with interesting features. One of these is a month-long anomaly in 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 level, 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 that on 7 June the flow velocity off Brisbane suddenly dropped from normal values to near-zero, where it stayed until 11 July. During 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 explained 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) and 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 a large amount of cross-shelf exchange (as can be seen in the chlorophyll images as well as the u-component velocity timeseries) with potentially major consequences for transport of fish larvae and everything else in the water - near Brisbane and farther south, and 2) turbines harvesting energy from the flow would be affected.

Where are Australia's fastest ocean currents?

This honour probably goes to one of the passages of the Buccaneer Archipelago. Sunday Strait, for example, is the main shipping route into King Sound and the town of Derby. Hydrographic chart AUS 733 notes that flow speeds of ~10kt (~5m/s) occur there. We are unaware of any instrumental measurements to verify that estimate, but offer this observation: At 1630WST on 29 July 2014 (close to the time of maximum spring ebb tide), it took two 250hp 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 flow speed to be 9kt.

Malaysia Airlines MH370: estimated drift of buoyant wreckage

Australia coordinated the search of the sea surface for wreckage of Malaysia Airlines flight MH370 from 18 March to 28 April, 2014. The Drift Working Group assembled by the Australian Maritime Safety Authority used several techniques, throughout the search, to estimate the drift of potential wreckage as part of the optimisation of the conduct of search operations, which involved as many as 12 aircraft and 12 vessels from a number of nations. The focus of efforts shifted north-eastward along the '7th arc' from early March when it was thought the plane 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 been distributed on 18 March when the search off Australia commenced. These simulations use the ATSB estimate of the 8 March crash region, and CSIRO trajectory modelling based on the Bureau of Meteorology winds and ocean current (left panel), and IMOS OceanCurrent surface current (right panel). The red, black and blue dots represent items with three degrees of exposure to the wind, drifting downwind at 2.8%, 1% and 0% of the 10m wind velocity, in addition to the two estimates of the velocity of the water. By 28 March, when the estimated location of the crash site was close to the updated location used here, our 90km x 700km rectangle of tracked particles is dispersed into a region of approximately 300km x 500km. 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 the 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 simulations, this is indeed possible, but very unlikely. A few of the 2700 tracked particles are now (29 June) estimated to be at the edge of the continental shelf. Winter westerly winds could conceivably bring those ashore. The vast bulk of the tracked particles, however, are 900-2500km west of Australia.

Pacific Ocean cool surface

For several months now, the surface temperature of the Pacific Ocean between New Caledonia and Australia has been a degree or more cooler than usual, as shown at right (click to enlarge). The satellite temperature image is rich in detail but it is far from being the whole story. Firstly, note how the cool patches are all depicted as rotating clockwise. We infer this rotation from the sea level map which features depressions at those places, consistent with a positive anomaly of the depth-integrated density, which could be caused by lower temperature and/or higher salinity. One hypothesis worth exploring is that reduced rainfall in that region has increased the 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 temperature 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 upper 100m, but it does not explain the measurements by nearby float 5904257 which sampled a more-normal vertical profile. Here, the isotherms and isohalines are shifted down by about 100m, thus reducing the depth-integrated density anomaly and raising the surface elevation, even though the temperature anomaly at the surface is zero or even slightly negative. This downward shift could only have been caused by a local accumulation or convergence of EAC waters, such as might result from the net transport southwards along the shelf being temporarily reduced. The question is why this advective deepening of the surface layer was not accompanied by any increase of surface temperature, but a reduction instead.

Rottnest Channel Swim: Don't expect the usual northward current flow

Recent satellite and HF radar observations, and forecasts of wind, waves and ocean currents (at right, click to enlarge) indicate that conditions for the 2014 Rottnest Swim (22 Feb) could contribute to relatively fast times.

Modelled ocean currents from the Oceans Institute of the University of 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 usual 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 by 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 the edges of the coverage (as evidenced by the disagreement where they overlap) and are subject to many sources of interference. The Rottnest channel is at the very edge of the coverage, so the north/south component of the flow is very uncertain.

FFI, or to provide feedback, contact Roger Proctor 0400 153 449 or email.

Extreme Bonnie Coast Upwelling

Winds associated with the combination of low atmospheric pressure over SE Australia and high pressure west of Tasmania around 4 February 2014 caused an extremely strong upwelling event stretching from the Eyre Peninsula to Portland, so this season can certainly be characterised as one that started early (see below) and included at least one extreme event. The surface velocities associated with the 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 upwelling region, with just a small component directed offshore to drive the upwelling. The dynamics is very different, and more complex, outside the continental shelf in the deep ocean, where eddies exist and the wind-driven current is not constrained to follow the coast.

5 March update:

13 June update:

Ocean current analysis for the Sydney to Hobart yacht race

This year's fleet will negotiate three significant ocean eddies. The first, seen at right south-east of Jervis Bay on 20 Dec (click to enlarge), is unfavourable, but may dissipate or move farther offshore between now and Boxing Day. Skippers will want to keep inshore of this, other factors being equal. The 2nd is more important. Well-exploited all the way from 36S to about 39S, it could give yachts a significant advantage (1-4h) over those that do not find this river of southward flowing tropical water. Finally, an isolated warm-core eddy is off Tasmania's Freycinet Peninsula (42S), potentially giving a small boost to yachts keeping a bit farther east than others.

Bonnie Coast upwelling: a strong, early start of the 2013-14 season

The first upwelling of the 2013-14 season occurred from 11-17 November due to strong south-east winds for several days. This was a strong event, especially for so early in the season, with the NOAA satellites seeing upwelled water at 12-13° C, the HF radar seeing surface flows of 0.3m/s to the NW and tidegauges seeing coastal sea level depressed by 0.2m. This occurence of an early, strong upwelling in 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 excellent agreement with the HF radar data, and an upwelled plume of 15° C water.

22 Nov Update:

25 Nov Update:

26 Nov Update:

28 Nov Update:

Throttling of both the Leeuwin and East Australian Currents

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 force is 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 achieving a similar effect. In both cases, some of the flow continues southward after completing a detour around the eddy but it is clear that there are impacts downstream (incuding the end of the warm spell off NSW discussed last month) when these diversions occur, as well as local impacts inshore of the eddies where there is suddenly much less tropical water. Both eddies have been sampled near their centres by Argo profilers. The 70m downward 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

Surface waters over the NSW continental shelf and nearby in the Tasman Sea have been much warmer than usual since early August. Shelf waters off the Sydney-Woolongong shelf are presently 22°C or more in places as shown at right, about 4°C warmer than usual. A few Argo floats have sampled these anomalous conditions at key locations, confirming the satellite estimates and showing that the warm anomaly extends down to 600m at places. As is usually the case, however, the warmth of the regions directly affected by the East Australian Current is contrasted by a nearby cold anomaly that exceeds 3° between depths of 300 and 500m in the centre of a cyclonic eddy farther offshore. The cold anomaly is not great at the surface, so its location is less obvious in the thermal imagery. It showed up more in the altimeter data, as a sea level depression exceeding 0.6m in August.

Highlights of the ANMN regional (shelf) ADCP data

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 reveal about events in the ocean, as a continuation of the list below.

Southern GBR, April 2012:

Coff's Harbour, Feb 2013:

Southern shelf, May 2011, 2012 and Sept 2012:

Ocean currents too strong for NSW lobster fishers

A warm-core eddy shed recently [April-June animation AVI format, use Internet Explorer] from the East Australian Current is making life difficult for lobster fisher Steve Drake of Kiama (34.7S) because his marker buoys are being dragged under by strong flow along the shelf break near the 200m isobath [16 June map]. Mr Drake said the strong southward flow has persisted since 8 May. The image for 16 May 2013 (at right) shows the flow off Sydney-Jervis Bay in mid-May to be part of the East Australian Current going around an anticyclonic meander in the manner typical of incipient eddy-shedding. The new eddy became completely detached from the EAC around 31 May when the cyclonic eddy discussed here earlier came close to the shelf break off Newcastle (33.5S). The new warm-core eddy is likely to persist for months off the southern NSW coast, possibly reaching Tasmania in some form in summer. The presence of a warm eddy off NSW does not always result in warm coastal water temperatures, because the strong southward flow can cause coastal upwelling (due to two processes; the bottom Ekman layer and thermal wind). But if the eddy, for whatever reason, comes very close to the coast there will be no cold water to upwell. This 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 under by strong northward flow. This is the first time he has seen this persist for more than a few days. The image for 23 April shows that the northward flow was associated with the southern cyclonic eddy also mentioned here earlier. That cold-core eddy was 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

The 22 June 2012 Technical News item listed some of the best MODIS Chlorophyll-a images for the initial 2011-2012 batch of MODIS imagery, and compared these with the recent re-processing using SeaDAS6.4. Here, we list some of 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 might least expect it, is clear:

• 18 Aug 2002 Winter: low-moderate (for here) surface chl with weak gradients.
• 17 Dec 2002 Early summer: clear waters everywhere.
• 30 Apr 2003 Leeuwin season: warm core eddies off shelfbreak 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 streaks of 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 flow, and 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 separated 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 conditions.
• 26 Sep 2009 Post-winter conditions: many eddies but small horizontal gradients of chlor-a.
• 23 Nov 2009 As above, but water clearing as summer starts.
• 16 Jan 2010 Mid summer, clear waters, Leeuwin not flowing.
• 24 Feb 2010 As above, but with filaments off Abrolhos
• 18, 19, 24, 27 Apr and 5 May 2010 Zoom-in on Perth region showing pinch-off of an eddy (radar just installed).
• 2 Aug 2010 Winter conditions again: higher min chlor-a, lower maxima.
• 22 Sep 2010 As above.
• 8 Oct 2010 As above, water clearing.
• 31 Oct 2010 As above, anticyclonic eddies retaining winter levels of chlor-a.
• 13, 15, 16 Mar 2011 a thin filament of high-chlor-a water is drawn (or pushed?) offshore from the Abrolhos Islands.
• 11 May 2011 Strong early-Leeuwin conditions: anticyclonic (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 progress on that yet, but we have now added 'gallery' index pages to all the images with least cloud. E.g. see the [gallery] link at the SNSW date index.

Fourteen loops and counting: why do cyclonic eddies trap drifters?

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 clockwise loops in a 120km-diameter cyclonic eddy. The 'parent' cold-core eddy of the present, smaller eddy has existed for a long time but between 13 March and 16 March it became elongated to the NNW. The drifter did its first 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) chlorophyll-a, in contrast to the other cyclonic eddy positioned a similar distance offshore, off Jervis Bay at 35S. Stepping back through time reveals that the difference between the eddies' chlorophyll concentations is explained by their different origins; the southern one having been formed over the continental shelf near Sydney-NewCastle around 13 March. The tendency of drifters to remain in eddies has been noted before but we cannot recall an instance of a drifter doing so many loops in a cyclonic eddy in this region before. Does it matter? Yes, because it tells us something about the exchange of water between the eddy and its surroundings. It also suggests there may be some convergence at the surface and therefore downwelling at the 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

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 these 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 page for 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 2011 (starting at 200m then finally reaching the surface) and remains low until 6 July 2011 when it suddenly 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 level map for 7 June (a day in the middle of the event when Cryosat data is dense near the mooring, so the eddy is well observed) with the map for 12 April when comparable Cryosat coverage shows normal conditions before the event, and 16 July, when the eddy has moved north, away from the mooring.

• Coffs, May 2012:

  Why is the flow suddenly northward, and bottom temperature raised, on 16-18 May 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 to be 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 the 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 and also SST and altimetry. Most of the EAC was still flowing around the cyclonic eddy off the continental 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 near-surface flow had a complex horizontal structure [20] [21] [25] and [28] 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 northward 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 impact of a large Coastal Trapped Wave that we could see (in tidegauge data) travelling up the NSW coast. ANMN mooring data clearly show now that the anomalous northward pulse was indeed about 0.5m/s as anticipated, off Sydney around 10 April, as shown in the depth-time plots of SYD100 , SYD140 and CH070 ADCP data. These data are also shown now in the SNSW map view.

• Sydney, Jan 2011:

  On 22 Jan 2011 we wrote about an extreme warm-core eddy that was forming off Sydney. 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].

• Adelaide, June 2011:

  The South Australian Current in full swing, June 2011, as evidenced by ADCP, HF radar, a surface drifter, altimetry and SST. Watch 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 flow on 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 just because of that. The beautiful SST image for 28 Feb shows the possible influence of the cold, northward Capes Current.

• Perth, July 2012

  A cyclonic eddy was off Perth in early July, 2012 forcing the strengthening Leeuwin up onto the upper slope. The Two Rocks 500m ADCP measured very 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 of the mooring, while the animation for the month shows how the 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 the 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 that had stronger current speed than the response to LC Lua and much longer-lived and complex in the vertical dimension [animation of April].

• Port Hedland, July 2012:

  Perhaps the most intriguing flows recorded by the Pilbara ADCPs (during their first 6 month deployment) occurred during July 2012 after cold air off the land had cooled the inner-shelf waters, causing plumes of 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, which is the interval between the times when the flow velocity measured by the KIM400 ADCP at 50-70m was directed 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 rotation 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 inferred velocities meaningless, so ignore the altimetric vectors on those maps).

• Darwin, May 2012:

  The strongest south-westward flow through the Timor Passage, as recorded by the ITFTIS 500m ADCP in the upper 100m, occurred around 13-16 May, quite likely because of strong easterly winds in the tropics associated the high pressure system passing over the southern states. The strongest counterflow occurred between 150 and 300m, especially from 12-26 Feb, for which we can offer no explanation at present. The SST animation is unhelpful as usual in that region during the NW monsoon.

Strong northeastward currents on the SE Tasmanian shelf generate a lee eddy behind Tasman Island:- IMOS glider confirms a model prediction

An IMOS Slocum glider sampling the Storm Bay line out from Hobart encountered extremely strong currents towards the NE recently, sweeping it up the Tasmanian coast towards Maria Island as shown at right. The glider's track also included a few unplanned loops. One of these was east of the southern tip of the Tasman Peninsula, over the mid-shelf between Tasman Island and the Hippolyte Rocks. A CSIRO experimental near-real-time model simulation has an anti-cyclonic eddy, at about the same location as the glider's loop, that appears to be a topographic lee eddy resulting from the 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

First major upwelling of the season for the Bonney Coast

The Bonney Coast (Portland, Vic, to Robe, SA) is possibly Australia's best-known upwelling hotspot with a well-documented upwelling season from December to March. The upwelling this year, however, has until recently been quite weak. The sea level and pressure maps for 1 Mar, SST image for 3 Mar and chl-a image for 5 Mar document the season's first really significant upwelling, with the usual features all present: a high atmospheric pressure system west of Tasmania caused strong SE winds along the Bonney Coast, which set the sea level down and thermocline up. Cold water surfaced where the shelf is narrowest. The injection of nutrients to the photic zone caused the phytoplankton to grow.

15 April Update:

Ocean impact of STC Rusty monitored by IMOS gliders

Severe Tropical Cyclone Rusty made landfall 100km east of Pt Hedland on 27 Feb 2013. An IMOS glider has recorded, for the first time in Australian waters, the impact on various water properties under cyclonic forcing conditions. Possibly the most notable of these impacts is that turbidity exceeded the intrument's range all the way to 30m (the bottom), as shown in Fig. 10 of a WAIMOS note . 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 of Meteorology ocean model forecast, as shown at right.

Strong upwelling in North-Eastern Bass Strait

The far eastern coast of Victoria has one of Australia's upwelling 'hotspots'. An example of intense localised upwelling occurred in the last few 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 of 16°C on 17 Feb and the cold plume extended 80 km to the south west. Observers at nearby Beware Reef measured surface temperatures as low as 15°C. The cold upwelling triggered a phytoplankton bloom that was clearly evident in a MODIS chl-a image for 21 Feb and even more evident to the Beware Reef diver pictured, who noted only a few m visual range down to 20m where it was quite dark, and only 12°C. The upwelling event appears to be due to the combined effect of 1) the local wind which was upwelling-favourable from 10 Feb, and 2) dynamic uplift from the along-shelf flows associated with a warm-core EAC eddy. The event has been simulated quite well in near-real time using the Bluelink-developed nested-model system but the dynamics have not yet been fully explored. [model estimates of SST and currents] Photo credit: Friends of Beware Reef.

The Sydney-Hobart yacht race:- pre-race notes on the ocean currents

Firstly, a reminder of a change to the way we are showing the strength of the surface current for panels such as the one for Sydney-Hobart. We have opted to double the number, in the east-west direction, of current vectors but halve the scale factor. So a 1kt current now shows as a shorter arrow, as indicated by the key. Bear this in mind if you compare this year's maps with previous years' ones.

Now, the state of the East Australian current and its eddies: The 'speed hump' in Bass Strait that presented navigators with a difficult decision last year is not there this year. Instead, the rhumbline appears to be fairly free of any strong currents as far as we can tell from the latest satellite information. To the east 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 a 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 race 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 200m isobath. Over the shelf, however, there was evidently a favourable flow of about 1kt on 15 Dec, as can be 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 the speed of the flow was. Navigators should watch the imagery for any evidence of this flow persisting til race time, which is quite possible but certainly not guaranteed. As usual, we wish all competitors an exciting but safe race and apologize in advance if the imagery on our website stops updating at any point due to a computer failure while our IT staff enjoy their holidays.

An even-higher sea level event in the Great Australian Bight

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, topping 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 the effect of this can be seen in the 24 August data from the HF radar, which senses the total near-surface current velocity. But why is the along-shore component of the radar velocity clearly less than the estimate derived (by geostrophy) from the sea level maps? The answer to this probably includes several considerations. One, demonstrated so clearly off Perth in June, is that strong onshore winds raise coastal sea level ageostrophically, i.e., the sea level slope sets up to balance the force of the wind. Geostrophy, in contrast, is when the sea level slope is in equilibrium with the Coriolis force acting on the along-shore component of flow. This balance takes longer to establish than the set-up due to wind, which is why we low-pass filter the coastal sea level before inferring the alongshore flow from the map of sea level. It may be that in this case this approach did not remove enough of the sea level slope due to the onshore component of 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 incomplete correction for the effects that large waves have on those measurements.

The Day the East Australian Current vanished

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 happened between 8 and 14 Aug, as shown in the larger-scale map for 11 Aug 2012. One is that the cold core eddy at 30S 155E, over the continental slope off Coffs Harbour, was restricting the southward flow of the East Australian Current at that point, so most of the flow went offshore around a big warm core eddy 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 NSW, centered at 34S 159E on 11 Aug. Strong southerly winds drive coastal currents northward and raise coastal sea level, as shown in the tidegauge data, but that would not change the surface temperature away from the coast. The cold southerlies cool the warm surface layer by extracting heat, but possibly more importantly, also by increasing the surface turbulence, mixing warm shallow layers with the water beneath. In summary, we think that the winds being so strong, and the flow volume so reduced, are what made the surface exression of the EAC off NSW disappear so fast. 22 August update: a fresh nose of warm surface waters can now be seen flowing south again between Coffs and the eddy offshore.

April 2013 update:

Extremely high sea level at Perth and in the Great Australian Bight

On 10, 11 and 13 June 2012, the Bureau of Meteorology National Tidal Facility tide gauge at Hillarys, Perth, recorded non-tidal sea level anomalies of up to 0.8m above mean sea level. The WA Department of Transport tide gauge at Fremantle Boat Harbour recorded a similar surge but also a ~3h-period oscillation that contributed to the highest sea level ever recorded at Fremantle: 1.35m above mean sea level. Fortunately, these surges came at neap tide (click images to expand - Hillarys data courtesy of BoM, Fremantle data and image courtesy of WA DoT). The total sea level might have been an 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 associated with the low pressure systems passing south of the mainland was the main cause. A consequence of these strong winds 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. This will be confirmed when the radar data processing resumes. Along the southern coast of Australia, where the winds 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 surge has now passed through Portland, Vic (non-tidal anomaly 0.3m) and will possibly also be measurable soon east of Bass Strait.

P.S.

A radar view of an intense cold eddy off Perth

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 radar. The drifter has made several clockwise orbits of the eddy center but the radius of these orbits is so small (~10km) that the frequency of the position fixes are sometimes inadequate. The clockwise rotation of these very small eddies has been seen before in SST imagery and by drifters but this recent example is the first we have seen in real-time using HF radar. The tight radius of the eddy is well-resolved by the radar and the correspondence of the radar data with the SST imagery is striking. We have coloured the 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 shelf. 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.:

Warm beach temperatures at Sydney

High sea level on Vic, NSW and Qld coasts, 8-15 April 2012

18 March 2013 Update:

2 July 2013 Update:

Cold water at Byron Bay

Tasmanian beaches: as warm as Bondi?

Update:

Sydney to Hobart yachtsmen: mind the speedhump in Bass Strait

The Leeuwin Current and East Australian Current are both still extremely strong

Extreme eddies off Sydney

18 March 2013 Update:

Rolex Sydney Hobart 2010:

Delayed impact of Ului on Barrier Reef

update on STC "Ului"

update on Sydney

Update:

Beachtime for Sydney

Severe Tropical Cyclone "Ului"

Centaur- success!

Centaur update.

Eddy off Brisbane - good news for Centaur search

The upwelled water off Fraser Island has become a cold-core eddy, around which the East Australian Current has been deviating. The flow is seen by animating a few high-quality SST images that weather conditions made possible on [8 Dec]. The (fairly rare) flow deviation is a welcome development for David Mearns who is searching for AHS Centaur.

Strong upwelling off Fraser Island

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 November causing uplift of the isotherms (see Argo profiles on 8 and 27 November).

Cold-core eddy off Ningaloo:

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

An especially strong cold core eddy off Sydney: [fli animation of SST March 14-15] [.AVI format] [News item]