.
.

South-East Fishery ocean movies

David Griffin, Madeleine Cahill, Chris Rathbone and Neil White
CSIRO Marine Research
Version 1.4.3 17 January 2002
[updated (www) version] [obtaining a CD or poster, revision history]
[Cataloguing-in-Publication entry of the CD-ROM version]

Installing a movie player
Movies with frames at intervals of:

5 days, showing sea surface temperature, height and current averaged over 10 days, on 5km and 18km grids, respectively. Trajectories of satellite-tracked, free-drifting buoys are also included.

5 days, showing seasonal anomaly of sea surface temperature, otherwise as above

~6-hours, showing more detail of surface temperature

8 days, showing chlorophyll-a instead of temperature
Oceanographic discussion
Appendix: Technical details
References
Acknowledgements

1.1 Sea surface temperature, height and current at 5-day intervals

[1990] [1991] [1992] [1993] [1994] [1995] [1996] [1997] [1998] [1999] [2000] [2001] [Contents]
Click on a year to play a movie, but first read [Installing and running a movie player]

What is shown

Each map shows the sea surface temperature composited over a 10-day period. The temperature scale shows that bright red, for example, means the water was about 22°C (at this time of the year - the scale shifts to follow the average seasonal cycle). Black arrow heads show the ocean current velocities which we infer from the sea level map shown as contours. Closely-spaced purple arrow heads show the positions and velocities of Argos satellite-tracked free-drifting buoys, at 12-hour intervals. Purple arrows on a 1.9° (latitude and longitude) grid are 5-day averages of the wind stress (approximately proportional to the square of the wind velocity).

Some explanation

Satellites measuring sea surface temperature have been flying since the 1980s, so many people are familiar now with the sort of images they produce. The AVHRR is a scanning instrument, so each overpass of the satellite returns a 2000km-wide swath of data. However, it cannot see through cloud, so it ends up taking typically 10 days to piece together a complete ('composited' - see Appendix) map of any particular region. If cloud is very persistent, of course, the resulting composite can be incorrect in places.

(www visitors only: [1993] [1994] [1995] [1996] [1997] [1998] [1999] [2000] ) [Contents]
The more novel feature of the movies is the inclusion of estimates of the surface current. These are derived from maps of sea level in the same way that meteorologists determine the winds from maps of atmospheric pressure. An example colour-coded map of tidal-residual sea level is shown above. The scale at the bottom shows that red, for example, means the sea level is 0.4m higher than the tidal prediction. In September 1992, a satellite was launched with a radar altimeter that measured sea level accurately enough for this to be done. The arrow heads show the geostrophic current going clockwise around a sea level depression, and anticlockwise around a high. The current is fastest where the sea level changes suddenly, for the same reason that winds are strongest where the isobars on a weather map are close together. The accuracy of the satellite 'altimetric' current can be gauged by comparing its estimates with the trajectories of the drifting buoys. The drifter going around an eddy at 44° 15'S, 149°E, for example, agrees with the satellite estimate. A quantitative comparison of the drifter velocities with altimetric estimates shows that the typical difference is 0.25m/s, or half a knot. Most of that is because an altimeter can only measure sea level directly under itself. The Topex/Poseidon altimeter takes 10 days to complete a global grid of lines 250km apart, by which time the ocean has changed somewhat. The fairly wide spacing of the satellite tracks makes it impossible to see all the detail of the sea surface, so the maps are smoother than reality, and currents often weaker. Flows inshore of the 1000m isobath are much less well estimated than further offshore, as discussed below in the appendix. The relative error (error divided by the speed) is least where the current is strong: drifter speeds of 1.5m/s (3 knots) are quite accurately estimated by altimetry.

1.2 Sea surface temperature seasonal anomaly, current and wind at 5-day intervals

[1990] [1991] [1992] [1993] [1994] [1995] [1996] [1997] [1998] [1999] [2000] [2001] [Contents]
These movies are the same as those above except that the temperature map shows the difference from the mean seasonal cycle. Our estimate of the mean seasonal cycle is provided by the CSIRO Atlas of Regional Seas (CARS), which draws on decades of sampling from ships to build up a description of the average seasonal cycle of temperatures, salinity etc, around Australia. The spatial resolution of the atlas is 0.5°. In each grid box, the mean, annual and semi-annual cycles are estimated.

2. Sea surface temperature and current at several-hour intervals

[1995] [1996] [1997] [1998] [1999] [2000] [least cloudy frames] [Contents]
(www visitors: these files are big, click on the image to see a short example)

What is shown

The arrow heads again show the satellite altimetric current. Drifter positions are shown every 6 hours from 5 days before the image time. The sea surface temperature anomaly, however, is processed differently, to retain more detail. When you run the movie, you can see small features moving along with the current, where there are long enough gaps in the cloud cover. To retain the detail of the raw data, we've not averaged them over time. That is, we've shown an image for every individual satellite pass. That raises the problem of what to do about clouds, and other atmospheric degradations of the data. Whenever the data are flagged as clearly 'bad', the image simply freezes at the most recent 'good' estimate of the sea surface temperature.

Some explanation

In movie type 1, the gaps in sea surface temperature images due to clouds were interpolated (estimated between observations) by seeking only to estimate the average temperature over a 10 day period. But that process smears out a lot of the detail, so the movie showed how large bodies of water changed their position, but not the movement of water within each body. To see movement of small features, we have to be content to see a very incomplete picture, which also has errors. One reason for the errors is that not all clouds (especially at night) are easy to detect automatically, since their apparent 'temperature' can be close to that of the ocean. A human observer can usually tell cloud from ocean better than today's automated processing. For these maps, which are intended solely for visual inspection (rather than ingestion in a numerical ocean model), we've only blanked out the really obvious areas of cloud. Nor have we blanked out areas where a 'warm skin' has formed on the sea, as happens during light winds. We've relaxed these quality controls so that less good data is erroneously discarded, so each image is as complete as possible.

3. Chlorophyll-a at 8-day intervals

[1998] [1999] [2000] [Contents]
These movies show SeaWiFS estimates (available since September 1997) of the amount of chlorophyll-a in the water, which is determined from the colour of the water. Each image is a composite over 8 days. The map above, however, is not a composite over 8 days, but shows the data from a single pass of the satellite. Note the scale is logarithmic, so 0 (orange) means 1 mg per cubic meter of chlorophyll. The deep blue waters of the East Australian Current are then plotted as deep blue, while greener, more productive waters are shown as green to red.

The image above is a particularly striking one, showing a fascinating example off Hobart of a cyclonic (cold-core) eddy that has little chlorophyll right in the centre, but a ring of high chlorophyll water rotating clockwise around it. Off southern NSW is an example of an anti-cyclonic (warm-core) eddy rotating anti-clockwise, with the highest chlorophyll concentration right in the centre. See also the plume of high chlorophyll off the Bonney coast where upwelling of cold, high-nutrient waters often occurs, as discussed below.

4. Oceanographic discussion

We include here only a few comments on some of the phenomena to be seen when viewing the movies. For more information about the oceanography of the region, see CSIRO's EAC leaflet, CSIRO's Sydney-Hobart (2000) www site and Cresswell (2000).

The East Australian Current (EAC) and its eddies

The EAC flows all the way down the east coast of Australia, bringing waters of the tropical Pacific ocean as far south as Tasmania in summer. Most of the flow, however, leaves the Australian coast somewhere off NSW to head east along the Tasman Front towards New Zealand. The extent of summer southward flow varies considerably from year to year. In January 1999, for example, the EAC was particularly strong. Winter is when the flow is weakest, and penetrates least southerly, allowing temperate water masses to flood the shelf (eg [10 August 1996], [26 August 1998] ). The cold water on the shelf off southern NSW is often (eg [28 August 2000]) clearly from Bass Strait. The separation from the Australian coast is a very unstable process and sometimes large amounts of EAC waters pinch off from the flow to form warm-core, anti-clockwise rotating (anti-cyclonic) eddies. The satellite altimeter sees these eddies as regions of raised sea-level. The image above for 1 September 1998 shows three such warm-core eddies (the 3rd is centred at 43° 20'S 151° 30'E). Cold-core, clockwise-rotating (cyclonic eddies) are also sometimes formed (eg [15 August 1997], [20 January 1997]).

The Zeehan Current and its eddies

The boundary current flowing along the continental slope from the Great Australian Bight to Tasmania is very much weaker than the East Australian Current, and much less well documented. South of Bass Strait, it is known as the Zeehan Current (Baines et al., 1983). It appears to be mainly a winter current. Good images of it are for [27 July 1996], [17 July 1999] and [2 August 2000]). Note that it is also an unstable current, with streamers carrying water off the continental shelf to accumulate in anticyclonic eddies in deep water.

Upwelling off South Australia to eastern Victoria

Perhaps one reason that the Zeehan Current does not flow in summer is that that is when winds are often in the opposing direction. When they are strong enough, they bring cold water to the surface in a process known as coastal upwelling. This happens off the Eyre Peninsula, the Bonney Coast (Robe to Portland) and eastern Victoria (Lakes Entrance to Croajingalong). Good images of it are for [2 March 1995], [12 January 1997], [10 December 1997] and [1 February 1999].

Other

See also the good image of a large anti-cyclonic eddy west of Tasmania on 9 February 1999. Step the 5-daily movies backwards from that date and you will see that that eddy was formed off south-east Tasmania in September 1997. Step forwards and you can follow it until it goes off-screen to the west in April 2000.

Also in 1999 off Tasmania, but off the east coast, occurred another notable event. This was a region of cold surface water that first appears in the temperature anomaly movie on 25 March 1999 at 42° 30'S 150° E. It is fully developed by 28 April 1999. After encountering the continental shelf, it propagates northwards, reaching northern Bass Strait by 20 May 1999, to collide with a warm core eddy at 39°S 149°E on 4 June 1999.

Appendix: Data sources and methods - technical details

Here we provide a very brief outline of the data used. The dates in parentheses refer to the span of the data used for the movies.

Sea level maps (Jan 1993-Dec 2000)

Data from up to three altimeters (CNES/NASA Topex/Poseidon and European Space Agency's ERS-1 and ERS-2) at a time, and coastal tide gauges were interpolated on a 25km grid every 5 days by an optimal interpolation. We include the coastal tide gauges in the analysis because altimeter data are not useful over the continental shelf for a number of reasons including orbit errors near land, incorrect tidal corrections and the increased intensity of rapid (ie daily) variations of sea level. However, for the SEF region shown here, there are an insufficient number of tidegauges installed to make the maps valid everywhere over the shelf. Open circles along the coastline of the example height map show the locations where estimates of coastal sea level are used in producing the map. These are interpolated along-coast between adjacent tide gauges. Altimeter data were obtained from NASA Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory / California Institute of Technology and AVISO at Collecte Localisation Satellites. For 1999 and 2000, we show the (10 daily, slightly coarser resolution) gridded "MSLA" sea level maps produced by AVISO because we have not yet processed all the data required to do our own gridding. Tide gauge data (Jan 1993 - Dec 1998) were obtained from the National Tidal Facility .

Currents

These are determined from the sea level maps by assuming a cyclostrophic momentum balance, which is close to a geostrophic balance but includes a small correction for the centrifugal acceleration, which is responsible for a warm core (anti-cyclonic) eddy having a smaller sea level anomaly than a cold core (cyclonic) eddy of equal angular velocity. For more information see Griffin et al. (2001). It is important to note that away from the locations of tide gauges, estimates of currents over the continental shelf are unlikely to be useful, and that near the tide gauges, the estimates are an average over the entire continental shelf, and over a 10-day period. To reduce crowding on the plots, velocities are only shown at every 2nd grid point (ie 50km spacing).

Sea surface temperature (Jan 1990 - Dec 2000)

These data were received and processed by CSIRO in Hobart. For the post-February 1996, five-daily composite maps, each pixel is the 65th percentile value of all unflagged data within a 10 day, 6km data window. Data are flagged as atmospherically-affected if there is too much brightness in the visible band or too much absorbtion in the thermal band (detected as the difference between channels 4 and 5). No cloud-flagging was done prior to February 1996. Accordingly, each pixel of the composite image is the 95th percentile of all the raw data in the same data window. For the 'snapshot' (a few hours rather than 5-day composite) images, data are also flagged as 'bad' if the temperature difference from the 10-day composite is outside of -6 to +2°.

Chlorophyll-a (Sept 1997 - Dec 2000)

These data are from the SeaWiFS project at the NASA Goddard Space Flight Center. The processing version is SeaDAS3.

Wind (Jan 1990 - Dec 1999)

Daily averages of the wind stress were provided by the NCEP/NCAR 40 Year Reanalysis Project . These data are used to compute the Ekman velocity which we add to the altimetric geostrophic current estimate, eg for making comparisons with drifter velocities, or modelling the movement of sub-surface larvae.

Drifters (Jan 1990 - Jun 1999)

Drifter data were obtained from the World Ocean Circulation Experiment Surface Velocity Program Data Assembly Center via MEDS. The drifters have sea-anchors centred at 15m depth, and hence are less subject to the effects of the wind than objects right at the surface.

References

Baines, P.G., Edwards, R.J. & Fandry, C.B. (1983). Observations of a new baroclinic current along the western continental slope of Bass Strait. Aust. J. Mar. Freshw. Res. 34: 155-157.

Cresswell, G. (2000). Currents of the continental shelf and upper slope of Tasmania. In Banks, M.R. & Brown, M.J. (Eds): TASMANIA AND THE SOUTHERN OCEAN. Pap. Proc. R. Soc. Tasm. 133(3): 21-30.

Griffin, D A, J L Wilkin, C F Chubb, A F Pearce and N Caputi (2001). Ocean currents and the larval phase of Australian western rock lobster, Panulirus cygnus. In press at Marine and Freshwater Research.

Acknowledgements

The production of this CD was supported by the AFMA Research Fund (Project No. 99/1505). We are also very grateful to the institutions discussed above for data provided, and to many CSIRO colleagues for help with numerous tasks.

[Contents]