RADARSAT scenes of Australian and adjacent waters

George Cresswell and Paul Tildesley

CSIRO Marine Research

Hobart, Tasmania, Australia 7000

Phone: (61) 3 6232-5222

Fax: (61) 3 6232-5123

E-mail: george.cresswell@marine.csiro.au

ABSTRACT

We examine a number of RADARSAT scenes of a tropical river plume, a high Reynolds number flow through an Indonesian strait, internal wave in an oil and gas field of northwestern Australia, and possible internal wave wakes from seamounts in the East Australian Current. In situ observations and satellite infrared imagery are used to complement the scenes wherever possible.

1. Introduction

The Canadian Space Agency / Agence spatiale canadienne kindly provided us with 22 RADARSAT SAR scenes of regions that we selected in the waters of Australia, Papua New Guinea and Indonesia where we have oceanographic work underway, or where we have worked and have a feeling for the main oceanographic features.

Many of the scenes gave exciting new oceanographic insights, particularly when combined with data from other tools, such as ships, moorings, drifters and other satellites. Others were difficult to interpret, being dominated by meteorological effects.

In this paper we report on our work on the Sepik River outflow in Papua New Guinea, the effects of the throughflow of water from the Pacific to Indian Oceans through Lombok Strait in Indonesia, and internal waves on the Northwest Shelf oil and gas field in Australia. RADARSAT scenes of other regions collected as part of this project will be discussed in later papers. The complete set of scenes can be viewed on:

http://www.marine.csiro.au/~cresswel/RADARSAT/index.html

2. Results

The Outflow the Sepik River in Papua New Guinea

RADARSAT, with its large-area, fine-resolution, all-weather "seeing" ability, was a key addition to the suite of shipboard, moored and drifting instruments used in 1997/98 to study the outflow of the Sepik River as part of project TROPICS (Tropical River-Ocean Processes in Coastal Settings). The aim of TROPICS is to understand the effects that fast-flowing, tropical rivers with heavy loads of sediments and solutes have on the coastal waters and seafloor and, further downstream, on the ocean basins that they feed into.

The research vessel Franklin surveyed the bathymetry and oceanography of the river outflow region in May 1997 during the SE monsoon. A 150 m deep canyon starts just out from the mouth of the river and continues 6 miles down the sloping topography at least to the 800 m isobath. It appears to have been carved by the river outflow. The survey found that the turbid surface plume from the Sepik was only about 1-2 m thick and that it slid across the marine water for 10 miles before being turned to the west by the underlying westward-flowing New Guinea Coastal Current, which had speeds up to 0.8 ms^-1.

The study region was characterised by stacks of near-mixed layers that may have formed elsewhere. The interfaces between the layers were roughly related to current speed maxima with different flow directions for different depths. One would also expect the interfaces to be places of strong internal wave activity. Suspended sediment loads, as indicated by a transmissometer, peaked at some of the interfaces, being greatest at the head of the canyon and becoming progressively depleted with distance offshore. The loads were, however, much less than those in the surface plume.

A RADARSAT SAR scene on 17 May 1997 during the ship survey shows the Sepik River plume and its curved front of roughened water, a lesser plume from the Ramu River a little to the east, and ocean wakes to the west of the offshore volcanic islands in the New Guinea Coastal Current (Figure 1a). The island wakes are white streamers, suggesting a steepening or breaking of small waves. There are also smooth regions where the local winds are slight. The research vessel Franklin appears as a small white spot just below the centre of the scene.

What was happening to the Sepik River surface plume at other times of the year? During the SE monsoon RADARSAT showed that it continued to flow westward in July and August (Figures 1b, c). There were wind shadows as well as ocean wakes behind the offshore islands and outflows could be seen from the lagoons to the west of the Sepik River. While there has been anecdotal evidence in the past to suggest that the Sepik River plume turns to the east during the NW monsoon, RADARSAT answered the question very elegantly by showing that both the Sepik and the Ramu River plumes turned to the east in a scene from March 1998 (Figure1d). The scene also showed that there were wind shadows, certainly as long as 30 km, to the southeast behind the offshore islands.

The interpretation of the in situ data for TROPICS continues and further surveys are planned for the year 2000.

Figure 1 RADARSAT scenes of the Sepik River, PNG, outflow region during the SE monsoon, (a) 17 May 1997, (b) 28 July 1997, (c) 21 August 1997 and the NW monsoon, (d) 1 March 1998.

The Pacific-Indian Ocean Throughflow in Lombok Strait

In previous studies with NOAA AVHRR we have found that the rapid southward throughflow of warm water past the island of Penida in Lombok Strait induces both upwelling in its lee and perhaps vortices up to 100 km downstream. This can also be seen in a CZCS image of the Bali region for 20 May 1981. The Reynolds number for Penida (20 km across) in the flow of at least 1.5 ms^-1 (Murray and Arief, 1988) can be calculated from

Re = ud / Kx

to be 150, where u is the speed of the flow, d the diameter of the obstacle, and Kx the eddy viscosity, ~ 2x10² m² s^-1 (Barkley, 1972, Tomczak, 1988). Since a flow past an obstacle with Re > 70 will spin off eddies into a vortex street, the AVHRR observations should come as no surprise.

What are the effects of such a high Reynolds number flow on the sea surface? The RADARSAT scene for 2154 UTC 7 August 1997 during the SE monsoon (Figure 2a) showed extensive areas of rough water in the strait, a phenomenon no doubt well-known to local mariners, but never mapped synoptically. The scene also shows the surface expressions of internal waves, as well as regions of smooth water. Of these, the one northwest of western Lombok was perhaps due to a wind shadow; while the ones south of Penida seem to occur at the edge of a current front.

The wake southward from Penida to the lower edge of the scene appears turbulent as if the Reynolds number of the flow is very high. The current speed at the time may well have greatly exceeded 1.5 ms^-1.

A clear NOAA14 AVHRR image was obtained 15 hours earlier at 0619 UTC 7 August 1997 (Figure 2b). It reveals warm, 24—C, water from the north jetting 60 km past both sides of Penida. The rapid flow induced overturning or deep mixing of the waters in the lee of Penida and in the lee of western Lombok, with the result that cool 17—C water upwelled to the surface.

Figure 2 (a) A RADARSAT scene for 7 August 1997 showing fast currents flowing through Lombok Strait. (b) A NOAA AVHRR image of Lombok Strait 15 hours prior to the SAR scene.

Northwest Shelf Oil and Gas Field

Internal waves were first detected on the Northwest Shelf of Australia in satellite imagery by Baines (1981) using Landsat. The fine resolution of RADARSAT reveals the surface expressions of internal waves and even oil and gas platforms (Figure 3). The two 70 km-long prominent lines of disturbed water may be due to solitary internal waves. Elsewhere there are many packets of internal waves with wavelengths decreasing back from their leading convex wavefronts. In general they appear to have been propagating towards shallow water ù an exception being the wavefronts immediately south of Rankin Bank, which appear to have been propagating eastward. Further south there are wave packets that have apparently come across the steep shelf edge in the west of the image, thence to cross over wave packets that have come from the north.

Woodside Petroleum Pty Ltd has current meter arrays installed for operational purposes at its North Rankin platform and has kindly made some of the records available for the day of the RADARSAT scene. The platform is on the 125 m isobath and current meters were positioned at depths of 7, 22.5, 36, 50, 65, and 95 m.

Figure 3 A RADARSAT scene of the Northwest Shelf (Australia) oil and gas fields for 12 February 1997 showing the surface expressions of a variety of internal waves (with depth contours overlaid on right).

At the time of the scene (0545 WST 13 February 1998) there were surface expressions of internal waves near the North Rankin platform. The currents at the upper two instruments at this time were roughly opposite (280—) to the direction of propagation implied by the curved wave fronts (110—). The relative speeds of the currents and the waves would determine whether or not the waves were progressing relative to the sea floor. The current vectors at the different instruments at the time of the scene generally decreased and rotated anticlockwise with depth. The temperature structure at the time of the scene was near- linear from 29—C at the 7 m instrument through the other depths to 21.3—C at the 95 m instrument.

In examining the records from different depths for many hours before and after the time of the RADARSAT scene we noted that they were quite dissimilar, with only one oscillation five hours before the scene being felt throughout the water column. Its effect on temperature was greatest, 3—C, at the instrument at 50 m, corresponding to a depression of the temperature structure of about 30 m. It had a variety of effects on the vectors measured by the different current meters. We surmise that an event like this would occur with the passing of the long prominent lines of disturbed sea surface in the RADARSAT scene.

East Australian Seamounts

One of our earlier scenes with ERS-1 SAR suggested that undular and V-shaped features on the ocean surface above the continental slope and shelf off eastern Australia near 31—S were expressions of transverse and oblique internal waves of wavelength about 1 km caused by the influence of seamounts and canyons on an energetic East Australian Current, EAC (Cresswell et al., 1996). The phase speed of 1 km wavelength internal waves was estimated from an assumed ocean density structure to be a little over 1 ms^-1 ù close to the expected speed of the EAC. This meant that transverse waves could become "anchored" behind canyons and ridges aligned across the EAC flow. Single peak seamounts appeared to produce oblique waves with half angles generally around 45—, but sometimes less. Some of the seamounts causing the internal waves may have been as deep as 600 m.

Figure 4 A RADARSAT scene of central eastern Australian waters for 21 February 1998 showing the edge of the East Australian Current (the disturbed region that appears white) and, near the bottom of the scene, some linear wakes that we suggest are the surface expressions of internal wave wakes from seamounts. Some smooth slicks can be traced back to headlands.

In five other scenes, however, the internal wave expressions were absent, probably because the EAC was either not strong, or had meandered further out to sea. With RADARSAT our experience was similar: only one of several SAR scenes revealed a hint of the type of surface expressions that we associate with seamounts. The RADARSAT scene (Figure 4) also revealed the edge of the EAC as a broad band of confused seas that we often experience on our research vessels. Nearshore there were smooth seas associated with coastal upwelling seen in an AVHRR image. Thin slicks up to 70 km long started from some of the major headlands.

3. Conclusions

SAR scenes of the quality provided by RADARSAT are an important tool for the regional oceanographer to employ along with his/her more conventional ones. On the one hand, in a glance the interpretation of in situ data sets becomes vectored towards the truth. On the other, new and unsuspected features of the ocean's behaviour becoming blindingly apparent.

References

Baines, P.G. (1981). Satellite observations of internal waves on the Australian North-west Shelf. Australian Journal of Marine and Freshwater Reseaarch 32, 457-463.

Barkley, R.A. (1972). Johnson Atoll's Wake. Journal of Marine Research 30, 201-216.

Cresswell, G. , Z. Changbao, P.C. Tildesley, and C.S. Nilsson. (1996). SAR observations of internal lee and wake waves from sea mounts. Marine and Freshwater Research 47, 489-495.

Murray, S. P. and D. Arief. (1988). Throughflow into the Indian Ocean through the Lombok Strait, January 1985 ù January 1986. Nature 333, 444-447.

Tomczak, M. (1988). Island wakes in deep and shallow water. Journal of Geophysical Research 93, 5153-5154.