Voyage Plans and Summaries

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Franklin Voyage Summary No. FR06/2001

Title

Tasman-Coral Sea mass and heat transport / Regional Ocean Observing Network design — Part 2.

Itinerary

Departed Apia 1000hrs, Sunday 8 July 2001 local (9 July 2001 EST)
Arrived Brisbane 1000hrs, Monday 23 July 2001

 Principal Investigators

Ken Ridgway (Chief Scientist)
CSIRO Marine Research (CMR)
GPO Box 1538
Hobart TAS 7000
Tel: 03 6232 5226
Fax: 03 6232 5000
Email: Ken.Ridgway@marine.csiro.au

Rick Bailey, CSIRO Marine Research

Ian Barton, CSIRO Marine Research

Richard Coleman, CSIRO Marine Research/University of Tasmania

Phil Sutton, New Zealand Institute of Water and Atmosphere (NIWA)

Scientific Objectives

This cruise is the final component of a multi-cruise experiment to enclose the Coral-Tasman Sea. The two previous cruises FR04/2001, FR05/2001 and a section east of New Zealand to be performed by colleagues at NIWA complete the box around the region.

The following objectives relate to both the individual part (FR06/2001) and the combined set of cruises:

• to measure the seasonal, interannual and decadal fluctuations in the transport of mass, heat and freshwater into the Tasman Basin,

• to survey the East Australian Current and associated eddies and fronts using in-situ and satellite data,

• to investigate the relationship between oceanic heat transport into the Tasman Sea, SST anomalies and regional climate fluctuation,

• to provide appropriate in-situ data for testing and validation of remote sensing techniques as well as ocean only and coupled ocean-atmosphere models,

• develop and validate multidisciplinary elements of a Regional Ocean Observing Network (ROONet) in support of an Australian Ocean Observing System (AOOS),

• to contribute to the development of an operational marine monitoring and nowcast system for Australian waters which also forms part of the Australian Ocean Observing System (AOOS),

• to gain an understanding of the basin-scale ocean variability within the Tasman and Coral Seas as part of the GOALS objective of the international Climate Variability program (CLIVAR).

Cruise Objectives

To occupy the PX30 section (Brisbane-Fiji) as part of the long-term monitoring of the mass and heat transport in the Tasman Sea and obtain closed volume observation around the SW Pacific boundary to compare direct transport closures with estimates derived from satellite observations.

To "value add" to the 10 years of high density XBT data (1991-2001) collected along this route from merchant vessels by achieving the following:

• assess the errors involved in obtaining dynamic height from XBT temperature profiles along the section,

• quantify the contributions to the baroclinic flow below 800m along the section,

• assess the validity of steric height and temperature extrapolation schemes along this section,

• determine a revised sampling strategy to enable the section to be maintained in the long term with a combination of satellite and in-situ data collection,

• assess the validity of inferred subsurface temperatures from surface satellite altimeter/SST observations and improve existing vertical projection schemes,

• assess the capabilities and utility of a real-time oceanographic analysis system under operational conditions,

• assess a range of data collection methods for application to ‘Ship of Opportunity ‘ programs,

• acoustically map and monitor at suitable and spatial scales the critical water column plankton/nekton and seabed habitats of the region,

• obtain validation data for sea surface temperature (SST) for MODIS, ATSR-2 and AVHRR satellite instruments,

• collect data to assist with the development of ocean colour algorithms using SeaWiFS, MODIS, and other ocean colour data,

• calibrate and evaluate XBT and XCTD instrumentation against the CTD.

Cruise track

The cruise track is shown in Fig. 1. The first section between Apia and B was a steaming leg which brought the ship to the end of a transect occupied during the previous cruise (FR05/2001). The next segment from B to C was necessary to connect the previous components to the final section. A short transit around the southern coast of Fiji then brought the ship to a point just south of Nadi (D). The main section of the cruise then followed the typical route of the XBT high-density merchant vessels between Fiji and Brisbane (DE, PX30).

Figure 1. The full red line shows the cruise track for FR06/2001. CTD stations are indicated by blue crosses. The red black squares indicate locations where ARGO floats were released.

Results

The cruise was successful in collecting the data required to address all of the objectives outlined above. The Fiji-Brisbane CTD section is particularly valuable, as it both provides a further ‘benchmark’ for the 10 years of XBT transects and also the section itself may be understood in the context of this XBT time series. We note that the processing procedures for Franklin data have been streamlined, so that much of the data are now readily available either during or soon after a cruise. For example, the CTD data are now processed during the cruise and the calibration with bottle samples is now completed by the end of the cruise.

Figure 2. Surface height field from real-time TOPEX/POSEIDON anomalies added to mean height field from CARS climatology.

Figure 3. Temperature and Salinity along section from CTD.

One of the main objectives of the cruise was to obtain a full-depth CTD transect between Fiji and Brisbane. This followed the track taken by container ships which have participated in a high-density XBT project for nearly 10 years. The temperature and salinity sections are presented in Figure 5. At the eastern end of the section we observed a surface lens of fresh water which persisted to the region south of New Caledonia. A far greater volume of water was contained in this feature compared to that observed on a previous occupation of this section in April 1999. The lens results from fresh water outflow from the PNG north coast and its increased volume is related to the monsoonal rains to the northwest. At a depth of 100-200m the presence of Sub-Tropical Lower water (SLW) was indicated by a salinity maximum. This decreased to the west, with maxima observed at several locations east of 172^oE. A comparison with the earlier data showed that the surface influx of freshwater pushed the salinity maximum to greater depths. In addition, the SLW showed less penetration to the west in this section — presumably due to seasonal forcing.

In the vicinity of New Caledonia we again observed energetic internal waves in both the T and S sections (Figure 5). However, although the waves appeared to be locked to the bathymetry the extent of their vertical amplitudes was less than previously observed. These features have been also observed in the XBT temperature profiles and so the CTD’s were closely spaced to resolve the waves in some detail.

In addition to the CTD’s a completely independent temperature section was obtained from XBT data. When compared with CTD full-depth sections these data enable several uncertain aspects of the results gained from the previous time series of XBT sections to be assessed. In particular, we are able to determine the accuracy of the T-S method for calculating steric height (and transport) directly from XBT temperature profiles and to quantify the extent of sub-800m baroclinic flow and the validity of vertical extrapolation schemes.

The typically strong East Australian Current (EAC) was observed just on the continental slope with surface currents greater than 1.0 ms^-1 and a volume transport magnitude of 30 Sv. The ADCP results showed evidence of substantial eddying activity throughout the section, with alternating currents of order 0.5 ms^-1. The strong southeastward flow adjacent to New Caledonia has currents approaching 1.0 ms^-1 (see Figure 2).

Validation of Satellite-Derived Sea Surface Temperatures.

Two infrared radiometer systems were deployed during the cruise to measure the skin temperature of the ocean surface outside the disturbed waters of the ship's wake. The twin TASCO system used on the two previous cruises (FR04/2001 and FR05/2001) was supplemented by the CSIRO custom-built DAR011 radiometer. The TASCO radiometers were logged on a Datataker DT50, and the DAR011 measurements were logged on a Gateway Laptop computer. The radiometers were mounted on the "Hero" platform outside the bridge door on the starboard side and data logging was performed inside on the bridge.

Persistent rain showers, and periods when sea spray was reaching the radiometers, meant that the radiometer viewing ports were often covered for protection. Persistent clouds also meant that limited opportunities were available for comparison with clear-sky satellite images. Four balloon-borne radiosondes were launched at times when the European ERS-2 satellite was overhead. Full analysis of the satellite and ship data is now waiting on delivery of the satellite data from Europe.

The data collected throughout the cruise will be useful in defining techniques for future data collection, as well as providing more information on the energy budgets at the air-sea interface.

Figure 4 shows a preliminary analysis of the cruise data and shows a radiometric skin temperature that is some tenths of a degree cooler than the bulk temperature measured with the ship thermosalinograph. This is consistent with the data collected on previous cruises.

Figure 4 . Radiometer measurements collected during FR06/2001.

XBT Comparisons

Just over 180 XBT’s were dropped between Western Samoa and Brisbane. The CSIRO Mk12 XBT system was used throughout the cruise, with XBTs deployed coincident with every CTD. XBT’s were also deployed while underway, midway between most CTD stations.

Several Mk-9 XBT systems were used in parallel with the Mk12 XBT’s, to evaluate a possible bias between the various systems. Twelve XBT’s (predominantly T-7’s) were deployed using the CSIRO T1200 software at the beginning of the cruise. New software being developed by CSIRO was then trialled but only one profile was successfully recorded. It was later found that the software was unable to translate the offset and scale values of the Mk9 cards onboard. Finally, another twelve Deep Blue XBT’s were deployed using the BoM Compaq laptop. In all cases, the Mk9 XBT’s were deployed coincident with Mk12 XBT’s and CTD stations.

A visual comparison of the Mk9 (BoM and CSIRO T1200) with the Mk12 XBT data showed very good agreement between the two systems in most cases. However there was also evidence of a small temperature bias in both Mk9 systems, with these results being warmer than the Mk12 data. Initial calculations indicate that these differences are less than 0.2 degrees C.

Figure 5. Simultaneous XBT casts using CSIRO system, MK 12 interface (Black) and the BOM system, MK. 9 interface (Red).

Summary of Data Collected

CTD Profiles

A total of 70 full-depth CTD profiles were obtained using 12 bottle rosettes. Samples of oxygen, salinity, NO₃+NO₂, SiO₂ and PO₄ were obtained.

XBT’s

A total of 180 XBTs were dropped along the entire cruise track. These consisted of 46 T7s, 11 T5s and 123 deep blue.

Underway

Observations were collected from the ADCP, GPS (ASHTEC), thermosalinograph, and a suite of meteorological sensors for the duration of the cruise.

SST

Two infrared radiometers were operated almost continuously during the voyage and five radiosonde balloons were launched when crossing ERS2 satellite tracks.

Argo Floats

Seven floats were deployed at roughly uniform spacing along the section as part of the global ARGO program on behalf of the Scripps Institution of Oceanography.

Cruise Narrative

We departed Apia, Samoa at 1000hrs on Sunday July 8^th (Western Samoa local time). The day was warm, the sky clear and the view looking back to the harbour was quite spectacular.

We proceeded on a southwestward course which brought us to the end of a zonal section occupied by the previous cruise (FR05/2001). However, just 8 hours from Apia the first task was to deploy the first of the ARGO floats. This was performed successfully from the stern. The launch procedure was carefully followed exactly as specified in the instructions provided. This method proved to be difficult to prevent the float from swinging around on the single tether and sustaining damage on the stern of the ship.

The first station was reached just after midday the following day. Due to the crossing of the International Dateline we lost Monday 9^th completely and so we reverted to Tuesday 10^th July. A test launch of the CTD was then performed down to 1000m. We used the new Seabird instrument with the large frame although only twelve 5 litre Niskin bottles were used. The section proper was then commenced as we completed the full station. The day was fine and clear and we anticipated that this would persist throughout the cruise. During the cruise we received regular up-dates of the real-time altimeter maps which helped to place our results into a wider context of the basin circulation

An associated program was also commenced at this point which consisted of an independent XBT transect. A further set of XBT deployments were carried out which were designed to test various combinations of probe type, recorder interface and acquisition software. The standard XBT system for VOS deployments was used as the reference section, with the (T7) probes launched from an appropriate point on the aft deck.

The following segment of the cruise consisted of some 13 stations which closed the gap in the overall ‘Tasman Box’ between the previous ‘dateline’ section and the final high-density XBT transect from Fiji to Brisbane. As we steamed southwestward along the track the winds maintained a steady 10-15 knots from the southeast and the cloud cover increased. On Wednesday (July 11) a further ARGO float was deployed about halfway along the section (BC) in the relatively deep water (2000m) just to the east of the Lau Ridge. On this occasion we modified the launch protocols so that the single tether was dispensed with and the float was simply dropped into the water well clear of the ships stern. This seemed to produce an acceptable result and reduced the possibility of the float contacting the ship during the deployment.

The end of the first segment of the cruise, just to the south of Fiji, was reached in the evening of Thursday (July 12). The opportunity to enjoy cocktails at the Australian High Commission was rejected in the interests of maintaining the cruise schedule. Despite the mild weather for the first segment we had only just maintained our very tight schedule and so were rather concerned at falling behind over the next section. The cruise track then followed around the southern flank of Fiji — unfortunately for the would-be spectators, under the cover of darkness.

The start of the main Fiji-Brisbane section, slightly to the west of Nadi, was reached at 0400hrs on Friday 13^th. The initial plan was to occupy the same stations as those completed along the same section during a previous cruise 2 years ago (FR02/1999). However, due to the very tight time restrictions in several cases we were forced to increase the spacing of the stations in order to keep on schedule. The next phase of the XBT comparisons was commenced. This involved testing the new windows based acquisition software developed at CMR. Unfortunately, despite the best efforts of all concerned only one successful drop was completed. An independent set of XBT data were collected along the section. This included probes launched at the CTD stations as well as between stations. To be consistent with the routine high-density sections, ‘deep blue’ probes were used. The next 2 ARGO floats were successfully launched in depths around 3000m. It was interesting to note that after being launched the floats varied their behaviour. In some cases they immediately disappeared from sight while we also observed situations where the antenna was visible for several seconds before slowly sinking below the water.

We crossed the vast depths of the New Hebrides Trench early on Monday (July 16). By this time we were some hours behind schedule so the CTD was only deployed to 4500m in what was a maximum depth of 6800m. The section then began to shallow as we approached the Norfolk Ridge. The station spacing was greatly reduced in this region to resolve the finescale structure of the energetic internal waves that are commonly observed there. We passed just to the south of the strange flat-topped Walpole Island which the maritime guide described as being overrun by spiders.

Several problems were encountered with no fire and no acknowledgement from the rosette unit. On July 21 (ctd#57) a further problem with the water sampling occurred, the software accepted the sample command but continually reported sample #13 taken and whilst bottle #15 did close, the last 4 bottles remained open. The problem continued into the following diagnostic test station, where it was noticed that the sampling counter did not reset to the home position on starting, but after reloading the CTD program (ctd_main), the problem disappeared. A laptop terminal was inserted in the modem serial line between the deck unit and annex port to determine whether the problem was in the Deck Unit/Rosette or the Annex/Computer side. During the next cast an unrecognised ASCII character (dec152) was received on the terminal after sample 18 on ctd#62. Thereafter, the communication from the terminal was disrupted although it was still possible to reset the modem to home position and close the remaining bottles from the deck unit. The deck unit was then replaced to eliminate another possible error in the link, and the fault did not recur for the remainder of the voyage.

An important component of the cruise was the SST validation exercise. This involved continuous monitoring of the sea surface temperature (SST) for MODIS, ATSR-2 and AVHRR satellite instruments. An infrared radiometer measured the surface skin temperature while another radiometer assessed the surface heat and radiation budget. Several weather balloons (with standard radio-sonde attached) were deployed at satellite crossing points.

Once clear of New Caledonia we were on the final portion of the transect crossing the Lord Howe Rise and then the abyssal waters within the narrow endpoint of the Tasman Basin. The surface height map from the altimetry satellite showed relative weak eastward flow over most of the region beyond New Caledonia. Within 400km of the western boundary we encountered a substantial anticyclonic high with northwest currents of ~ 0.6ms^-1 on the eastern side. The EAC was in full flow on the western flank and the largest currents of the cruise (up to 1.8 knots) were observed.

As we proceeded towards the Brisbane endpoint of the section we encountered consistent southerly conditions. This pattern derived from the presence of a persistent low which was located over New Zealand. The wind and seas had built up to some extent which reduced our top speed and made conditions rather more uncomfortable. The build-up of wind and seas continued and combined with the tightness of our schedule the CTD sampling was reduced slightly. We noted that the range of ADCP collected deteriorated in concert with the change in weather conditions. Fortunately over the final 24 hours conditions moderated slightly and we managed to gain time over the last few stations approaching the continental slope. The section was completed at 2000hrs on Sunday July 22. We arrived in Brisbane slightly ahead of schedule at 0900hrs on July 23.

Summary

The cruise was very successful and a high quality data-set was obtained which met all our initial objectives. All instruments performed well with no major problems. The CTD functioned very well and the CTD vs. sample comparisons indicated that the data were of the highest quality. We note that the reduced time allocated to the cruise (ie. no bad weather days) meant that the schedule was very tight and some rationalization of the cruise plan became necessary so that we could reach Brisbane on schedule. Perhaps the cruise proposal guidelines should be expanded to include further information on cruise length and provision for adverse weather etc. The full depth CTD section, especially when combined with the previous 2 cruises, will provide a very valuable complement to understanding and interpreting the 10-year time series of high-density XBT sections.

Data Distribution

Following a reasonable period for data processing and reporting of results, the data collected from this cruise will be made freely available to international scientific programs (e.g. CLIVAR), national and international data archives, including those of Samoa, Tonga, Fiji, New Caledonia and Vanuatu

Personnel

Scientific Participants

Franklin Crew

Acknowledgments

The ORV staff displayed their usual professionalism and managed to cope with all problems with equipment and software. I would also like to thank all members of the scientific staff who worked so very well as a team, obtaining data of the highest quality. The ship’s officers and crew provided an efficient and well-run ship at all times ensuring that the scientific requirements of the cruise were addressed. In general, the cruise was a most satisfying experience both on a scientific and a personal level.

We gratefully acknowledge the support provided by the CSIRO Earth Observation Centre which has been central to the success of this cruise in collecting such a valuable set of simultaneous satellite and in-situ data.

Ken Ridgway
Chief Scientist

Updated: 31/01/03