Mooring Array Design Changes for 2005-2006 S Cunningham

16.1 Introduction

The RAPID-MOC array is a pre-operational prototype. As such, the array design is evolving, driven by practical demands – such as a response to mooring losses to deep-sea fishing, and scientifically – as each set of retrieved data suggests improvements to the measurement array to improve our estimates of the Atlantic meridional overturning circulation.

16.2 Changes in the Eastern-Boundary Mooring Array in Response to Mooring Losses Due to Deep-Sea Fishing

During the year 2004-2005 three moorings in the Eastern Boundary were lost through fishing activity: EBADCP, EB2, and the top part of EB3.

The EBADCP mooring was a total loss and was not recovered. As the releases could not be located we suspect the mooring has been relocated by trawling. Fishing activity is suspected, but no definite proof is available. Another ADCP was deployed in Spring 2005 and survived only a few days. This instrument was recovered and inspection suggests that the mooring was dragged before breaking beneath the release. Deploying again in Autumn 2005 the mooring has a weak link below the release to the anchor. This is to ensure that fishing will break the mooring allowing the instrument to surface and be recovered.

Mooring EB2, deployed in February 2004 surfaced in early October 2004 some 100km north east of its deployment position. Again, no definitive evidence of fishing has been obtained but the displacement distance to much shallower water suggested trawling.

On recovering the EB3 mooring we found that the mooring had been cut at 1000m, with clear damage on the wire indicating damage from a fishing cable.

To combat losses to the principal tall boundary moorings we have moved them much further offshore, adding two more near bottom moorings crawling up the continental slope.

16.3 Pressure Inverted Echo Sounders (PIES)

PIES measure bottom pressure, and travel time to the surface and back to the seabed instrument. The travel time measurement can often be interpreted as a measure of the density structure. Being a seabed instrument the PIES should be less prone to

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fishing activity. However, to interpret the travel time estimates as vertical density, comparisons between density and travel time are required. We will obtain this by deploying one PIES at the EB1 mooring site offshore so we can test the method. A second PIES will be deployed in 1000m depth, giving estimates of the variability in the thermocline density structure very close to the eastern boundary.

16.4 Bottom Pressure Measurements

Based on recent analysis of bottom pressure records (Johns et al, 2005; and Kanzow et al, 2006) annual deployments and recoveries of BPRs does not allow estimation of bottom pressure fluctuations on longer than annual timescales, because of problems with pressure sensor drift and the difficulty of levelling bottom pressure measurements from separate deployments. To combat this we are focussing our BPR measurements at specific sites, two sites at the western and eastern boundaries and one site on each side of the mid-Atlantic ridge. At each site BPRs have been deployed for a two year duration. At the start of year two a second BPR will be deployed at each site, giving BPR records that overlap by one year. This should help eliminate the problem of drift and levelling the pressure records between separate deployments, ensuring we obtain effectively continuous levelled bottom pressure records across the Atlantic.

16.5 Vertical Resolution of CTD Measurements on Moorings

In the papers and thesis (Kanzow, 2004; Johns et al, 2005; and Kanzow et al, 2006), the importance of increased vertical resolution relative to our present

resolution, and making measurements at matching depths on moorings either side of the Atlantic is emphasised. Both improved vertical resolution and matching depths helps reduce biases in the estimates of horizontal gradients. We have adopted the following standard levels: 50m, 100m, 175m, 250m, 325m, 400m, 500m, 600m, 700m, 800m, 900m, 1000m, 1100m, 1200m, 1400m, 1600m, 1800m, 2000m, 2500m, 3000m, 3500m, 4000m, 4500m, and 5000m.

16.6 Limiting use of McLane Moored Profilers

In the first and second years of the RAPID-MOC array we have had limited success with MMPs, recovering only 1078km out of 4086km of vertical CTD profiles (the 2 MMPs from MAR4 and EB2 that are currently deployed are not included in this calculation). This has been due to the total loss of two instruments through fishing (the original EB2 in 2004, and WB4 in 2005); the implosion of vehicle buoyancy on mooring MAR4; and the failure of the CTD pressure sensor and drive train on mooring WB4. Consequently, we conclude that in fishing regions the MMP mooring type is extremely vulnerable. Any wire break is likely to lead to total loss: in contrast vertically distributed instruments on a mooring with backup buoyancy are likely to be partially recoverable, as illustrated by the recovery of EB3 in 2005 despite loss of the top 1000m of mooring. Also, if there is a failure of the CTD then data loss is total. Our remaining two MMPs will be used only in the eastern boundary in the offshore region where fishing is less likely to be a problem. The moorings on the MAR and western boundary have been replaced by vertically distributed CTDs.

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16.7 Changes to Mooring Locations

The principal moorings across the array and where BPR measurements will be made, WB2, MAR1, MAR2, EB1 and EB2 have been relocated slightly so that they lie beneath cross over points of the TOPEX satellite.

16.8 References

Johns, W. E., T. Kanzow, and R. Zantopp, 2005: Estimating ocean transports with dynamic height moorings: An application in the Atlantic Deep Western Boundary Current. Deep Sea Research.

Kanzow, T., 2004: Monitoring the integrated deep meridional flow in the tropical North Atlantic, Mathematics and Natural Sciences, University of Kiel, IFM- GEOMAR, 140.

Kanzow, T., U. Send, W. Zenk, A. Chave, and M. Rhein, 2006: Monitoring the deep integrated meridional flow in the tropical North Atlantic: Long-term performance of a geostrophic array. Deep Sea Research, accepted.