The Design Process, Evolution and Deployment of the Rapid-MOC Moorings in the Atlantic at 26.5ºN Darren Rayner and Rob McLachlan National Oceanography Centre, Southampton

Introduction to the Rapid-MOC project Combines array of moorings across the Atlantic at 26 N, with winds from satellite scatterometry and the Florida Straits flow from a disused telephone cable between Florida and the Bahamas Collaborative project between NOCS, RSMAS (University of Miami) and AOML (NOAA, Miami) Mooring array first deployed in 2004 and serviced annually Array will continue to 2014 through funding of Rapid-WATCH project Evolving array design Project aim: To provide a 10 year time series of the strength and structure of the Atlantic Meridional Overturning Circulation

UK contribution currently consists of: 21 moorings (9 of which tall) 12 BPR landers 2 Inverted Echosounders US contribution currently consists of :- 3 moorings (in WB sub-array) 4 BPR landers (in WB sub-array) Florida Straits Cable 3 sub-arrays - Eastern Boundary (EB), Western Boundary (WB) and the Mid-Atlantic Ridge (MAR) Introduction to the Rapid-MOC project

Bottom Pressure Recorders (BPRs) 1st year tall moorings deployed using a drop off mechanism Non-uniform drift in pressure sensor - looks exponential but detrending may remove signal were trying to detect 2nd year of drift more closely approximated to linear Difficult to join subsequent time- series due to pressure drift so now use overlapping time-series Now use a lander tripod separate from the rest of the mooring and deployed for two years

Example of 1st year mooring design Main hold-up from single large steel sphere Very little support/backup buoyancy used Light anchor (900kg) 3/16 diameter wire used throughout 9 Microcat CTDs for 4700m of water column BPR on drop-off mechanism

Example of current mooring design Staged top design with 2 steel spheres and buoyancy above this too Much more support/backup buoyancy Heavier anchor (twice as heavy) Mixture of 4 different wire diameters (4mm, 5mm, 3/16 and 1/4) 16 Microcat CTDs No BPR

Multi-stage top design Top floats can be cut off and 24 SS will support the upper section Weakest link is above main buoyancy Assumes actual breaking load close to manufacturers stated minimum breaking load 3 x Trimsyns 50m depth 24 Steel Sphere 90m depth 37 Steel Sphere 150m depth 8 x 17 glass 780m depth 40m of 4mm wire 960kg MBL 60m of 4mm wire 960kg MBL 630m of 5mm wire 1500kg MBL 3/16 wire 1814kg MBL 25kg 66kg 371kg 543kg (at 2000m) In-situ tension 935kg 894kg 1129kg 1271kg Reserve Weakest Link

Design Process - Flowchart Current profile Site info Science requirements Rough idea/sketch of mooring.csv text file of mooring design Outputs: Backup buoyancy Launch tension Knockdown (max depths) Stretch (min depths) Required anchor weight In-situ tension Database of materials Design OK? Working design Mooring package No Yes Adjust design Previous experience

Backup buoyancy MAR MAR

Launch tension Peak launch tension calculated from modelling drag as the mooring falls to the seabed following an anchor last deployment. Design package gives warning if launch tension is over 50% of breaking load Heavier anchor = higher launch tension WHOI safe anchor weight determined from buoyancy, current profile and drag using: Wet Anchor Weight = 1.5 x (V A + H A /0.6) V A = vertical anchor load H A = horizontal anchor load Need to convert to dry anchor weight for material being used

Knockdown Knockdown (Subduction) calculated from mooring drag and current profile Design package produces simple reference plot Aim to modify routines further to warn if exceeds maximum operating depth Instrument tilt can be calculated (routines been modified to automatically do this)

Summary Brief intro into the Rapid-MOC project. Discussed changes in the mooring designs – BPR landers – tall moorings Example of MAR1 design from 1st year and present Development of the multi-stage top design Ran through the design process and gave example outputs from the design package I use.

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