RRS Discovery Cruise D359, 17 Dec 2010-14 Jan 2011. RAPID moorings cruise report

Abstract

This cruise report covers scientific operations conducted during RRS Discovery Cruise D359. Cruise D359 departed from São Antonio, Cape Verde on Friday 17th December 2010 arriving Santa Cruz de Tenerife Friday 14th December 2011.

The purpose of the cruise was the refurbishment of an array of moorings on the mid-­Atlantic Ridge and off the Moroccan Coast at a nominal latitude of 26.5°N. The moorings are part of a purposeful Atlantic wide mooring array for monitoring the Atlantic Meridional Overturning Circulation and Heat Flux. The array is a joint UK/US programme and is known as the RAPID-­?WATCH/MOCHA array. Information and data from the project can be found on the web site hosted by the National Oceanography Centre Southampton http://www.noc.soton.ac.uk/rapidmoc and also from the British Oceanographic Data Centre http://www.bodc.ac.uk.

The RAPID transatlantic array consists of 24 moorings of which 21 are maintained by the UK, and 20 bottom landers of which 16 are maintained by the UK. The moorings are primarily instrumented with self logging instruments measuring conductivity, temperature and pressure. Direct measurements of currents are made in the shallow and deep western boundary currents. The bottom landers are instrumented with bottom pressure recorders (also known as tide gauges), measuring the weight of water above the instrument.

The RAPID naming convention for moorings is Western Boundary (WB), Eastern Boundary (EB) and Mid-­Atlantic Ridge (MAR) indicating the general sub-­regions of the array. Numbering increments from west to east. An L in the name indicates a bottom lander, M indicates a mini-­mooring with only one instrument, H indicates a mooring on the continental slope. During D359 we recovered: MAR0, MAR1L4, MAR1, MAR2, MAR3, MAR3L4, EB1, EB1L7, EBHi, EBH1, EBH1L7, EBH2, EBH3, EBH4, EBP2, EBH5, EBM5. We did not recover EBM1, EBM4, EBM6, EBH1 and MAR3. We deployed: MAR0, MAR1L7, MAR1, MAR2, MAR3, MAR3L6, EB1, EB1L7, EBHi, EBH1, EBH1L8, EBH2, EBH3, EBH4, EBP2, EBH5. A sediment trap mooring NOGST was also recovered and redeployed for the Ocean Biogeochemistry and Ecosystems Group at the NOCS.

CTD stations were conducted at convenient times throughout the cruise for purposes of providing pre and post deployment calibrations for mooring instrumentation and for testing mooring releases prior to deployment.

Shipboard underway measurements were systematically logged, processed and calibrated, including: waves (spectra of energy and significant wave height), surface meteorology (air pressure, temperature, wind speed and direction and radiation (total incident and photosynthetically active), 6m-­depth sea temperatures and salinities, water depth, navigation (differential GPS measurements feeding two independent and different receivers, heading, pitch and roll from a four antenna Ashtec ADU5 receiver, gyro heading and ships speed relative to the water using an electro-­magnetic log). Water velocity profiles from 15m to approximately 500m depth were obtained using a ship mounted 75 kHz acoustic Doppler current profiler. Sea-­water samples from CTD stations and of the sea-­surface were obtained for calibration and analysed on a salinometer referencing these samples against standard sea water. For velocity data (wind and currents) measured relative to the ship considerable effort was made to obtain the best possible earth-­referenced velocities.

Four APEX argo floats supplied by the Met Office were deployed at pre-­assigned locations, filling gaps in the network.

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