Satellite Relay Tagging Program - Southern Ocean - MEOP Quality Controlled CTD Profiles

Brief description

CTD (Conductivity-Temperature_Depth)-Satellite Relay Data Loggers (CTD-SRDLs) are used to explore how marine mammal behaviour relates to their oceanic environment. Loggers developed at the University of St Andrews Sea Mammal Research Unit transmit data in near real-time via the Argo satellite system. Data represented here was collected in the Southern Ocean, from both Elephant Seals and Weddell Seals. Data parameters measured by the instruments include time, conductivity (salinity), temperature, pressure and depth. The data represented by this record have been Qc'd and are the Australian subset of the MEOP-CTD database (MEOP: Marine Mammals Exploring the Oceans Pole to Pole), complemented with the most recent Southern Ocean deployment data. This Australian subset of the Southern Ocean database represents about one quarter of the entire MEOP-CTD database, which currently is about 52,000 profiles obtained from 275 CTD-SRDL tag deployments. The Australian dataset originated in 2004, and was initially collected by Mark Hindell's team based at the University of Tasmania, and in later years his data has formed part of the Animal Tracking Facility of Integrated Marine Observing System (IMOS).

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Maintenance and Update Frequency: asNeeded

Statement: The CTD-Satellite Relay Data Loggers (CTD-SRDLs) are built by the Sea Mammal Research Unit (SMRU, University of St. Andrews, UK), incorporating CTD sensors developed by Valeport Ltd (Devon, UK). The sensor head consists of a pressure transducer, a platinum resistance thermometer, and an inductive cell for measuring conductivity. The temperature and conductivity sensors have a precision (repeatability) of 0.005oC and 0.005 mS/cm, respectively. Before being taken into the field, devices are calibrated in the laboratory by Valeport. Some of the CTD-SRDLs (about half) were also tested at sea against a ship-based CTD before the deployment. CTD-SRDLs record hydrographic profiles during the ascent of seals, retaining only the deepest dive in each six-hour time interval, and transmitting profiles in a compressed form (between 10 and 25 data points per profile, depending on the tag program) through the Advanced Research and Global Observation Satellite (ARGOS) system. The accuracy of ARGOS geo-positioning is typically better than +/-5 km, because seals never stay at the surface more than a few minutes. New methods are currently under development by the ARGOS team, which should substantially improve the quality of positioning. Hydrographic profiles are post-processed using a unified procedure of editing, adjustment, and validation (Roquet, F. et al. Delayed-mode calibration of hydrographic data obtained from animal-borne satellite relay data loggers. J. Atmos. Ocean. Technol. 41, 787-801 (2011)). Each individual CTD-SRDL dataset is post-processed separately, as each tag has different technical specifications and a different life history. A standard set of tests, adapted from Argo standard quality-control procedures (Wong, A. et al. Argo Quality Control Manual v2.9. Argo Data Management, 54pp, http://www.argodatamgt.org (2013)), is first run to remove bad profiles, spikes, and outliers. For CTD-SRDLs with profiles in frozen areas, a temperature offset was estimated using the local freezing temperature. A salinity adjustment was also estimated, consisting in a pressure dependent linear correction. This bias is mainly induced by an external field effect on the conductivity sensor, which cannot be corrected a priori because it depends on how the tag has been attached on the seal?s head. Adjustments parameters were estimated for each CTD-SRDLs separately by comparisons of salinity measurements with available data in the World Ocean Database. Because the southern ACC region (south of 55oS) is associated with a large-scale upwelling of circumpolar deep waters near the surface, the salinity at depth is very stable there, with a low natural variability highly suitable for use as a reference. Salinity data cross-comparisons between different CTD-SRDLs were also used to estimate suitable adjustments for CTD-SRDLs having no profiles available in the southern ACC region. Temperature and salinity adjustments vary from tag to tag, and they are applied identically to all profiles from a given tag.Once calibrated, the accuracy of post-processed CTD-SRDL measurements was estimated to be +/-0.03oC in temperature and +/-0.05 psu (practical salinity unit) or better in salinity for CTD-SRDLs built after 2007. The achieved accuracy is highly dependent upon availability of ship-based CTD comparisons, and the type of water masses sampled during the deployment time. In best cases, an accuracy of +/-0.01 oC and +/-0.02 psu can be obtained. Pre-2007 CTD-SRDLs (about 5% of profiles) used an older technology with a poorer accuracy roughly estimated around +/-0.1 oC and +/-0.1 psu.

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Credit
Australia's Integrated Marine Observing System (IMOS) is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). It is operated by a consortium of institutions as an unincorporated joint venture, with the University of Tasmania as Lead Agent.

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Antarctic Climate and Ecosystem Cooperative Research Centre (ACE CRC)

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Australian Antarctic Science (AAS) program

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Marine Mammals Exploring the Oceans Pole to Pole (MEOP) Consortium