The 2019 Atlantic to Mediterranean (ATL2MED) mission is a multifaceted field study including inputs from 12 oceanographic research institutions from seven countries. The mission began with an eddy survey off the coast of West Africa. During the second phase of the mission led by the Integrated Carbon Observation System (ICOS) Ocean Thematic Center (OTC), two Saildrone unmanned surface vehicles (USVs) will collect calibration and validation data at nine fixed ocean stations in the along the route from Cabo Verde to Trieste, Italy.
ICOS is a European Research Infrastructure (RI) that aims to understand the planet’s greenhouse gas balance by collecting high-quality measurements of carbon and other greenhouse gasses in the atmosphere, ocean, and terrestrial biosphere. The mission aims to determine the biogeochemical variability in undersampled areas of the Atlantic Ocean and Mediterranean Sea as well as assess saildrones as an efficient tool for validation purposes and the extent that they could complement the existing network of ocean observing systems.
On the surface of the ocean, measurements of air-sea carbon (CO2) exchange are performed using fixed stations and ships of opportunity (volunteer commercial vessels equipped with CO2 instruments and sensors). The validation and calibration of both data and instruments is of vital importance to ensure data is of the highest possible quality. Validation is usually performed by collecting discrete water samples in the vicinity of the fixed station; water samples are analyzed for variables of interest (inorganic carbon, alkalinity, and pH) in an on-shore laboratory. Results of the analyses are compared with the sensor CO2 measurements from the fixed station.
“While we have a good grip on validation and calibration of the CO2 instruments installed on ships of opportunity, this is not the case for the fixed ocean stations. Currently, no fixed station within the ICOS ocean network is able to perform in situ calibration of the CO2 sensors,” said Dr. Ingunn Skjelvan, a research scientist at NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, and principal investigator for station certification at ICOS OTC.
The saildrones will visit nine fixed ocean stations in the Atlantic and Mediterranean: CVOO (Cabo Verde), ESTOC (Gran Canaria), LION (France), ANTARES (France), DYFAMED (France), W1M3A (Italy), E2M3A (Italy), PALOMA (Italy), and Miramare (Italy). Currently, only CVOO and the four Italian stations are currently part of ICOS. The stations selected are all situated along the route from Cape Verde to Trieste and have an established time series of observations.
In addition to the standard Saildrone suite of sensors, which collect data above and below the sea surface, SD 1030 is equipped with an ASVCO2—a CO2 instrument developed by the National Oceanic and Atmospheric Administration (NOAA)’s Pacific Marine Environmental Laboratory (PMEL). Saildrones are environmentally friendly wind and solar-powered vehicles capable of long-term ocean deployment, up to 12 months. While the payload of individual vehicles can be customized according to a particular use case, the data collected can serve a wide range of science objectives beyond the specific objectives of the mission.
“The saildrone data is not only important for validation purposes, it will also contribute additional data at the sites, increasing the value of the entire data set. A data set consisting of many variables can be used in more ways than a data set of fewer variables. And thus, the outcome will be new knowledge,” said Skjelvan.
During the eddy survey, led by the Helmholtz Centre for Ocean Research (GEOMAR), one of the two saildrones, SD 1053, suffered biofouling due to the warm, tropical waters, and returned to Mindelo, Cabo Verde, for maintenance. SD 1030 sailed north to the CVOO buoy and sailed a series of patterns around the buoy for calibration and validation. The two saildrones are expected to rendezvous before entering the Mediterranean Sea.