OVIDE-FOX

Argo-Oxygen Floats Deployments and XBT sections 

in the subpolar gyre of the North-Atlantic

by Virginie Thierry*^(a), Gilles Reverdin ^(b) 

^*Corresponding author : Virginie Thierry 

^(a) : LPO, IFREMER Brest, France

^(b) : LOCEAN/IPSL, CNRS/UPMC, Paris, France 

The establishment of a global network of measurements of dissolved oxygen in the ocean became apparent in recent years, as a necessity by the entire international community. Oxygen is, in addition, an important parameter for the OVIDE project for understanding the formation and ventilation processes of the intermediate and deep waters, monitoring and understanding the variability of water mass properties and ocean circulation in the context of global climate change.

The deployments of Argo floats equipped with oxygen sensors, complemented by high-resolution XBTs sections in the North Atlantic, open the door to new scientific progresses with qualified and validated data basis. Supported by the GMMC Coriolis-Mercator since 2007, this project fits into regional (CPER-Argo, Region of Brittany), national (N-Argo France, PABIM2 PPR), European (Euro-Argo) and international (ARGO) dynamics.

The scientific context 

Started in 2002, the OVIDE project contributes to the observation of the circulation elements in the subpolar gyre of the North Atlantic, to better understand climate variability of the North Atlantic Ocean and its interactions with the atmosphere in a warming climate. Some features are particularly monitored in this region because they are essential for their impact on the climate of Europe : the magnitude and the variability of the meridional overturning circulation are followed, as well as the mechanisms and sites of formation of subpolar mode waters.

Made in june/july 2002-2004-2006-2008 and 2010, the OVIDE sections of hydrography/geochemistry (fig.1) between Greenland and Portugal helped characterize these climate indices, including highlighting a significant change in the meridional overturning circulation between OVIDE-2002 and NOCS-1997, and a significant variability between 2004 and 2006.

Figure 1 : OVIDE section between Greenland and Portugal.

The OVIDE-FOX project aims to complement this OVIDE observation network by the establishment of a network of oxygen measurements. Dissolved oxygen is indeed a marker of choice for tracking oceanic water masses and their displacements. It is also an important parameter for the study of convection in the North Atlantic and associated biogeochemical processes, and the formation of intermediate and deep waters (Körtzinger et al., 2005). Finally, oxygen seems to respond very sensitively to ocean global warming due to an increased dissolved oxygen degassing with increasing temperature (Claustre et al., 2010).

In addition to this new Argo-Oxygen database, data from XBTs are collected at high resolution in order to help maintain the global ocean observing system. These data are still needed, especially for estimating bias in temperature between surface and subsurface. They must also be able to give an idea ofthe interannual variability of the mixed layer thickness in the subpolar gyre region.

Methodology

The OVIDE-FOX project is divided into four stages according to :

• an initial phase of deployments with the launching Argo profiling floats equipped with oxygen sensors. 

Since 2002, 90 floats were deployed in the North Atlantic in the OVIDE project. These floats contribute to the Argo global network, with a programming that follows international recommendations : parking depth of 1000 meters, T / S profiles from the surface to 2000m and a cycle every 10 days. The sample provided by OVIDE-FOX maintains the Argo network in the area of a complex dynamically subpolar gyre.

• a second step of delayed quality controls and validations of Argo data :

Since 2008, the OVIDE-FOX team is actively working to establish management procedures and quality control on oxygen data rarely obtained from autonomous profiling (therefore impossible to recalibrate after launch. A good knowledge of Argo floats and a better understanding of how oxygen sensors, together with tests in pool and sea, should help identifing errors in the data to make the corrections, a preceding necessity for their scientific exploitation.

• a third step affects the realization and validation of high-resolution XBTs data :

As part of its contribution to ocean observing networks, the XBTs are still used, but with a sampling strategy modified since OceanObs'09 (high resolution sections, crossing the ocean and all done on a seasonal basis) . XBTs data along the OVIDE section complete a dense network of XBTs measurements since 1985, including the Reykjanes Ridge. Informations obtained from XBT profiles are often difficult to be interpreted, on the one hand because of difficult conditions in which they are collected, and on the other hand because of fairly systematic biases observed in these data. An important work of qualification and validation of these data must go on to ensure their scientific exploitation.

• Finally, the last step is the scientific analysis of these new data (Argo-oxygen, XBTs)

Data obtained for OVIDE-FOX contribute to a global network for monitoring the variability of subpolar gyre in the North Atlantic, with the scientific objective to better understand mechanisms of variability in a very complicated region with a key role in the context of global warming.

Preliminary Results

Concerning the in situ network of measurements funded by GMMC for OVIDE-FOX, 16 Argo floats have been deployed in june 2008 during the OVIDE-2008 cruise, among which two floats were equipped with oxygen sensors. In parallel, and in the framework of the SURATLANT project, 150 XBTs probes have been deployed the same year from Nuka Arctica merchansing ship  between Shetland Islands and Cape Farewell. In 2009, 16 Argo floats and 192 XBTs probes have been deployed during OVIDE-FOX. In june-july 2012, 13 Argo floats (among which 10 floats equipped with oxygen sensors) have been deployed during the CATARINA campaign along the OVIDE section, and 192 XBTs probes deployed from R/V Nuka Arctica and Reykjafoss between Shetland Islands and the Cape Ferewell, and between Terre-Neuve and Irland. Other XBTs sections funded by GMMC have been realized during OVIDE campaigns in 2004 (2), 2008 (7), and 2010 (10).

On Argo oxygen data management, and in relation with the Argo community, a management protocol of oxygen data (Thierry et al.2010) common to all Argo Data Centers (DAC) has been adopted by the international Argo community. This step ensures data management consistency by each DAC. A long work of system reliability and understanding of sensors behavior allowed to identify inherent errors for each sensor type, to estimate biases and to achieve an international consensus on how to apply the corrections.

In parallel, a work for XBTs data qualification goes on for OVIDE-FOX. In particular, the study by Reverdin et al., 2009 showed a bias temperature of -0.03 ° C to -0.05 ° C, while a more comprehensive statistical approach led by Hamon et al. 2011 resulted in an equation fall very close to the overall statistical results, indicating a difference in temperature between 0 and 200 meters below 10°C. The XBTs high-resolution data collected for OVIDE FOX has identified more precisely the subsurface signature of surface fronts (T) in an attempt to establish seasonal signatures (Despres et al., 2011).

Scientifically, the Argo data in the North Atlantic have brought important scientific advances for understanding the mechanisms of variability in this region. A heat balance determined from Argo data in the mixed layer in the Iceland basin (Boisséson et al., 2010) showed that beyond the air-sea fluxes, advection and turbulent mixing at the basis of the mixed layer are essential processes for determinating properties of the mixed layer. In this area also, the joint use of Argo data and hydrographic data cruises showed warming and salinization of modal waters located on the Reykjanes Ridge since the mid-90s (Thierry et al. , 2008). Finally, Argo data helped to highlight a resumption of deep convection in the subpolar gyre of the North Atlantic during winter 2007-2008 (Vage et al., 2009).

The network of OVIDE-FOX Argo-oxygen data has also lead to the development of indicators of the state of the ocean meeting the objectives of the working group on Oceanic Hydrography (WGOH) of the International Council of the Exploration of the Sea (ICES). He also contributed to the deep currents estimation for the construction of the ANDRO atlas (Ollitrault et al., 2013), and the reconstruction of global T and S fields in the North Atlantic (Gaillard et al., 2009), allowing the analysis of large-scale seasonal to interannual variability in the North Atlantic (von Schuckmann et al., 2009). Finally, these data were also used to reconstruct (Forget et al., 2008a, b) or validate (Ferron, 2011) hydrology and ocean circulation in the North Atlantic by variational assimilation.

Related links 

OVIDE project: http://www.ifremer.fr/lpo/ovide

CATARINA project : http://catarina.iim.csic.es/en

International Council for the Exploration of the Sea : http://www.ices.dk

SO-Argo France : http://wwz.ifremer.fr/lpo/SO-Argo

CREST Argo : http://wwz.ifremer.fr/lpo/SO-Argo/Activities/CREST-Argo

References

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