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Float activities within MerSea

Why Float Observations? How does it work? Deployment and Scientific results 1- Nordic Seas. 2- Atlantic Ocean 3- Southern Ocean . Provor Float equipped with a Seabird salinity sensor

Atlantic Ocean

Warm and saline subtropical water is carried northward by the North-Atlantic Current in the upper part of the meridional overturning cell (MOC). Due to heat loss to the atmosphere the water gets progressively colder and denser and is transformed in sub-polar mode water, intermediate water and eventually in deep water by deep convection in the Labrador and Nordic Seas. The deep water then returns to the south as part of the lower part of the MOC. The MOC is associated with the northward heat transport to the North Atlantic which contributes to the mild climate of western Europe. The variability of this heat transport could be related to changes in the atmospheric forcing but could be also a trigger mechanism for climate change.

Fig. 1: Bathymetry and topographic features in the North-Atlantic. (Red dots) CTD Station positions for the 2006 OVIDE cruise.

The ARGO profiling floats contribute to the observation of key components of the circulation in the sub-polar gyre of the North-Atlantic. Indeed, they allow the monitoring of the water mass properties and their variability from seasonal to inter-annual time scales and help understand the relation between this variability, the atmospheric forcing and the mean circulation.

Float deployments:

A total of 16 ARGO Floats were deployed during the Ovide cruise between Greenland and Spain in June 2006.These deployments conclude the float deployments under the MERSEA programme in this ocean region.

Fig 1:Float deployments during the Ovide cruise. All were Provor floats.

 Technical results

All the Provor floats are profiling to 2000m every 10 days and all are still active and working according to specification.

Fig 2 Map of CTD stations added to the reference DB (left) and map of the floats submitted in delayed mode by France and Germany (on the right). The color legend applies to the left panel only.

 At the Coriolis Data Center, the delayed mode data have been implemented in the database; the generation of the corresponding files has been done and an update of the float time-series is regularly done. German contribution (AWI,BSH,IFM-Geomar) were delivered to GDAC at the autumn 2006. An update of the float time-series for the BSH contribution was sent in March 2007. The locations of the Argo delayed mode data from the different institutes are shown on Figure 2 .

 Scientific results

By combining Argo data with CTD data from hydrographic cruises (mainly from the OVIDE and A01E sections), we estimated the interannual variability of the core property of the Subpolar Mode Water as observed on the Eastern Flank of the Reykjanes Ridge during the period 1990-2006 (Figures 3 and 4). In this region, the density compensated tendency for cooling and freshening observed in the early 1990s was interrupted in 1996, with a later increase in both properties (and a decrease of density) until 2003 (but notice the data gap in 1998-2001). Since 2003, the data suggest a decrease of the temperature and salinity of the mode water core accompanied with an increase in density. Individual extreme values observed in 2002 (potential temperature and potential density greater than 8°C and less than 27.4, respectively) are due to an eddy sampled during the Ovide cruise. During that entire period, the data do not permit to identify significant modifications in the depth of the mode water core, except possibly in 2003-2004, when the mode water core is anomalously shallow.

The Argo data are not only fully consistent with data from hydrographic cruises, they are also indispensable to complement the time series and will be essential in the future to continue the monitoring of the mode water property. The Argo data also provide interesting information on the mode water formation. For instance, data along the trajectory of the float 6900452 clearly show the ventilation of the 27.4-27.5 isopycnals in February 2007 (Figure 6) and the entrainment in the mixed layer of colder and fresher waters from the underlying layers.

Figure 3. : Time evolution of the properties of the Subpolar mode water found on the east flank of the Reykjanes Ridge (box on Figure 4. (a) potential temperature; (b) salinity, (c) potential density, (d) depth. Individual data are from CTDs (crosses), and from ARGO floats (circles for real time data and stars for delayed mode data). The median of the data for a given year is reported by a square. The vertical bars report the NAO index (Hurrell, 1995). Note that the temperature time series is very close to the February SST from Reynolds product at the same location (not shown).

Figure 5 Trajectory of float 6900452. The black box indicates the area where the variability of the Reykjanes Ridge Mode Water is estimated (Figure 3.1.12).

Figure 6: Potential temperature, potential density, potential vorticity and salinity along the trajectory of float 6900452.