Mixed-Layer Evolution in the Subpolar North Atlantic measured by Argo floats (2011)

Linn Schneider(a)* , Dagmar Kieke(a), Monika Rhein(a), Birgit Klein(b) 

   

(a) : IUP Universität Bremen, Germany

(b) : BSH Hamburg, Germany

* Corresponding author : Linn Schneider

Context

The mixed layer (ML) is the site of ocean-atmosphere interactions. It has a crucial role in the climate system : 1) - the mixed layer depth (MLD) establishes the volume of water over which the surface heat flux is distributed, controlling the air-sea heat exchange ; 2)- in area where deep convection occurs, winter mixed layer conditions set the properties of the new formed deep and intermediate water masses.

Due to its contact with the atmosphere, atmospheric seasonal changes result in changes of the mixed layer. These changes were examined for the year 2007 from 4293 quality controlled Argo profiles in the subpolar North-Atlantic region. The algorithm of Holte and Talley (2009) is used for the MLD determination, and tested to clarify its advantages to threshold and gradient methods. Then, the ML temporal and regional evolutions are analysed in relation with air-sea interaction processes. Finally, a mixed layer heat budget is calculated for the year 2007 in this region.

 

Data & Method

Argo float temperature, salinity and density profiles from the year 2007 are used to study the mixed layer in the subpolar North Atlantic [40-67°N] (figure 1). Quality controls are operated on those Argo profiles, first by checking spikes and gradients, then by deleting profiles with possibly incomplete ML resolution, and finally by avoiding, through a visual control, profiles with more than 10 missing values. About 4293 profiles from the 4775 profiles initially available are kept for this study, alllowing a good data repartition in time and space after the QC.

 Figure 1. Data availability of the study region 2007 in space and time before and after the quality controls. WGC : West Greenland Current ; EGC : East Greenland Current ; NAC : North Atlantic Current ; IC : Irminger Current ; LC : Labrador Current ; C: Convection. Blue narrows for warm/salty waters, yellow narrows for cold/fresh waters.

 

Wind forcing and heat and moisture fluxes induce turbulent mixing, and determine the Mixed Layer Depth (MLD). Various methods exist for the MLD determination. Threshold or gradient criteria mainly are sources of over-or underestimation of MLD because of difficulties to find reference and threshold value appropriate for every region and season.

The MLD in the study region was determined using an algorithm developed by Holte and Talley (2009) which combines physical profile features like profile maxima and minima as well as threshold and gradient criteria to detect realistic MLDs. This algorithm is compared to other methods for identifying the MLD, and turned out to be the most appropriate method for the study region. From the resulting MLD, the temporal evolution as well as the regional evolution of the mixed layer was examined, and possible causes of these evolutions were explained by air-sea interaction processes through the analysis of surface and momentum fluxes calculated from the NCEP/NCAR reanalysed atmospheric data. Finally, to investigate the importance of air-sea heat exchange and oceanic factors influencing the ML heat content respectively, a ML heat budget is calculated according to Dong et al., 2007.

 

Results

The application of the Holte and Talley (2009) algorithm to Argo data from the subpolar North Atlantic shows an important improvement to MLD estimates from single threshold and gradient criteria.

The temporal and regional evolution of the resulting algorithm MLD is analysed for year 2007 (figure 2). Deepest MLDs appear in winter in Irminger and Iceland basin for depth above 500 meters, and in the Labrador Sea for depths above 300 meters. Shallow MLDs, less than 50 meters, are observed in summer for all the region, while spring and autumn are transitional periods. To explain the MLD evolution resulting from ocean-atmosphere interaction, the buoyancy loss was calculated showing that deep MLDs in winter coincide with regions of high buoyancy loss while shallow MLDs in summer coincide with buoyancy gain. The regional and seasonal MLD evolution can be explained by air-sea heat and moisture exchanges.

Figure 2. Seasonal MLD evolution in year 2007

Finally, the calculated ML heat budget (figure 3), based on the Argo float MLD confirms that air-sea exchange is the post important factor for the ML heat content, and therefore for the temporal MLD evolution in 2007. Entrainment impacts ML heat content in early and late summer, and autumn when MLD is shallow (week 20-22,31-41), while advection and diffusion play a minor role. The total heat budget is not closed, supposing some uncertainties on MLD estimation from Argo data in some regions of low data repartition.

Figure 3. Mixed Layer Heat Budget for year 2007 in the region, according to Dong et al.,2007

 

References
  • Dong S., S.T. Gille and J. Sprintall : An Assessment of the Southern Ocean Mixed Layer Heat Budget, J.Climate, Vol.20(17), 4425-4442, doi:10.1175/JCLI4259.1
  • Holte, J. and L.Talley : A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation. J. Atmos. Oceanic Technol., 26, 1920–1939. doi: http://dx.doi.org/10.1175/2009JTECHO543.1, 2009
  • Schneider, L. : Entwicklung der Mixed Layer im subpolaren Nordatlantik, Diplomarbeit, Institut für Umweltphysik, Universität Bremen, 2011.
  • Schneider L., D. Kieke, M. Rhein and B. Klein 2011, Mixed layer evolution in the subpolar North Atlantic measured by Argo floats, Geophysical Research Abstracts13, EGU2011-2594.