Water mass transformation in the deep basins of the Nordic Seas (2016)

Katrin Latarius (1,2)* and Detlef Quadfasel (2)

     

(1) Institute of Oceanography, University of Hamburg, Bundesstr. 53, 20146 Hamburg, Germany

(2) Alfred Wegener Institute for Polar and Marine Research, Climate Sciences Department, Bussestraße 24, 27570 Bremerhaven, Germany, present address

* Corresponding author : Katrin Latarius

Full paper: Latarius K. and D. Quadfasel 2016, Water mass transformation in the deep basins of the Nordic Seas : Analyses of heat and freshwater budgets. Deep Sea Research 114: 23-42, doi.org/10.1016/j.dsr.2016.04.012.
Context

This study explores the contribution of the Nordic Seas (NS) to the deep overflows into the North Atlantic, focusing on the four deep basins of the NS : the Greenland Sea (GS), the Lofoten Basin (LB), the Norwegian Basin (NB) and the Icelandic Plateau (IP). The NS are a major site of high latitude water mass transformation, where strong vertical mixing induced by heat loss to the atmosphere transforms the most part of the incoming subtropical warm and saline Atlantic Water into the dense overflow waters. These waters leave the area across the Greenland-Iceland-Scotland Ridge, contributing to the formation of North Atlantic Deep Water. 

Figure 1: Schematic of the circulation and water mass transformation in the Nordic Seas. Colored arrows indicate the advection of major water masses in topographically steered currents. Transformation and vertical mixing takes place mainly in the cyclonic gyres of the deep basins, indicated by the vertical cylinders. Eddy fluxes between the interior and the boundary current as well as injections from the rim currents facilitate the export of newly formed waters via the boundary current. © Latarius and Quadfasel, 2016

The circulation in the NS is distributed into two distinct components (fig. 1) : swift currents circumnavigating the Nordic Seas (and the Arctic Ocean) as boundary currents and following submarine ridge systems in the interior (Rudels et al., 1999), and the regional cyclonic circulation cells in the interior of the NS, following the topographic steering of the four major deep basins.

The present study concentrates on the deep basins in the interior of the NS, where Argo-type profiling float measurements are carried out since 2001. In the first part, a description of the seasonal development of hydrography in the four basins during the past decade is analyzed. In the second part, the information about hydrography development is combined with air-sea fluxes to establish annual and seasonal heat and freshwater budgets for the individual basins.  The Budget calculations yield a contribution of 18% to the total temperature decrease and 6% to the total salinity decrease in the Arctic Mediterranean, although the basins account for only 4% of the total area. The density increase nearly exclusively takes place in the eastern basins, whereas the Greenland Sea plays an important role in matching the temperature and salinity characteristic of the overflow water.

 

Data & Method

Data from Argo floats deployed in the NS from March 2001 to April 2011 are used to investigate the development of the hydrography in the deep basins (fig. 2). The whole data set consists of 4030 hydrographic profiles from 78 floats (70 APEX floats and 8 NEMO floats). The floats operated with Argo standard parametrization (10-day cycle, 1000 or 1500 dbar parking depth and 2000 dbar profiling depth), except for the shallower IP basin (profiling depth at 1300 dbar).

  Figure 2. (left) Positions of all float profiles in the Nordic Seas in the time span March 2001 to April 2011; colors of the dots indicate the deployment basin: green- Norwegian Basin (NB), yellow- Lofoten Basin (LB), red – Greenland Sea (GS), blue - Icelandic Plateau (IP). The yellow contour-lines show the f/H-characteristic, which are used to define the areas of the four basins. (right) Number of floats per month in the individual basins; color code as left. © Latarius and Quadfasel, 2016

The lifetime of the floats was between 0.5 and 6 years, with a mean of 2.4 years. Most floats were trapped in their deployment basins for about half of their lifetime. The average residence time in the NB was 1.3 years, in the LB 0.5 years, in the GS 1.2 years and on the IP 2.1 years. Measurements from floats are equally distributed during the year. On average, a mean of 3-4 floats occupy each of the deep basins providing order of ten hydrographic profiles per basin per month. This allows resolving the seasonal development of the hydrographic structure. The analyses base on the monthly means for each of the basins. 

Results
Hydrography and seasonal signal

Time series of potential temperature, salinity, potential density and mixed-layer depth (MLD) are analyzed for the period 2002-2012, for the eastern (NB and LB) and western (GS and IP) parts of the Nordic Seas (according to the order the inflowing AW passes the areas with the cyclonic circulation). 

In the eastern Nordic Seas (figures 3) :

In the NB the upper 400 m and in the LB the upper 600m are dominated by the warm and saline AW. Below this Atlantic layer, a salinity minimum (at 500 m depth in the NB and 1000 m depth further to the north in the LB), characterizes the Norwegian Sea Arctic Intermediate Water (AIW) which forms in the GS and on the IP by wintertime convection and spreads into the Norwegian Sea. 

A seasonal cycle, forced by heat fluxes between the atmosphere and the ocean, is visible in the NB and the LB in the temperature time series in the Atlantic layer. Heat is accumulated during summer near the surface and lost to the atmosphere during winter. Larger winter MLDs are found in the LB because of the divergence of current branches south of the basin, leading to larger residence times and enhancing the effect of winter vertical overturning there (Nilsen and Falck, 2006). 

A seasonal cycle is also visible in the near-surface salinity in the NB and to a lesser extent in the LB, with a maximum salinity from late summer to the turn of the year and a minimum salinity in spring. This observation can be explained by wind-driven intrusions of coastal water, which is freshest during the spring maximum in coastal runoff (Nilsen and Falck, 2006).

    

Figure 3. Time series for the eastern part of the Nordic Seas (left : NB - right : LB)  for (a) potential temperature, (b) salinity, (c) potential density, (d) MLD and (e) number of float measurements. © Latarius and Quadfasel, 2016

In the western Nordic Seas (figures 4) :

In the GS and on the IP, the upper 50 m are dominated by cold and fresh water fed laterally from the west from the East Greenland Current which carries Polar Surface Water (PSW) out of the Arctic Ocean (fig.1). In the GS, waters of Atlantic origin are still observable below the surface layer (100-400 m) but the signatures were regularly destroyed during winter convection. 

AIW occupy the depth range 600 - 1600 m, just above the intrusion of deep water masses from the Arctic Ocean (Somavilla et al., 2013). On the IP, AW is not observed below the surface layer. Instead we found the locally formed AIW there.

In both basins, a seasonal cycle is visible in temperature as well as in salinity. In the GS, at least the upper 500 m are involved in the seasonal cycle but homogenization due to winter cooling can reach much larger depths in individual years. Strong variability occurs in the near surface salinity and the MLD of the GS. On the IP, in contrast, the amount of freshwater accumulated in the basin in late summer does not show strong interannual variability. Mean wintertime buoyancy fluxes on the IP are less than half of that in the GS (Latarius, 2013; Moore et al., 2015) and consequently the resulting typical winter MLD is shallow (~150 - 200 m).

Regarding to long-term development, the time series in the GS show increasing temperatures from below the surface layer to the profile depth of 2000 m. In the Atlantic layer, the warming holds until 2010. The signal is transferred into deeper layers by winter convection and temperatures stay high there until the end of the time series in 2012. For the NB, the increasing temperatures are visible over the whole depth rate and time span, but only the Atlantic layer (upper 500 m) shows increasing salinities. 

    Figure 4 : Time series for the western part of the Nordic Seas (left : GS - right : IP)  for (a) potential temperature, (b) salinity, (c) potential density, (d) MLD and (e) number of float measurements. © Latarius and Quadfasel, 2016

Heat and Freshwater budgets

The seasonal cycle in all four basins is characterized by a stratification phase during summer, when the ocean gains heat from exchange with the atmosphere, and a homogenization phase during winter, when the ocean loses heat to the atmosphere. The heat loss induces a redistribution of heat and freshwater in the water column by vertical mixing, when the water in the surface layer reaches the density of the layers below. Freshwater fluxes between the ocean and the atmosphere as well as horizontal fluxes of heat and freshwater also shape the seasonal cycle. 

Based on the float measurements heat and freshwater budgets are estimated for the basins. The changing heat and freshwater content (HC and FWC respectively) of a water column (see schematic fig. 5) is determined by the heat and freshwater fluxes between atmosphere and ocean  (SurfFIH and SurfFIFW respectively), the horizontal inputs of heat and freshwater (HIH and HIFW respectively) by lateral advection (mean currents) or horizontal mixing (eddies), and the vertical mixing of heat and freshwater within the water column (VMH and VMFW respectively).

Figure 5. Schematic picture for the calculation of the heat (or freshwater) budget calculation during summer (stratification phase) and winter (homogenization phase). The green components are unknown and have to be calculated from the black components. During winter the calculation is done from the deepest layer to the top. Explanation for the Abbreviations in the equations: SurfFL – surface flux, HI- horizontal input, VM – vertical mixing, δHC/δt – development in time of heat content (or freshwater content) in the basin. © Latarius and Quadfasel, 2016

The respective heat and freshwater contents are directly calculated from the hydrographic times series of the float data. Surface fluxes from the NCEP data set with corrections according to Renfrew et al. (2002)  are used to estimate the mean cycles of the fluxes between atmosphere and ocean for the time span of float observations in each basin. The lateral exchange as well as the vertical mixing are calculated as the residuum between the two previous terms. The lateral exchange in 50 to 800 m gives the contribution of the basins to the water mass transformation from Atlantic water to overflow water. The vertical mixing quantifies the destabilization of the water column induced by the heat loss to the atmosphere. Similar kind of budget calculations were carried out in Latarius and Quadfasel (2010) only for the GS.

In the eastern NS (figures 6) : 

The heat budgets for the NB and the LB look similar, with accumulation of heat from the atmosphere and from horizontal input below 50 m in summer which is redistributed by winter vertical mixing.  But also differences emerge between the NB and the LB. The amount of heat accumulated in summer is of the same order for both basins (NB: 130 W/m2, LB: 144 W/m2), but the influence of the Atlantic Water is stronger in the LB (more horizontal input). Also the structure of lateral input and vertical mixing is different within the upper 50-500 m. During winter, a larger portion of the accumulated heat gets lost to the atmosphere in the LB than in the NB, which reflects the harsher weather conditions to the north (Latarius, 2013). For both basins the exchange of freshwater between the atmosphere and the ocean is of minor importance in the overall freshwater budget.

  Figure 6 : Heat (left) and freshwater (right) budgets for the Norwegian Basin (left panel) and the Lofoten Basin (right panel). In each panel: (top) for summer (May to October), (middle) winter (November to April) and (bottom) for the whole year. These are average budgets for the time span 2002 to 2010. Heat and freshwater transfers take place in the direction of the arrows. Red numbers in the interior mark heat/freshwater gain, blue numbers heat/freshwater loss. The numbers to the right of the column denote the depth; the maximum mixed-layer depth is written in red. © Latarius and Quadfasel, 2016

In the western NS (figures 7) : 

The budgets for the GS look again similar to the ones from the NB and LB with heat and salt accumulation from horizontal input during summer and release to the atmosphere and redistribution within the water column during winter. But there are some important differences. In the GS heat and salt are accumulated over a wider depth range. About 20 % of the heat and 37 % of the salt are brought into the basin below 500 m. In contrast to the eastern Nordic Seas Atlantic Water is found at larger depths here as it spreads below the Polar Surface Water, being denser due to cooling on the way through the eastern Nordic Seas (Rudels et al., 2005; Rudels et al., 2002). Also the redistribution during winter reaches much larger depths (1500 m). 

Freshwater is imported into the basin in the upper 50 m during the whole year with highest values during winter. This lateral input is approximately two orders of magnitude larger than the freshwater flux between the atmosphere and the ocean and about twice as high as in the NB and LB in the eastern Nordic Seas. It originates from the fresh Polar Surface Water, which is transported with the East Greenland Current from the Arctic Ocean towards the subpolar North Atlantic (Dickson et al., 2007; Rudels et al., 2005). 

In contrast to the other basins on the IP the atmospheric heat fluxes from summer and winter are almost balanced such as the lateral fluxes above and below 50 m. For freshwater the import from the atmosphere and laterally below 50 m is balanced by the export of freshwater in the surface layer.

  Figure 7. Heat (left) and freshwater (right) budgets for the Greenland Sea (left panel) and the Icelandic Plateau (right panel). In each panel: (top) for summer (May to October), (middle) winter (November to April) and (bottom) for the whole year. These are average budgets for the time span 2002 to 2010. Heat and freshwater transfers take place in the direction of the arrows. Red numbers in the interior mark heat/freshwater gain, blue numbers heat/freshwater loss. The numbers to the right of the column denote the depth; the maximum mixed-layer depth is written in red. © Latarius and Quadfasel, 2016

Conclusion

This study  investigates the role of each deep basin of the Nordic Seas for the water mass transformation from inflowing Atlantic Water to deep overflow water. Thanks to Argo-type measurements collected in the area since 2001, data are available equally distributed throughout the year with a high resolution in time. They enable us to resolve the seasonal cycle within the upper 1300 m (IP) to 2000 m (NB, LB, GS) of the water column in the basins and thus directly observe the water mass transformation there. 

In combination with air-sea fluxes from NCEP reanalysis, heat and freshwater budgets provide information about the lateral exchange between the basins and the surrounding as well as about the vertical mixing within the basins. 

Globally, the analysis of the budgets shows that the four deep basins of the NS contribute 17% to the total temperature decrease and 7% to the total salinity decrease of the transformation from Atlantic inflow to deep overflow in the Arctic Mediterranean, although they account for only 4% of the total area. The GS in the western part of the NS is important for matching the characteristic of the overflow water, while the eastern part (NB and LB) is important for the densification.

Acknowledgments

Support for this study was provided by Deutsche Forschungsgemeinschaft (SFB 512 E2 and FOR1740), Bundesministerium für Bildung und Wissenschaft (Nordatlantik Projekt) and European Union (MERSEA , THOR - 7th Framework Programme (FP7 2007-2013) under grant agreement n.212643, NACLIM – 7th Framework Programme (FP7 2007-2013) under grant agreement n.308299, and Euro-Argo). Part of the floats in the Nordic Seas were financed by Deutsche Forschungsgemeinschaft (SFB 512 E2) and Dansk Forskningsråd. Globally Argo data are collected and made freely available by the International Argo Project and the national programs that contribute to it (http://www.argo.ucsd.edu,http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System. The meteorological data set is made available by the National Centres for Environmental Prediction, National Oceanic and Atmospheric Administration, Boulder, USA (NCEP).

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