The heat and fresh water content in the Norwegian Sea the last 15 years using Argo data

Kjell Arne Mork, Institute of Marine Research and Bjerknes Centre for Climate Research, Norway

Reference: Mork, K.A., Ø. Skagseth, B. Berx, H. Søiland, and H. Valdimarsson. Heat and freshwater content in the Norwegian Sea (to be submitted).
Introduction

The Nordic seas (Norwegian, Greenland and Iceland Sea) is the major region of water mass transformation in the northern loop of the global thermohaline circulation. Atlantic Water (AW) is here transformed, through intense cooling, into a water mass that is dense enough to feed the lower North Atlantic Deep Water (Figure 1).

Here, we present the use of Argo data from the Coriolis Global Data Assembly Centre (http://www.coriolis.eu.org/) to monitor and describe the climate variability in the Norwegian Sea, on time scales ranging from monthly to decadal. The climate variability was investigated in terms of ocean heat and freshwater contents, and in Mork et al. (2017) both air-sea heat fluxes and advective mechanisms responsible for the variability were explored. The World Ocean Atlas 2013 was used as climatology in which anomalies of the heat and freshwater contents were calculated from.

Results

Time series of the heat and fresh water content anomalies (HCA and FwCA, respectively) reveal interannual and pentadal time scales overlying the seasonal variability (Fig. 2). In average, the HCA are larger in the Lofoten Basin than in the Norwegian Basin and opposite for the FwCA, meaning that in the last 15 years Lofoten Basin is warmer and saltier compared to the Norwegian Basin relative to the climatology. The timing of maximum HCA is different within the two basins (Norwegian Basin: 2014, 2016; Lofoten Basin: 2009/2010, 2015) while timing in minimum FwCA is similar for both basins (2010-2011). Noteworthy is that the Lofoten Basin has become warmer and fresher since 2011 while this is less evident for the Norwegian Basin. The variability of the heat and freshwater contents in the upper 200 m follow those in the upper 1000 m depth for the Norwegian Basin and less for the Lofoten Basin.

Conclusion

A reasonable number of Argo floats can resolve seasonal variation in the Norwegian and Lofoten Basin. Reliable results for shorter time scales would require a higher number of floats to reduce the contribution from meso-scale eddies. On seasonal and interannual scale the two basins in the Norwegian Sea should be covered separately when climate variability is studied or monitored as they are influenced differently by the various processes and upstream sources. The warming and freshening during 2012-2016 in the Lofoten Basin may have been caused by, at least partly, fresher inflowing Atlantic Water from the south and a positive air-sea heat flux anomaly (less ocean heat loss to the atmosphere).