Reduced efficiency of the Barents Sea cooling machine

Abstract

Dense water masses from the Barents Sea are an important part of the Arctic thermohaline system. Here, using hydrographic observations from 1971 to 2018, we show that the Barents Sea climate system has reached a point where ‘the Barents Sea cooling machine’—warmer Atlantic inflow, less sea ice, more regional ocean heat loss—has changed towards less-efficient cooling. Present change is dominated by reduced ocean heat loss over the southern Barents Sea as a result of anomalous southerly winds. The outflows have accordingly become warmer. Outflow densities have nevertheless remained relatively unperturbed as increasing salinity appears to have compensated the warming inflow. However, as the upstream Atlantic Water is now observed to freshen while still relatively warm, we speculate that the Barents Sea within a few years may export water masses of record-low density to the adjacent basins and deep ocean circulation.

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Fig. 1: The Barents Sea cooling machine.
Fig. 2: The Barents Sea temperature changes.
Fig. 3: Ocean heat loss in the Barents Sea.
Fig. 4: BSW changes.

Data availability

This study covering the period 1971–2018 included 322,812 vertical profiles of T and S from the joint Norwegian (IMR)–Russian (PINRO) ecosystem surveys performed annually during fall (August–September). The Norwegian data are available at https://www.nodc.noaa.gov/OC5/WOD13/. The Russian data are available for this study through a bilateral data exchange but are generally not publicly available. The ERA–Interim atmospheric reanalyses are available at https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim. The time series of volume transport Supplementary Fig. 3 are available at ftp://ftp.nmdc.no/nmdc/IMR/fastesnitt/indeks/fluks_mnd_1997_2017.xls.

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Acknowledgements

This work received funding from the Norwegian Research Council projects Pathway (grant 263223) and Nansen Legacy (grant 276730), the Fram Centre project ICEHOT and the Trond Mohn Foundation (project BFS2018TMT01).

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Authors

Contributions

Ø.S. did the data analysis. All authors contributed to the ideas and the writing of the manuscript.

Corresponding author

Correspondence to Øystein Skagseth.

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The authors declare no competing interests.

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Peer review information Nature Climate Change thanks Vladimir Ivanov, Kerstin Jochumsen and Zhenxia Long for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Hydrographic data coverage.

The Barents Sea is divided into a 1° longitude by 0.5° latitude grid and we show the fraction of years to the whole (1971-2018) period where at least one observation exists within 55 km distance from the grid point. The unit is in [%]. The black lines show the areas used in Figs. 2 and 4 and Supplementary Fig. 2.

Extended Data Fig. 2 The Barents Sea along-stream cooling.

Temperature difference between regions R1-R2, R2-R3, and R3-R4 in a) the layer 0-30 m, and b) in the layer 100-200 m. The regions R1-4 are shown in Fig. 2. A Statistical test if the along-stream cooling of Atlantic Water is significantly different during the two periods 1971-1999 and 2000-2018 are given in Supplementary Table. 1.

Extended Data Fig. 3 Atlantic Water inflow to the Barents Sea.

The estimates represent one-year mean values centred about 1st Jan and are based on an array of current meters in the western Barents Sea at about 20o E between 71o 30′–73o 30′N. AW is defined as water with temperature exceeding 3oC (see for example Ingvaldsen et al.29). The trend (dashed line) is 0.2 Sv over the period 1998 to 2018 and is not significant. Vertical bars indicate a guesstimate of the standard error. To estimate this we use total monthly error attributed to the individual current meters. The standard error of the annual means is then given by: \({\boldsymbol{e}}_{{\boldsymbol{annual}}} = \sqrt {\frac{{\mathop {\sum }\nolimits_{{\boldsymbol{i}} = 1}^{\boldsymbol{N}} {\boldsymbol{e}}_{\boldsymbol{i}}^2}}{{\boldsymbol{n}}}}\), and depends on the number of current meters (N) that varies over time and the time averaging, here number of months n=12. Sv=106 m3/s.

Extended Data Fig. 4 Statistical test of changes in the along-stream cooling of Atlantic Water.

The table show p-values based on a Welch test to investigate if the mean values are significantly different during the two periods 1971-1999 and 2000-2018. p-values < 0.05 are in bold (significant at the 95% level), and ± indicates if the cooling has become stronger/weaker during the recent compared to the early period. The definitions of the ‘Cold’ and ‘Warm’ periods (based on Supplementary Fig. 2) is subjective. However, as long as we avoid extending the ‘Warm’ period with many years into the preceding ‘Cold’ period the statistical results appear robust.

Supplementary information

Supplementary Information

Supplementary information, data, methods, data availability, Figs. 1–3 and Table 1.

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Skagseth, Ø., Eldevik, T., Årthun, M. et al. Reduced efficiency of the Barents Sea cooling machine. Nat. Clim. Chang. (2020). https://doi.org/10.1038/s41558-020-0772-6

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