Reduction in ocean heat transport at 26°N since 2008 cools the Eastern Subpolar Gyre of the North Atlantic Ocean

Northward ocean heat transport at 26°N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large—approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009–16) on an annual average basis associated with a 2.5-Sv (1 Sv ≡ 106 m3 s−1) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26°N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2°C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport.

10.1175/JCLI-D-19-0323.1

0894-8755

1677-1689

Bryden, Harry L.

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Johns, William E.

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King, Brian A.

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Mccarthy, Gerard

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Mcdonagh, Elaine L.

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Moat, Ben I.

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Smeed, David A.

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1 March 2020

Bryden, Harry L.

7f823946-34e8-48a3-8bd4-a72d2d749184

Johns, William E.

e09ba73d-f275-4ff3-a07c-dfb8f1622b21

King, Brian A.

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Mccarthy, Gerard

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Mcdonagh, Elaine L.

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Moat, Ben I.

9cb7be4b-9e0e-42cc-a015-bf8dfa5ef27d

Smeed, David A.

2f7e2df1-c899-4d7a-94ef-d47ddb1cc494

Bryden, Harry L., Johns, William E., King, Brian A., Mccarthy, Gerard, Mcdonagh, Elaine L., Moat, Ben I. and Smeed, David A. (2020) Reduction in ocean heat transport at 26°N since 2008 cools the Eastern Subpolar Gyre of the North Atlantic Ocean. Journal of Climate, 33 (5), 1677-1689. (doi:10.1175/JCLI-D-19-0323.1).

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Accepted/In Press date: 15 November 2019

e-pub ahead of print date: 24 January 2020

Published date: 1 March 2020

Additional Information: Funding Information: Acknowledgments. The RAPID-MOCHA-WBTS project to monitor the Atlantic meridional overturning circulation and associated meridional heat and freshwater transports at 26°N is supported by the U.K. Natural Environment Research Council and by the U.S. National Science Foundation (Grant 1332978) and the U.S. National Oceanic and Atmospheric Administration (Fund Reference 100007298). Additional funding from the NERC ACSIS programme (Grant NE/N018044/1), from the European Union Horizon 2020 research and innovation programme BLUE-ACTION (Grant 727852), and by the A4 project (Grant Aid Agreement PBA/CC/18/01) supported by the Irish Marine Institute under the Marine Research Programme funded by the Irish government and cofinanced by the ERDF enhanced the analysis presented here. The RAPID dataset and associated heat and freshwater transports are made freely available online (http://rapid.ac.uk/rapidmoc). EN4 monthly mean subsurface temperature and salinity profiles are made freely available by the Met Office (http://www.metoffice.gov.uk/ hadobs/en4). The availability of these datasets allowed us to start this analysis. John Fasullo provided the Trenberth and Fasullo (2017) overall air–sea heat fluxes north of 26°N up to 2013, and Chunlei Liu kindly provided the Liu et al. (2017) gridded air–sea heat flux monthly values for our analyses. Elizabeth Kent provided access to bulk-formula climatologies from NOC, ERA, MERRA, and JRA, David Berry made uncertainty maps of NOC air– sea exchange over 5-year intervals, and Jeremy Grist provided access to the NCEP bulk-formula climatology. Publisher Copyright: © 2020 American Meteorological Society.