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The observation-based application of a regional thermohaline inverse method to diagnose the formation and transformation of water masses north of the OSNAP array from 2013 to 2015

The observation-based application of a regional thermohaline inverse method to diagnose the formation and transformation of water masses north of the OSNAP array from 2013 to 2015
The observation-based application of a regional thermohaline inverse method to diagnose the formation and transformation of water masses north of the OSNAP array from 2013 to 2015

The strength of the meridional overturning circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed regional thermohaline inverse method (RTHIM) to a region encompassing the Arctic and part of the North Atlantic subpolar gyre for the years 2013, 2014, and 2015. RTHIM is a novel method that can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv (1 Sv ≡ 10 6 m 3 s 21) during 2014–15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat, and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program (OSNAP) and show good agreement, lending validity to our results.

0022-3670
1533-1555
Mackay, Neill
04a0ec32-fec9-4876-a0c3-509177721555
Wilson, Chris
fadc83b7-f240-485b-8734-51099d02775a
Holliday, N. Penny
c4ef99c7-1be1-4148-ae23-0c678aa4f8f2
Zika, Jan D.
1843cce7-77ce-4ef6-9f79-bcf4f9db30e5
Mackay, Neill
04a0ec32-fec9-4876-a0c3-509177721555
Wilson, Chris
fadc83b7-f240-485b-8734-51099d02775a
Holliday, N. Penny
c4ef99c7-1be1-4148-ae23-0c678aa4f8f2
Zika, Jan D.
1843cce7-77ce-4ef6-9f79-bcf4f9db30e5

Mackay, Neill, Wilson, Chris, Holliday, N. Penny and Zika, Jan D. (2020) The observation-based application of a regional thermohaline inverse method to diagnose the formation and transformation of water masses north of the OSNAP array from 2013 to 2015. Journal of Physical Oceanography, 50 (6), 1533-1555. (doi:10.1175/JPO-D-19-0188.1).

Record type: Article

Abstract

The strength of the meridional overturning circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed regional thermohaline inverse method (RTHIM) to a region encompassing the Arctic and part of the North Atlantic subpolar gyre for the years 2013, 2014, and 2015. RTHIM is a novel method that can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv (1 Sv ≡ 10 6 m 3 s 21) during 2014–15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat, and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program (OSNAP) and show good agreement, lending validity to our results.

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e-pub ahead of print date: 13 February 2020
Published date: June 2020
Additional Information: Funding Information: Acknowledgments. NM, CW, and NPH acknowledge funding from the U.K. Natural Environment Research Council under the U.K. OSNAP Large Grant (NE/K010875.1). NPH and CW are additionally supported by the U.K. Natural Environment Research Council’s North Atlantic Climate System Integrated Study Program (NE/N018044/1) and Climate Linked Atlantic Sector Science (CLASS, NE/R015953/1). NPH is also supported by NERC U.K.-OSNAP-Decade (NE/T00858X/1 and NE/T00858X/2). JDZ acknowledges funding from Australian Research Council Grant DP1603130. NM is additionally supported by NERC Grant NE/P021298/1, with thanks to Professor Andrew Watson. The OSNAP data are available for download from o-snap.org. Publisher Copyright: © 2020 American Meteorological Society.

Identifiers

Local EPrints ID: 439837
URI: http://eprints.soton.ac.uk/id/eprint/439837
ISSN: 0022-3670
PURE UUID: 7343cfff-6201-467b-b081-224e3f05744d

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Date deposited: 05 May 2020 16:31
Last modified: 17 Mar 2024 05:31

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Contributors

Author: Neill Mackay
Author: Chris Wilson
Author: N. Penny Holliday
Author: Jan D. Zika

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