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Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean

Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean
Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean
Offline particle trajectories are obtained for a quasi-global isopycnic-coordinate OGCM using an analytical method, adapted for use with online time-integrated isopycnal and diapycnal mass fluxes. The method is highly efficient, allowing the calculation of large ensembles of such trajectories. These ensembles can be used to establish pathways and transformations associated with the global circulation of water masses on timescales which are well in excess of any feasible model integration length. The method is here used to investigate the important, yet poorly observed, transformation of North Atlantic Deep Water (NADW) through slow spreading, upwelling and diapycnal mixing (defined when and where density decreases below a threshold value). A fundamental problem arises through unsteadiness in the thickness of NADW layers (due to various model flaws and/or intrinsic variability). Particles converge on gridboxes where layers inflate during the online time-integration period. Depending on the degree of layer inflation, only a fraction of NADW particles can be diagnosed as transformed at some point along their respective trajectories. However, the unsteadiness of layer thickness decreases during a 50-year spin-up, implying fewer converged trajectories and an increased fraction of transformed NADW. Using trajectories to trace NADW southward across the equatorial Atlantic, with mass fluxes from years 10, 30 and 50 of model spin-up, the transformed percentage (of NADW exported from the North Atlantic) increases from 17–18% (with fluxes from years 10 and 30) to 41% (using year 50 fluxes). In the latter case, about 30% of the NADW upwells south of 30°S after 500–1000 years. Most of the remaining 70% upwells in the South and North Pacific after 1000–2500 years.
WOCE, OCEAN MODEL, NORTH ATLANTIC DEEP WATER, CONVEYOR BELT, ISOPYCNALS, DIAPYCNALS
1463-5003
27-53
Marsh, R.
702c2e7e-ac19-4019-abd9-a8614ab27717
Megann, A.P.
4fd92e77-5512-487d-bb28-8476afafdeaf
Marsh, R.
702c2e7e-ac19-4019-abd9-a8614ab27717
Megann, A.P.
4fd92e77-5512-487d-bb28-8476afafdeaf

Marsh, R. and Megann, A.P. (2002) Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean. Ocean Modelling, 4 (1), 27-53. (doi:10.1016/S1463-5003(01)00011-7).

Record type: Article

Abstract

Offline particle trajectories are obtained for a quasi-global isopycnic-coordinate OGCM using an analytical method, adapted for use with online time-integrated isopycnal and diapycnal mass fluxes. The method is highly efficient, allowing the calculation of large ensembles of such trajectories. These ensembles can be used to establish pathways and transformations associated with the global circulation of water masses on timescales which are well in excess of any feasible model integration length. The method is here used to investigate the important, yet poorly observed, transformation of North Atlantic Deep Water (NADW) through slow spreading, upwelling and diapycnal mixing (defined when and where density decreases below a threshold value). A fundamental problem arises through unsteadiness in the thickness of NADW layers (due to various model flaws and/or intrinsic variability). Particles converge on gridboxes where layers inflate during the online time-integration period. Depending on the degree of layer inflation, only a fraction of NADW particles can be diagnosed as transformed at some point along their respective trajectories. However, the unsteadiness of layer thickness decreases during a 50-year spin-up, implying fewer converged trajectories and an increased fraction of transformed NADW. Using trajectories to trace NADW southward across the equatorial Atlantic, with mass fluxes from years 10, 30 and 50 of model spin-up, the transformed percentage (of NADW exported from the North Atlantic) increases from 17–18% (with fluxes from years 10 and 30) to 41% (using year 50 fluxes). In the latter case, about 30% of the NADW upwells south of 30°S after 500–1000 years. Most of the remaining 70% upwells in the South and North Pacific after 1000–2500 years.

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More information

Published date: 2002
Keywords: WOCE, OCEAN MODEL, NORTH ATLANTIC DEEP WATER, CONVEYOR BELT, ISOPYCNALS, DIAPYCNALS

Identifiers

Local EPrints ID: 8012
URI: http://eprints.soton.ac.uk/id/eprint/8012
ISSN: 1463-5003
PURE UUID: 2c4b7a85-bb9f-4c09-b290-b88049273595

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Date deposited: 19 Aug 2004
Last modified: 08 Jan 2022 00:56

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Contributors

Author: R. Marsh
Author: A.P. Megann

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