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Use of Passive Tracers as a Diagnostic Tool in Coupled Model Simulations—Northern Hemisphere

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  • 1 CSIRO Atmospheric Research, Aspendale, Victoria, Australia
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Abstract

This study focuses on the uptake of a passive idealized tracer in the Northern Hemisphere oceans from two coupled ocean–atmosphere simulations: a standard horizontal diffusion case and the second case including the Gent and McWilliams (GM) eddy mixing parameterization. The results are compared with tracer uptake in stand-alone synchronous and asynchronous ocean simulations for the same cases. The GM set of integrations shows tracer penetration reduced from the standard set in all water mass formation regions. There is a strong similarity in the tracer distributions in the stand-alone ocean simulations in both the standard and GM cases. Changes in the velocity fields between the stand-alone ocean and coupled simulations explain many of the differences in the modeled tracer concentrations.

There is a particular focus in the study on the dynamics of modeled water mass formation for North Pacific Intermediate Water, Labrador Sea Water, northeast Atlantic mode water, and North Atlantic Deep Water. The model representation of these water masses is compared with observational data of passive tracers, and gives mixed results as the model water masses are on lighter density surfaces than the real ocean though the timing of the advance of the tracer plume within the water masses appears to be realistically modeled. For the coupled simulations, the North Atlantic and North Pacific Oceans have interdecadal signals that alter the circulation and hence tracer patterns. Some issues arising from the interdecadal signal are discussed in relation to tracer distribution on density surfaces within the ocean.

Corresponding author address: Dr. Siobhan O'Farrell, CSIRO Atmospheric Research, Private Bag No. 1, Aspendale, Victoria, 3195 Australia. Email: Siobhan.O'Farrell@dar.csiro.au

Abstract

This study focuses on the uptake of a passive idealized tracer in the Northern Hemisphere oceans from two coupled ocean–atmosphere simulations: a standard horizontal diffusion case and the second case including the Gent and McWilliams (GM) eddy mixing parameterization. The results are compared with tracer uptake in stand-alone synchronous and asynchronous ocean simulations for the same cases. The GM set of integrations shows tracer penetration reduced from the standard set in all water mass formation regions. There is a strong similarity in the tracer distributions in the stand-alone ocean simulations in both the standard and GM cases. Changes in the velocity fields between the stand-alone ocean and coupled simulations explain many of the differences in the modeled tracer concentrations.

There is a particular focus in the study on the dynamics of modeled water mass formation for North Pacific Intermediate Water, Labrador Sea Water, northeast Atlantic mode water, and North Atlantic Deep Water. The model representation of these water masses is compared with observational data of passive tracers, and gives mixed results as the model water masses are on lighter density surfaces than the real ocean though the timing of the advance of the tracer plume within the water masses appears to be realistically modeled. For the coupled simulations, the North Atlantic and North Pacific Oceans have interdecadal signals that alter the circulation and hence tracer patterns. Some issues arising from the interdecadal signal are discussed in relation to tracer distribution on density surfaces within the ocean.

Corresponding author address: Dr. Siobhan O'Farrell, CSIRO Atmospheric Research, Private Bag No. 1, Aspendale, Victoria, 3195 Australia. Email: Siobhan.O'Farrell@dar.csiro.au

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