Equatorial Pacific Subsurface Countercurrents: A Model–Data Comparison in Stream Coordinates

Kathleen A. Donohue University of Rhode Island, Narragansett, Rhode Island

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Eric Firing University of Hawaii, Honolulu, Hawaii

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G. Dail Rowe Accurate Environmental Forecasting Inc., Narragansett, Rhode Island

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Akio Ishida Japan Marine Science and Technology Center, Yokosuka, Japan

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Humio Mitsudera International Pacific Research Center, University of Hawaii, Honolulu, Hawaii

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Abstract

An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.

Corresponding author address: Dr. Kathleen A. Donohue, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197. Email: kdonohue@gso.uri.edu

Abstract

An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.

Corresponding author address: Dr. Kathleen A. Donohue, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197. Email: kdonohue@gso.uri.edu

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