Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent

Type Article
Date 2014-12
Language English
Author(s) Qiu Bo1, Chen Shuiming1, Klein Patrice2, Sasaki Hideharu3, Sasai Yoshikazu4
Affiliation(s) 1 : Univ Hawaii Manoa, Dept Oceanog, Honolulu, HI 96822 USA.
2 : CNRS Ifremer UBO IRD, Lab Phys Oceans, Plouzane, France.
3 : Japan Agcy Marine Earth Sci & Technol, Applicat Lab, Yokohama, Kanagawa, Japan.
4 : Japan Agcy Marine Earth Sci & Technol, Res & Dev Ctr Global Change, Yokohama, Kanagawa, Japan.
Source Journal Of Physical Oceanography (0022-3670) (Amer Meteorological Soc), 2014-12 , Vol. 44 , N. 12 , P. 3079-3098
DOI 10.1175/JPO-D-14-0071.1
WOS© Times Cited 112
Keyword(s) Circulation/ Dynamics, Eddies, Instability, Mesoscale processes, Observational techniques and algorithms, Altimetry, Models and modeling, Ocean models, Variability, Seasonal variability
Abstract Located at the center of the western North Pacific Subtropical Gyre, the Subtropical Countercurrent (STCC) is not only abundant in mesoscale eddies, but also exhibits prominent submesoscale eddy features. Output from a 1/30 degrees high-resolution OGCM simulation and a gridded satellite altimetry product are analyzed to contrast the seasonal STCC variability in the mesoscale versus submesoscale ranges. Resolving the eddy scales of >150 km, the altimetry product reveals that the STCC eddy kinetic energy and rms vorticity have a seasonal maximum in May and April, respectively, a weak positive vorticity skewness without seasonal dependence, and an inverse (forward) kinetic energy cascade for wavelengths larger (shorter) than 250 km. In contrast, the submesoscale-resolving OGCM simulation detects that the STCC eddy kinetic energy and rms vorticity both appear in March, a large positive vorticity skewness with strong seasonality, and an intense inverse kinetic energy cascade whose short-wave cutoff migrates seasonally between the 35- and 100-km wavelengths. Using a 2.5-layer, reduced-gravity model with an embedded surface density gradient, the authors show that these differences are due to the seasonal evolution of two concurring baroclinic instabilities. Extracting its energy from the surface density gradient, the frontal instability has a growth time scale of O(7) days, a dominant wavelength of O(50) km, and is responsible for the surface-intensified submesoscale eddy signals. The interior baroclinic instability, on the other hand, extracts energy from the vertically sheared STCC system. It has a slow growth time scale of O(40) days, a dominant wavelength of O(250) km, and, together with the kinetic energy cascaded upscale from the submesoscales, determines the mesoscale eddy modulations.
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