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

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 /308 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.

Submesoscale transition from geostrophic flows to internal waves in the northwestern Pacific upper ocean

“…Numerical simulation evidence supporting the co-existence of interior QG and mixed layer instabilities in the STCC can be found in ref. 35. Compared with the Kuroshio band, the internal wave motions in the STCC band expand to the longer wavelengths (Fig.…”

Submesoscale circulation in the northern Gulf of Mexico: Surface processes and the impact of the freshwater river input

“…30 29 28 27 26 30 29 28 27 26 -96 -94 -92 -90 -88 -96 -94 -92 -90 -88 (a) (b) In winter, between January and March, submesoscale eddies are most numerous, as commonly found in other regions of the world ocean (Callies et al 2015;Capet et al 2008a;Capet et al 2008b;Mensa et al 2013;Qiu et al 2014). In this season surface buoyancy gradients and vorticity filaments can intensify and undergo frontogenesis (McWilliams et al 2009b).…”

“…Those structures intrude into the shelf. other ocean regions, such as the North Atlantic (Callies et al 2015;Mensa et al 2013), the South American slope (Capet et al 2008a), or the North Pacific (Qiu et al 2014 where the intensity and relative importance of submesoscale processes have been found to be inversely proportional to the mixed layer depth. The summer peak dominates the yearly variability of vorticity for waters shallower than 200 m but deeper than the MLD.…”

The influence of mesoscale and submesoscale circulation on sinking particles in the northern Gulf of Mexico

“…Submesoscale features are stronger and more numerous when more available potential energy can be extracted. In general, for open ocean conditions, these characteristics imply a seasonal cycle of submesoscale activity that follows that of the mixed layer depth (Capet et al, 2008;Mensa et al, 2013, Qiu et al, 2014Callies et al, 2015). In the GoM, however, the submesoscale seasonality is further modulated by the riverine outflow.…”