Abstract
The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability.
Highlights
Submesoscale motions are known to be ubiquitous in the World oceans [1]
A kinetic energy (KE) peak at submesoscale is observed in winter when the mixed-layer depth (MLD, H) is large enough for mixed-layer instabilities (MLI) to develop, which leads to a transformation of available potential energy (APE) into KE [2,3,4,5]
The interannual to decadal variations of the submesoscale KE in the Subtropical Countercurrent (STCC) region were examined by using the outputs from a submesoscale permitting hindcast simulation in the North Pacific from 1990 to 2016
Summary
Submesoscale motions (involving spatial scales from 1 to several tens km) are known to be ubiquitous in the World oceans [1]. Large-scale buoyancy anomalies (forced by surface turbulent fluxes) are stirred by mesoscale eddies, which produce strong buoyancy gradients (∇b) subsequently affected by MLI These mechanisms explain that the APE release by MLI is often parameterized as proportional to the product of the averaged horizontal buoyancy gradient (M2 = ∇b) squared and the mixed layer depth. In this region exhibits a peak in late winter followed by a EKE peak in May, i.e., two months later They investigated the contribution of other instability mechanisms but suggested the possible role of MLI and the subsequent inverse KE cascade to explain the EKE seasonal peak. ◦ N in this region (including the STCC and HLCC) may be attributed to the instability at mesoscale of temperature and salinity observations from the Argo program They found that the EKE decadal the interior zonal velocity induced by the surface heat flux variations.
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