The mesoscale variability of the sea surface temperature: An analytical and numerical model

Abstract

This study examines the emergence and evolution of a mesoscale sea surface temperature (SST) variability induced by a uniform and impulsive wind stress when an embedding quasigeostrophic (QG) flow is present. The SST variability which is triggered by the mixed-layer deepening closely resembles some characteristic properties of the QG flow, namely either the subsurface temperature or relative vorticity, depending on the amplitude of the deepening. The SST variance can have the same order of magnitude as the subsurface temperature variance. Within 10 days, the SST field, which is stirred only by the horizontal QG flow, displays a rapid spectral evolution characterized by the emergence of small-scale structures and the appearance of thermal fronts located in the QG jet areas. This evolution depends only on the deformation of the large-scale structures of the SST field, initially resulting from the mixed-layer deepening, by the QG strain field. In contrast with SST, later evolution of the mixed-layer depth is characterized by the emergence of large-scale structures. From these dynamical results, it is speculated that, when nonuniform initial conditions are considered, the resulting SST spatial variability should be more closely related to the subsurface temperature and the SST variance could be significantly increased.

Remote sensing of ocean surface currents: a review of what is being observed and what is being assimilated

“…where Q(x, t) are the heat fluxes, κ is the thermal diffusion, w e is the entrainment velocity at the base of the ML which is nonzero only if there is a deepening of the ML (e.g., see Klein and Hua, 1990) and T d is the temperature below the ML. In the ocean, the Péclet number is smaller than one, implying that the diffusion term can be removed from Eq.…”

“…Results show that the ML depth is a good indicator of the periods in which the phase shift between SSH and SST is minimal, but different from zero, and, consequently, the eSQG approach can be applied . The best situations correspond to deep ML, that are typically found in winter when smaller stratification favors the deepening of the ML (see Klein and Hua, 1990, for a discussion on the effect of ML deepening on SST). Notice that this approximation has a limited capability to reconstruct the vertical structure of the ocean (e.g., Isern-Fontanet et al, 2008;LaCasce, 2012) which has lead to proposals for improved models of the upper ocean dynamics (Wang et al, 2013;Chavanne and Klein, 2016).…”

Thermohaline structure in the California Current System: Observations and modeling of spice variance

“…[21] The presence of persistent, large-scale spice gradients provides a background gradient that can be stirred to produce thermohaline structure at smaller scales [Klein and Hua, 1990;Ferrari and Rudnick, 2000;Ferrari and Polzin, 2005]. The narrow poleward currents [Todd et al, 2011a] adjacent to similarly strong equatorward flows (Figures 4g-4i) create a horizontally sheared flow field that can stir the large-scale gradients to smaller scales.…”

The generation of phytoplankton patchiness by mesoscale current patterns

“…After a wind event, the mixed layer depth will initially reflect the generated eddy structure with a smaller mixing depth above cyclonic and a larger mixing depth above anticyclonic eddies, but horizontal advection will make the relation between mixed-layer depth and eddy field more complex over time. Klein and Hua (1990) observed a cascade of sea-surface temperature variability, generated by a wind-induced mesoscale eddy field, from small wavenumbers to large wavenumbers within a few days. The advective flow field is likely to affect the phytoplankton distribution as well, hence chlorophyll concentrations are not expected to be well correlated with the eddy field after more than a few days.…”