Enhancement in vertical fluxes at a front by mesoscale‐submesoscale coupling

Abstract

Oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean. Submesoscale, O(1) Rossby number, dynamics are particularly relevant for inducing the vertical (and lateral) flux of buoyancy and tracers in the mixed layer, but how these couple with the stratified pycnocline is less clear. Observations show surface fronts often persist beneath the mixed layer. Here we use idealized, three-dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities. We contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper. For the deeper fronts, we examine the effect of density stratification on the vertical coupling. Our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer. Eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals, thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts. The vertical enhancement of tracer fluxes through the mesoscale-submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean. A further implication for wind-forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior. The discrepancies are most severe within the mixed layer suggesting a potential role for Ekman-layer dynamics absent in existing submesoscale parameterizations.

Submesoscale turbulence in the upper ocean

“…Furthermore, future work will need to investigate whether and how the evolution and restratification is affected by the presence of the thermocline, which is expected to become increasingly important as mixed layer eddies grow larger (cf. Chapter 6; Ramachandran et al, 2014).…”

ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal

“…Thermal fronts can play a significant role in air-sea interactions, upper-ocean vertical structure, biological productivity, lateral and vertical mixing processes, and modification of cyclone tracks, but these features have received scant attention (Ramachandran et al 2014). Preliminary analysis shows that the highresolution (1 km) GHRSST daily mean product allows for detection of sharp frontal features during cloud-free conditions (Fig.…”

Submesoscale CO<sub>2</sub> variability across an upwelling front off Peru

“…Of all mechanisms presented, the gravity current is the only one that may account for the fast changes up to 50 m depth. In contrast, the stability analysis suggests that MLIs are mainly active in the mixed layer and could only effect lower layers by interacting with the deep mode (Ramachandran et al, 2014;Capet et al, 2016).…”

“…Still, submesoscale variability with large-amplitude variations of surface CO 2 concentrations on horizontal length scales of O(10 km) was observed. This variability was successfully reproduced by the modeling study of Resplandy et al (2009) but it does not seem to have a major effect on the model's overall CO 2 budget. The influence of submesoscale variability on the overall CO 2 budget might be stronger in the case of EBUS due to the ubiquitous existence of sharp fronts and filaments, i.e., in the case of a highly energetic (sub)mesoscale flow field (McWilliams et al, 2009;Colas et al, 2012).…”