Submesoscale processes promote seasonal restratification in the Subantarctic Ocean

Abstract

Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modeling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intraseasonal variability of the Subantarctic ML is still relatively unknown. We compare 5 months of glider data in the Subantarctic Zone to simulations of a 1‐D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater, and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down‐front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1‐D model estimates and the observations. However, during up‐front winds, the strength of springtime stratification increased over twofold compared to the 1‐D model, with a rapid shoaling of the MLD from >200 m to <100 m within a few days. The ML stratification is further modified under a negative Ekman buoyancy flux during down‐front winds, resulting in the destruction of ML stratification and deepening of the MLD. These results propose the importance of submesoscale buoyancy fluxes enhancing seasonal restratification and mixing of the Subantarctic ML.

Subsurface Fine‐Scale Patterns in an Anticyclonic Eddy Off Cap‐Vert Peninsula Observed From Glider Measurements

“…Furthermore, in spite of corrections (see section ), the measurement around the thermocline (~100‐m depth) may be less accurate. In any case, the study of submesoscale mechanisms in the surface mixed layer are at the limits of the experiment sampling (e.g., Brannigan, ; Du Plessis et al, ; Mahadevan et al, ; Thompson et al, ) and thus beyond the scope of this work.…”

When Mixed Layers Are Not Mixed. Storm‐Driven Mixing and Bio‐optical Vertical Gradients in Mixed Layers of the Southern Ocean

“…The high degree of variance we observe in mixed‐layer nighttime Chl‐a fluorescence and particle backscatter statistics is enhanced in the summer and not observed in hydrographic properties (Figure ), indicating that biological processes frequently dominate vertical mixing in the mixed layer. Submesoscale features within the mixed layer have been observed in the Southern Ocean (Adams et al, ; Bachman et al, ) and can be important in shaping the local distributions of bio‐optical properties and biogeochemical tracers in the Southern Ocean (du Plessis et al, ; Long et al, ; Read et al, ; Resplandy et al, ; Rosso et al, ), even in the summertime (Erickson et al, ; Viglione et al, ). Thus, submesoscale variability could explain some of the observed mixed‐layer bio‐optical variance.…”

Iron Supply Pathways Between the Surface and Subsurface Waters of the Southern Ocean: From Winter Entrainment to Summer Storms

“…These studies have focused on particular localized scenarios, and their relevance has not yet been contextualized within the broader seasonal mean state. The SO, in particular the SAZ, is a region of strong eddy (Frenger et al, ) and small‐scale frontal activity (du Plessis et al, , ) and strong winds from passing storms (Patoux et al, ; Yuan, ; Yuan et al, ). We hypothesize that storms increase mixing and advection during summer that are enhanced by wind‐mesoscale interactions and may result in nonnegligible physical iron supplies from the subsurface to the surface ocean to support phytoplankton production beyond what is possible by the once‐off winter supply.…”