Stirring of Sea‐Ice Meltwater Enhances Submesoscale Fronts in the Southern Ocean

Giddy, Isabelle; Swart, Sebastiaan; Plessis, Marcel du; Thompson, Andrew F.; Nicholson, Sarah

doi:10.1029/2020jc016814

Cited by 25 publications

(42 citation statements)

References 66 publications

(131 reference statements)

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“…The newly formed summer mixed layer overlies the cool and salty previous winter mixed layer (Figure 3), which then undergoes surface warming and freshening from air-sea fluxes that approximately double the mean stratification of the upper 100 m from 2.9 × 10 −5 s −2 on Yearday −12 to 5.2 × 10 −5 s −2 on Yearday 79 (Figure 6). The dominance of the thermal component in driving buoyancy gain during the post sea-ice melt summer was also observed by Giddy et al (2021). We showed here that despite increased stratification during summer, mixing below the MLD entrained cooler underlying WW (Figure 7c).…”

Section: The Marginal Ice Zonesupporting

confidence: 79%

“…To investigate this, we have compared the temperature and salinity profiles between the western and eastern ends of the bow-tie transects and find a propensity for slightly fresher (saltier) mixed layers at the western (eastern) ends of the bow-tie by about 0.02 g kg −1 (Figure S4 in Supporting Information S1), which scales with the observed daily salinity changes that the mixed layer budget cannot resolve (Figure 8i). Furthermore, Giddy et al (2021) used this data set to infer these gradients as stirring of submesoscale flows induced by the seasonal sea-ice melt. We found here that this stirring provided significantly more variability in S m than air-sea freshwater fluxes over the summer, while episodic entrainment events provided similar daily S m variations to the stirring of sea-ice induced gradients.…”

Section: Glider Sampling Considerationsmentioning

confidence: 99%

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The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

et al. 2022

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Water mass transformation in the Southern Ocean is vital for driving the large‐scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind‐driven processes on the intraseasonal (1–10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation—the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea‐ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea‐ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.

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Section: The Marginal Ice Zonesupporting

confidence: 79%

Section: Glider Sampling Considerationsmentioning

confidence: 99%

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

et al. 2022

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“…During our study period, this stratification appears to be relatively strong due to the influx of freshwater and low mean wind stress. Intermittent, strong wind events (>0.5°N m −2 ) did occur several times over the glider deployment (Figure 3c), which may have acted to destabilize the mixed layer, similar to the results of Bachman and Klocker (2020) or Giddy et al (2021). However, even if the submesoscale tracer fluxes are localized to the surface ocean, they still provide a continuous source of recently ventilated surface waters that are effectively stirred by the mesoscale, generating fine-scale gradients along density surfaces.…”

Section: The Case For Southern Ocean Ventilation Hotspotsmentioning

confidence: 53%

“…10.1029/2021JC017178 7 of 24 The core of the PF is associated with a local weakening of the stratification at the base of the mixed layer, which likely enables subduction in this region (Figure 4c). Near the PF, stratification at the base of the mixed layer is strongest and the MLD itself shallowest at the end of the austral summer (March), as a result of the stratifying effects of sea ice melt and increased solar radiation (Giddy et al, 2021). During austral fall and early winter (April-July), stratification at the base of the mixed layer decreases, corresponding to a reduction in the maximum vertical stratification (Dong et al, 2008).…”

Section: Regional Variabilitymentioning

confidence: 99%

Observational Evidence of Ventilation Hotspots in the Southern Ocean

et al. 2021

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Observations with high spatial and temporal resolution were collected from two ocean gliders following a Biogeochemical-Argo float.• Physical and biogeochemical tracer variance is enhanced within a standing me-10 ander of the Antarctic Circumpolar Current. 11• The standing meander shows evidence of heightened exchange of water proper-12 ties between the ocean surface boundary layer and the interior.

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“…Fox-Kemper et al ( 2008) provide a parameterization of the restratification mechanism by MLEs, which has been used in other studies in terms of equivalent stratifying heat flux, Q MLE (du Plessis et al, 2017(du Plessis et al, , 2019Giddy et al, 2021;Mahadevan et al, 2012). MLEs transform horizontal buoyancy gradients to vertical stratification, therefore Q MLE depends on the strength of horizontal buoyancy gradients and MLD as:…”

Section: Submesoscale Equivalent Heat Fluxesmentioning

confidence: 99%

Seasonal to Intraseasonal Variability of the Upper Ocean Mixed Layer in the Gulf of Oman

2022

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High‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone.

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Stirring of Sea‐Ice Meltwater Enhances Submesoscale Fronts in the Southern Ocean

Cited by 25 publications

References 66 publications

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

Observational Evidence of Ventilation Hotspots in the Southern Ocean

Seasonal to Intraseasonal Variability of the Upper Ocean Mixed Layer in the Gulf of Oman

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