The daily-resolved Southern Ocean mixed layer: regional contrasts assessed using glider observations

Abstract

<p>Water mass transformation in the Southern Ocean is vital for closing the large-scale overturning circulation, altering the thermohaline characteristics of upwelled Circumpolar Deep Water before returning to the ocean interior. Using profiling gliders, this study investigates how buoyancy forcing and wind-driven processes lead to intraseasonal (1-10 days) variability of the mixed layer temperature and salinity in three distinct locations associated with different Southern Ocean regions important for water mass transformation - the Subantarctic Zone (SAZ, 43°S), Polar Frontal Zone (PFZ, 54°S) and Marginal Ice Zone (MIZ, 60°S). Surface heat fluxes drive the summertime mixed layer buoyancy gain in all regions, particularly evident in the SAZ and MIZ, where shallow mixed layers and strong stratification further enhance mixed layer warming. In the SAZ and MIZ, the entrainment of denser water from below is the primary mechanism for reducing buoyancy gain. In the PFZ, turbulent mixing by mid-latitude storms result in consistently deep mixed layers and suppressed mixed layer thermohaline variability. Intraseasonal mixed layer salinity variability in the polar regions (PFZ and MIZ) is dominated by the lateral stirring of meltwater from seasonal sea ice melt. This is evident from early summer in the MIZ, while in the PFZ, meltwater fronts are proposed to be dominant 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 mixed layer thermohaline properties to small spatio-temporal processes, which suggests a similar regional dependence to surface water mass transformation in the Southern Ocean.</p>

Atmospheric Rivers Contribute to Summer Surface Buoyancy Forcing in the Atlantic Sector of the Southern Ocean

“…These events adjust salinity in the mixed layer as the precipitation is dissipated vertically rather than advected horizontally as shown in du Plessis et al. (2022). This contributes to an overall decrease in seasonal salinity over the time series.…”

“…Large events (>75th percentile of precipitation) accounted for 88% of the total precipitation observed during the 55 days of WG observations. The WG observations show a seasonal decrease in salinity of ∼0.03 g kg −1 over the duration of the deployment (Figure 1f), which is primarily due to the accumulated precipitation in the surface ocean over the summer (2.17 ± 3.89 mm day −1 , 120 mm total) (Nicholson et al., 2022; du Plessis et al., 2022). We observe large and rapid (<6 hr) responses in salinity during the large rainfall events with four events of salinity decreasing up to 0.05 g kg −1 occurring on 27 December 2018, 1 and 5 January, and 7 February 2019 (Figures 1d and 1f).…”

“…(2022) and assuming that the effects of advection and entrainment are negligible (which do not impact in the same way as precipitation in this region, see Figure 8e, du Plessis et al., 2022), we calculate a mixed‐layer salinity budget for the whole time series. Starting with a salinity of 34.11 g kg −1 , forcing it with the ERA5 precipitation, and using a mean mixed‐layer depth of 100 m (using observations from both Nicholson et al., 2022, and du Plessis et al., 2022), we see a decrease in the salinity of 0.04 g kg −1 , compared with the actual observed change of 0.03 g kg −1 . The difference between the computed and actual change could be due to active mixing when the mixing‐layer depth extends below the base of the mixed layer or the slight variability in the mixed‐layer depth during the time series.…”

“…Further, du Plessis et al. (2022) show no significant difference in the summer mixed‐layer buoyancy gain of the Southern Ocean when calculated using ERA5 and two other commonly used reanalysis products.…”

The Importance of Spatial and Temporal Scales in Understanding Chlorophyll Variability in the Southern Ocean

“…Satellite remote sensing, in particular, has been used to observe spatial and temporal trends in chlorophyll concentration (Sullivan et al., 1993, and many others) and net primary production (Arrigo et al., 2008). More recently, autonomous platforms, including profiling floats (Mohrmann et al., 2022) such as Biogeochemical‐Argo floats (Briggs et al., 2018; Claustre et al., 2020; Johnson et al., 2017), unmanned surface vehicles such as Saildrones (Sutton et al., 2021), and gliders (du Plessis et al., 2022; Giddy et al., 2021; Nicholson et al., 2022), have opened a window into monitoring biogeochemical and phytoplankton dynamics in the Southern Ocean. In order to capture changes, we need to make measurements at high spatial and temporal resolution.…”