The ocean mixed layer under Southern Ocean sea-ice: Seasonal cycle and forcing

Abstract

The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea‐ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea‐ice plays a key role for water mass transformation, the carbon cycle, sea‐ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under‐ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal‐derived, ship‐based, and Argo float observations, we describe the seasonal cycle of the ocean mixed‐layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea‐ice. Mixed‐layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed‐layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea‐ice freshwater flux for the salt budget and by air‐sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea‐ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large‐scale water mass formation and transformation with far reaching consequences for ocean ventilation.

An assessment of ten ocean reanalyses in the polar regions

“…Nevertheless, these observations remain relatively scarce, especially below the sea ice (Schmidtko et al 2014;de Lavergne et al 2014;Roemmich et al 2015;Roquet 2015;Pellichero et al 2017). The amount of in-situ observations for sea-ice thickness is also relatively limited (Worby et al 2008).…”

“…Reanalyses tend to underestimate MLDs around East Antarctica, whereas an ORA ensemble mean positive bias of more than 200 m is seen in the Ross and Weddell Seas. Pellichero et al (2017) note however that the Ross Sea sector is the least well observed region so the climatology must be used with caution in this area. Of individual ORAs, MOVE-G2i and, to a lesser extent, GLORYS2v4 and GECCO2, show signs of open ocean deep convection in the Weddell Sea ( Figure S15).…”

“…The total number of cases (n) on which the assessment is done is also given Fig. 18 Antarctic mixed layer depth for January (top) and August (bottom), in an observation-based climatology (OBS; Pellichero et al 2017), for the ensemble mean of the ORAs (MMM) and bias of the ensemble mean with respect to the climatology (MMM-OBS)…”

“…As the MLD is a relevant physical index of the vertical mixing intensity in the upper ocean (Toyoda et al 2017a), the MLDs simulated by the ORAs are evaluated against two observationbased products. These are the Monthly Isopycnal and Mixed-layer Ocean Climatology for the Arctic (MIMOC; Schmidtko et al 2013) and a recently published Southern Ocean mixed layer climatology (Pellichero et al 2017).…”

“…In addition, MIMOC includes data recorded by ice-tethered profilers in the Arctic Ocean, while Pellichero et al (2017) use observations from animal-borne sensor programs in the Southern Ocean ). These contemporary sources provide an unprecedented data coverage of the sea-ice regions over the entire seasonal cycle.…”