What causes winter water warming in the Southern Ocean?
September 10, 2024
Winter Water is a mode water that forms during austral summer in the Southern Ocean south of the subpolar front.
Spira et al. 2024, “Winter Water in the austral summer” Spira et al. (2024). The observed spatiotemporal variability of Antarctic Winter Water. Journal of Geophysical Research: Oceans.
In the winter it is the water that makes up the mixed layer and it thus cold from heat loss and salty from brine rejection. In the summer time, when sea ice melts, a fresh layer is formed above the winter water. This layer warms quickly and traps winter water beneath. Below the winter water is the warm (2°C) and salty Circumpolar Deep Water.
During summer, this winter water layer warms and erodes, changes its density class and mixes with the waters above and below. It is thus a key mediator of water mass transformation and is thought to to be intermediary water in the pathway of water transformation from Circumpolar Deep Water to Antarctic Intermediate Water.
“A slocum with a microrider”, Sarah Nicholson
The question addressed with this study is what physical process drives the warming of winter water in the summer time. We used exciting and novel observations of microstructure shear.
Turbulent processes
The intensity of turbulent mixing mediates the turbulent flux of a water property (like momentum, heat). By measuring the rate of dissipation of turbulent kinetic energy together with the background gradient of, for example, temperature, we can approximate the turbulent flux of heat.
\(F_H = K\frac{dT}{dz},\) where \(K = 0.2\frac{\varepsilon}{N^2}\).
Drivers of turbulent mixing and heat flux into the winter water layer
By diagnosing the sources of shear production we attributed enhanced mixing at the base of the mixed layer to wind driven shear (using a first order Law of the wall approximation) and enhanced mixing at the base of the winter water layer to double diffusive convection (evidence of temperature steps).
Following with a heat budget, we conclude that the warming trend is attributable to turbulent fluxes of heat.
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