Following todays discussion at the CM3 evaluation meeting I investigated further the 3 areas identified as convection areas in @AndyHoggANU plots of convection in year 50 of the “cm3-20-01-2026” run. Also briefly looked at year 40 and year 80. Only one of them meets the criteria that would normally used to describe open ocean convection by the community at ~170W 68S. The other regions were caused by coastal polynyas which occurred repeatedly at the same locations, so are driven by katabatic winds off the Antarctic continent so lead to deep mixed layer next to the coast. These may not be at the correct locations for polynyas but are not far from the correct locations (see work of Alex Fraser), which are regions of low, thin ice concentration, and high ice production so high levels of ocean mixing on the shelf and dense shelf waters and potentially off shelf flow if the conditions are right. The other locations picked up in the density derived Mixed layer depth field in mlotst are at the continental shelf break and represent genuine overflow conditions as represented by the “coarse’ 0.25 representation in CM3 in the Ross and Weddell Seas.
I will add a few plots, I have loads more (like a folders of > 70). One reason this was hard to detect, from just scanning ice cover each year for open ocean polynyas is that the open ocean convection at 170W occurs late in the season in October, as the ice starts to open up, and then refreezes during a cold outbreak rather than mid-Winter, Aug-Sep when ice conditions in the region are around 90%. The ice cover also has a local minima at 170W, but barely detectable in 1440 E-W CM3 grid points and the coastal polynya thin ice points are way more noticeable.
The cause of the polynya at 170W 68S is series of sub subsurface topography features upstream and from 160E-180W 68-70S.
The feature does contaminate the age tracer if looking to detect bottom water formation regions, but small scale coastal polynyas also are an issue as well.
Mixed layer depth year 50 of run in Ross sea showing deep mixing at 170W 68S, mixing at Ross ice shelf caused by polynya which opens each spring and along continental shelf break as dense waters overflow.
Mixed layer depth year 50 of the CM3 run in the Weddell Sea with intermittent coastal polynyas 0-25E and mixing at the continental shelf break as dense shelf water overflows.
I showed the age tracer in the meeting today along this section at 40W in the Weddell Sea here is confirmation from the density section that it represented local overflow rather than an upstream signal of an overflow of the dense shelf water as represented at 0.25 resolution.
Salinity section from year 40 of CM3 simulation polynya was still present at 25E, not all coastal polynyas found at all locations all model years, this a more common one, Recent brine rejection down to 1000m, mixing over last decade down to 3000m.
Temperature section of Open ocean convection in year 50 of CM3 along 170W in October, (September shows only mild activity, August none), showing active convection to 2500m in narrow chimney at 68S, surface signature in ice and SST minor.
Temperature section 10 years earlier shows convective activity was centred 72-69S again signal was only significant in October, barely present when ice was more compact and colder in mid-winter Aug-Sept conditions.
Final figure, of topography of region, along 68S at 160E two islands then there are small ridges/seamounts between 170E and 180E/W 67-69S rising to 1000m in height, which are the likely cause of the mixing, The region is also part of the Southern edge of the ACC, so has strengthening current flows.
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