sea ice changes

[PaulSpence]

evaluate how the sea ice responds ... link to changes in watermass transformation

[PaulSpence]

[PaulSpence]

Thicker sea ice on shelf when easterlies weaken, and less ice on shelf when they strengthen. A little counterintuitive give that stronger easterlies should move more ice towards land (onshore Ekman with easterly winds).

[adele-morrison]

Yes, this isn't what I would have expected. Also the DSW formation increases in the increased easterlies perturbation, which means there should be more sea ice production in the up case.

However, this is consistent (even down to the spatial pattern) with the SST change (see figure here). So maybe we need to figure out what's driving the SST change? Perhaps there's increased convection in the up case, which brings up more heat from below?

[adele-morrison]

I think my guess here was correct. In the first year, the SST change is as we expected for the fast Ekman response (cooling over shelf in increased easterlies case). However, by year 3 this is overwhelmed by surface warming (presumably upwelling/mixing from below?). Might be worth checking if the sea ice anomaly in the first year is consistent with this?

Opposite response in first year to the long term SST trends:

Map of first year SST anomalies:

[matthew-england-unsw]

Excellent! This is a very nice result - and reassuring to see / confirm that the fast time-scale Ekman response is there (sanity check that the wind anomaly sign was applied correctly haha) - then eventually overwhelmed by upwelling / mixing from below. Highly analogous to the John Marshall et al. argument around a two-time scale mid-latitude response in SST: i.e. the initial surface SWW-driven cooling eventually being overwhelmed by CDW upwelling.

[PaulSpence]

Seasonal ice thickness anomalies in first year (model year 2150). Losing ice with stronger easterlies, gaining ice with weaker. Seems opposed to the SST changes, with stronger easterlies having lower SST and less ice. Confusing to me because stronger easterlies should pile ice up onshore.

[PaulSpence]

Annual mean ice thickness change over first 3 years. The sign of change on the Weddell/WAP doesn't seem to match the SST pattern.

[PaulSpence]

Annual mean frazil growth anomaly (m?). There is also a congel (basal) growth term. Not sure of other ice growth terms. Seems to agree with more DSW, more ice growth with stronger easterlies (why thinner ice?).

[wghuneke]

Maybe looking at the sea ice movement helps. I expect the ice to be more divergent near the coast in the UP case, leading to higher sea ice production rates, deeper mixed layers and thinner sea ice.

[adele-morrison]

Ok, that makes sense that there's more ice production in the UP case, even though the ice is thinner. I would guess the thickness is just following the SST response (warming in the UP case). I think we need to get to the bottom of why the SST warms in the UP case.

[PaulSpence]

Ice velocity anomalies ...seems like the world is upside down and backwards.

[adele-morrison]

Maybe where there's more ice, it's moving south and the reverse (definitely in Ross and east Weddell)? Hard to tell what's going on in East Antarctica and West Antarctica though. Maybe a longer average would help?

[wghuneke]

Thanks, @PaulSpence , that was quick! Looking at the Ross Sea, this makes sense to me. Hard to tell what's going on with the narrower shelf regions. We might have to zoom in as for the surface momentum stress plots).

[PaulSpence]

[PaulSpence]

[PaulSpence]

Sea Ice Volume Anomalies on the Shelf over the first 3 years.

[PaulSpence]

Annual mean sea ice volume in control:

Annual mean Up sea ice volume anoms:

Annual mean Down sea ice volume anoms:

[StephenGriffies]

impressive symmetry between UP and DOWN.

[adele-morrison]

Also while preparing for my seminar I made this sea ice thickness and velocity figure (very similar to Paul's previous ones, but averaged over last 5 years, and I masked the velocities where the sea ice concentration was low at the northern edge).

Do you think this is good for Figure 7?

I guess it would look better if we could plot transports not velocities, but I don't think we have the right diagnostics to do that properly.

[PaulSpence]

Hi Adele,

Thanks for looking into the sea ice. I am working on it as well. Trying to regrid the data a bit to smooth the arrows.

P

On Mon, Nov 22, 2021 at 10:55 AM Adele Morrison @.***> wrote:

Also while preparing for my seminar I made this sea ice thickness and velocity figure (very similar to Paul's previous ones, but averaged over last 5 years, and I masked the velocities where the sea ice concentration was low at the northern edge).

Do you think this is good for Figure 7?

[image: sea_ice_thickness_velocity] https://user-images.githubusercontent.com/8506963/142784059-099764ee-9412-41e0-a857-1223db0ebae4.png

I guess it would look better if we could plot transports not velocities, but I don't think we have the right diagnostics to do that properly.

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[adele-morrison]

Yes, it would be nice if the velocities were smoother. I tried and failed at improving it using linear interpolation. Do you think it's worth trying to compute transports from (monthly volume) * (monthly velocity) and using those instead? That may remove the noise near the northern sea ice boundary and is probably actually closer to what we want for the mechanism centred around sea ice export.