Surface vs bottom contributions to SSH

[matthew-england-unsw]

Decompose our Fig. 2a plot (i.e. time-series of SSH in UP case say) into the contributions due to (i) Surface 100m (ii) Bottom-most 100m (not sure how we handle shelf regions shallower than 200m, maybe split in half?) and (iii) waters in between.

[matthew-england-unsw]

I guess I am still wondering about the strong coherent surface mixed layer T-S changes, driven by sea-ice formation and sea-ice advection anomalies, and how they in turn affect the SSH via steric height. The abyssal layer salinity changes are highly likely a small fraction of that surface S signature, and therefore a lesser player in why the initial Ekman response in SSH is reversed. I also think the advective time-scale to move the abyssal S signature around the circulpolar ocean is too slow to account for the SSH changes

[matthew-england-unsw]

PS: a cheap and easy way to sort this would be to plot something like [\alpha T + \beta S] anomalies ... as a ratio of the upper 100-m values to the bottommost 100-m values (taking care of any shelf regions shallower than 200-m). I know ratios can overstate regions of little change but it would be a first look-and-see. No rush of course, just sending before I forget.

[julia-neme]

These are the top (full line) and bottom 100m (dashed line) steric sea level changes for control, up and down:

And these are the anomalies of up and down with respect to control:

The magnitude of the anomalies here is ~mm, while the sea level anomalies in Figure 2 are of the order of ~cm. So I guess this means that the steric contribution is not very important as Ekman transport onto the shelf?

[julia-neme]

We talked a little bit about these SSH changes @matthew-england-unsw, and I think consensus was not to include them in Figure 2 for the following reasons:

• The magnitude of the changes is << than the total SSH changes
• In the anomaly plots you can see that the relative contributions of the upper 100m and the bottom 100m are of ~ magnitude

But maybe you see something else interesting in these time series worthy of inclusion?

[matthew-england-unsw]

Thanks Julia! Yes, I should’ve been clear, I never proposed this as a figure for the paper, my goal was merely to understand the relative roles played by bottom most temperatures vs surface temperatures. I felt the narrative of the paper was all around the dense shelf water changes and how that was controlling the response to wind changes. I think your analysis shows that changes right through the water column may be important, certainly the changes at the surface play as big a role as T-S changes in the abyss?

[matthew-england-unsw]

Oops typo's above: I meant ".... my goal was to understand the relative roles played by bottommost salinity vs upper ocean salinity". Given surface and bottom layers are of the same order of magnitude in contributing to SSH changes, when we describe the reversal of the SSH tendency (i.e. the reversal after the initial Ekman forced response), I think we shouldn't just describe this in terms of the DSW changes. It's likely that the salinity changes right through the water column are contributing here. Apologies I missed y'all earlier today. matt

[adele-morrison]

Matt I think we had the same thought yesterday too. As a result we’ve decided to add salinity changes to the age plots in figure 3.

On Thu, Nov 4, 2021 at 11:33 PM, Matthew England @.***> wrote:

Oops typo's above: I meant ".... my goal was to understand the relative roles played by bottommost salinity vs upper ocean salinity". Given surface and bottom layers are of the same order of magnitude in contributing to SSH changes, when we describe the reversal of the SSH tendency (i.e. the reversal after the initial Ekman forced response), I think we shouldn't just describe this in terms of the DSW changes. It's likely that the salinity changes right through the water column are contributing here. Apologies I missed y'all earlier today. matt

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[matthew-england-unsw]

Thanks Adele, sounds good!