Separate zonal/meridional perturbation simulations

[adele-morrison]

We decided yesterday to run additional perturbations where we apply only the zonal or meridional perturbations. Only a year and half so far, but looks like it is going to be interesting. The initial positive SSH on the shelf comes purely from the zonal perturbation.

I'll update here as the run progresses.

[adele-morrison]

I managed 7 years of each of the separate zonal and meridional perturbation runs before the end of the quarter. Here are some plots.

SSH anomaly. The summary here is that the initial positive SSH anomaly in the first 2 years comes entirely from the zonal wind perturbation. In the zonal only simulation, this positive anomaly persists beyond the first 2 years. The downward trend in SSH (linked to an increase in the abyssal cell, which increases the salinity on the shelf) comes from the meridional wind anomaly component. This is consistent with the sea ice advection hypothesis. The sum of the separate zonal and meridional perturbations approximately matches the full UP perturbation (and also the UP with no katabatics simulation).

[adele-morrison]

Bottom age anomaly (averaged over first 7 years of perturbations) also nicely shows the changes in the abyssal overturning cell from the different perturbations.

The meridional only component has younger waters in the abyss and along the overflow regions (i.e. the western Weddell Sea between the 1000m and 3000m isobaths), and older waters on the shelf (due to more CDW on the shelf when the lower cell increases).

The zonal only component has younger water on the shelf. Perhaps due to the Ekman mechanism depressing isopycnals? Or otherwise directly changing the CDW transport across the slope? I'm not entirely sure of the mechanism here.

The response seems roughly linear, with the sum of the two separate component anomalies matching the original UP simulation.

[adele-morrison]

Finally, here are the SWMT time series for each of the 4 DSW producing regions. In all regions, the meridional only perturbation matches the regular UP simulation pretty well, and the zonal only perturbation has very little influence on the DSW formation. In the Weddell and Adelie we do see a slight decrease in DSW formation in the zonal only perturbation.

[AndyHoggANU]

Seems really clear! Probably the last link in the chain is to show that the UP-Zonal case has similar Ekman pumping to UP? Or have we shown that already?

[matthew-england-unsw]

Very nice!! Thanks Adele :-) These 'decomposition' experiments do seem to complete the study nicely. Regarding the zonal(-only) wind response on the shelf, wouldn't it be both? i.e. Ekman depressing isopycnals, inducing an anomalous shelf overturning circulation [i.e. downwelling at the coast], and thereby also changing the CDW transport across the slope. This then changes the ratio of CDW to ASBW on the shelf. Could even be some Spence et al. [2017] BBL effects? This diagram attached below just pilfered via google isn't too bad at showing some of these processes, although it doesn't show SSH changes and it is a NH plot

[adele-morrison]

Further on this, it might be nice to plot up some transects with isopycnals from the zonal-only simulation. I think what you're suggesting here Matt is that the effect of the isopycnal depression at the coast could extend further down to the slope and hence inhibit CDW transport across the slope?

Andy, no we haven't split the Ekman pumping up into zonal and meridional components yet. I think Paul is onto it though.

[matthew-england-unsw]

Yes correct thanks Adele. Ekman pumping can have a surprisingly rapid and detectable impact at depth. Ekman effects might appear along the shelf break within a season or two. Agreed: Isopycnal snapshots over time would help visualize this. But when plotting I think we would need that technique of centring the x-axis along the 1000-m isobath as the zonal average is accumulated.

[adele-morrison]

Looks like our predictions of the temperature changes in the zonal/meridional perturbations were correct: Overwhelmingly cooling in the zonal perturbation (due to deepening isopycnals) and overwhelmingly warming in the meridional perturbation (due to heat brought onto shelf by enhanced overturning/CDW transport).

The main exception to this is in West Antarctica, where it also cools in the meridional experiment. Any ideas what's going on there? I guess West Antarctic transects may be helpful to understand.

Bottom temperature plots:

[matthew-england-unsw]

Thanks Adele. Do you mean the shelf bottom temperatures? Maybe it's in part because the A/B sector is unique in that it's the only shelf sector that isn't at, or downstream, of a DSW formation site? Even just west of the Peninsula (where meridional winds are poleward btw), that warming there is likely due to WSBW changes. But the rest of the A/B sector is more remote from DSW formation effects.

[adele-morrison]

Matt, yes I agree, the West Antarctic likely stands out because it’s far from a dense water site. But just interested in then what’s the mechanism driving the cooling there in the meridional wind case?