Seasonal variability in ASC speed and cross slope heat transport

[willaguiar]

Hello everyone

I have done a previous short analysis with ACCESS-OM2-01 regarding the ASC speed and cross slope heat transport, and thought it would be useful to post here to start a discussion for our first hackathon.

Using Wilma code, I obtained the monthly velocities along the 1000m-isobath (along-slope speed) as a measurement of the ASC, and also calculated the heat transport across the 1000-m isobath (HT).

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Spatial Variability

From the figure above we can see the mean cross slope heat transport (a), ASC surface speed (c), and their standard deviation ( from a 4-years-long period, monthly means). It seems that a few regions where the ASC is highly variable, also have a high variability in the cross slope heat transport which would suggest that ASC would have some control on the variability of the cross slope heat transport. e.g., in the eastern Weddell Sea, and West Antarctic Peninsula.

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Temporal Variability Because I used monthly output in the RYF simulation, all variability here is seasonal.

<<-old wrong fig, correct fig below check this comment for explanation on the correction.

Above we see the total average (depth and long/lat) ASC speed (positive = westward) vs the cross slope heat transport (negative = towards the South). Top is a circumpolar average, middle are averages for all points, except in the West Antarctic Peninsula, and bottom is the average only for the West Antarctic Peninsula (WAP). It is quite surprising how well the circumpolar (top and middle) mean ASC speed time series matches the cross slope heat transport.

A Pearson's correlation (above) between the ASC and HT in the circumpolar case is quite good (r=-0.77),and it suggests that the stronger the ASC, the weaker will be the cross-slope heat transport. However, this correlation is less robust in the case without the WAP, and reversed on the WAP (notice the speed in there is positive)

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This analysis creates some points that could be useful to discuss in the next meeting:

[a] The definition used here for ASC is based on fixed points following the 1000m-isobath (@wghuneke , correct me if I'm wrong). It would be interesting to discuss alternative methods that can track the ASC, so we can distinguish if a decrease in speed is actually a decrease in ASC speed or a displacement of the front/ ASC centre. This is important to separate if the agreement we see in the second fig is valid or just an artifact of the ASC meridional displacement.

[b] The lower agreement between the ASC speed and HT when we remove the WAP from the calculations could be a method bias too. The points used to define the ASC speed are different than the ones for the HT. This could generate discrepancies when making regional correlations. So it might be a good idea to define a single isobath contour for both ASC and HT calculations.

[c] As we can see in the top b figure, we don't have HT calculated for the East Antarctic Peninsula. That has to do with the binning method used to remove the zonal heat convergence in the HT calculations (I can explain that better in the meeting). This could probably be fixed if we find a way to calculate the heat convergence along slope instead of zonal (perhaps a task for a hackathon ).

Feel free to create additional suggestions :)

[PaulSpence]

Re: a) maybe track the max speed near the isobath to follow the ASC position? or an ssh/temp/vel gradient contour? b) does the ASC need to be calc at precisely the same location as the HT? c) for HT convergence do we need to consider uncertainty associated with the choice of an arbitrary reference temperature (Holmes et al., 2019; Forget & Ferreira, 2019)?

[PaulSpence]

I note that Matthis Augre has shown that going from 1/10th to 1/20 increases the std of SSH along the slope by almost a factor of 10. It also changes the processes of SSH variability from generally steric to more eddy style dynamics.

[adele-morrison]

As discussed at the meeting, we could extend / improve this analysis by:

• Using a daily climatology from the RYF to have a longer, more robust estimation of the seasonal cycle. Need to check what daily data we have for heat transport.
• Investigate seasonal correlations over different spatial regions, including point-by-point comparisons on the isobath (requires using exactly the same isobath for ASC and heat transport) an averaged over more different sectors of the slope (e.g. using fresh/warm/dense regimes, or just selecting individual bays/troughs).

[adele-morrison]

I had a quick look at relationship between seasonal variability in cross-slope heat transport and ASC speed in a few regions of East Antarctica. These results show a climatology averaged over 3 years of the RYF on the 1000m isobath. Looks like there is no relationship!

Some thoughts:

• Just because there is no relationship in the seasonality doesn't mean there won't be a relationship on interannual or shorter-than-seasonal time scales.
• The heat transport seems to peak (large, southward heat transport) around February - April, when the sea ice is minimum. I'm wondering if the heat transport is dominated by strong southward surface flow (i.e. Ekman transport when there's no sea ice), rather than deep CDW transport. I think this would be worth exploring for this one regional case study, so we can understand better what's causing variations we see in heat transport. To do this, we could split the heat transport into depth or density layers.
• The 2nd and 3rd plots at the top of this issue show that there is a seasonal relationship when integrated around the whole of Antarctica. Here I've just looked at a small sector in East Antarctica. It would be good to decompose the upper plots into more sectors to see which regions the correlation is coming from.