Reanalysis of hotspots of lower cell threshold crossing near the Drake Passage

[Midway-X]

This Issue is based on the dataset of SOBD research. Using the lower cell threshold from both sigma_0 and sigma_2, the Drake Passage is a significant region where particles gain density and become denser and enter the lower cell category.

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Overview of locations where particle crossed sigma_0 = 27.80 kg/m^3 (i.e., lower cell density threshold). In this comment, I calculated all events of particles crossed and entered LC threshold in each 0.1*0.1 degree grid box.

1. locations from surface released particles

If we consider the whole Southern Ocean, we have 17k particles of total 82k once crossed lower cell density threshold:

In the Drake Passage area (30-60W & 50-70S), we have 5.7k particles once crossed lower cell density threshold:

2. locations from 500m released particles

   Similarly, if we consider the whole Southern Ocean, we have 27k particles of total 76k once crossed lower cell density threshold:

In the Drake Passage area (30-60W & 50-70S), we have 6.4k particles once crossed lower cell density threshold:

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Testing use sigma_2 = 37.0625 27.80 kg/m^3 (i.e., original Lower cell threshold from MOC stream function). For Drake Passage area, choosing sigma_2 basically cannot decrease the number of lower cell threshold crossing events. Sigma_2 just switches these events from one area to another area. Here, I output the density map of the different between sigma_0 and sigma_2 (i.e., density sigma_0 - density sigma_2 for each 0.1 deg grid box):

1. Surface release in Drake Passage:

2. Surface release in Drake Passage and adjacent areas:

3. 500m release in Drake Passage:

4.500m release in Drake Passage and adjacent areas:

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This Video shows 40 sample trajectories from surface released particles in map and sigma_0, sigma_2, depth, temperature and salinity profiles:

https://user-images.githubusercontent.com/74391012/162143018-65aa0324-3c48-45c9-84f0-ae6f1447f9a9.mp4

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This Video shows 40 sample trajectories from 500m release particles in map and sigma_0, sigma_2, depth, temperature and salinity profiles. These trajectories are from 4 sub areas near the Drake Passage:

https://user-images.githubusercontent.com/74391012/162123564-8bd9b002-5c67-473f-b860-9946eb5fb39b.mp4

[PaulSpence]

This is great info Yinghuan! What is your assessment of the Drake Passage hotspot? My very quick assessment leads me to :

i) DP is a region of great mixing. Some particles that first travel along the shelf, or downstream in the open ocean ACC get mixed into the lower cell in DP. ii) Using a lower cell rho threshold from a zonally averaged MOC is a little problematic in DP. iii) Some of the parcels that 'first' cross the lower cell threshold in DP, have spent quite a bit travelling just above the threshold ... but still at fairly steady depth.

[Midway-X]

I think you explain these trajectories better than me :) From above plots, my understanding is:

1. The lower cell threshold from MOC may be not appropriate in the DP. We may need a slightly light one (just what I saw from these trajectories and maybe not correct).
2. A lot of the above particles come from the ASC in the Weddell sea. I have not counted the number, but it seems like Weddell sea particles are more than West Antarctica particles.
3. The mixing in the DP does create some of the bottom water because it gives water mass density.