willaguiar
commented
1 year ago Shelf WMT by sector
We can see that most of the shelf SWMT changes happens in the Weddell Sea, where the WMT curve moves to denser values.
Most of this change in Weddell WMT is driven by salinity changes:
Looking to shelf properties during the winter , WS=Weddell Sea
We can see in the Age at the bottom anomalies [a] that CM2atm has stronger DSW export to the abyss in the WS than in the ctrl case (21mbath) ([a] is the only plot here that is not the winter average, but the values for the last month of the simulation). The WS shelf also has higher salinities in CM2atm, both in the bottom [b] and surface [c] .
In [d] we have the freshwater fluxes from sea ice growth/melt, and the yellow line is the contour of 0.2 salinity anomaly from [c]. We can see that, when compared to the ctrl, more water exits the ocean in the inner WS in CM2atm. These negative anomalies in freshwater flux match the contour of high surface salinity anomalies suggesting that the stronger freshwater outflow is causing the WS inner shelf to increase salinity. This increase salinity/increased freshwater outflow region matches the region with increased ice growth seen in [e].
ATM and sea ice changes during winter
Here are a few options of what could be causing this change in WS.
OPTION 1: When looking at Tau_x on the control case [a] we can see there is some convergence between eastward stress in the Antarctic Peninsula (red box) and Westward stress in inner/west WS (blue box). Looking at the anomalies in tau_x in [b] we see that this convergence is decreased. This decreased convergence could make sea ice thinner in the WS ( and that can be seen in ice volume anomalies in [e]). Thinner winter sea ice due to weaker convergence would allow more ocean-atm heat exchange during winter, fostering local sea ice formation (i.e., brine rejection, freshwater outflow) and therefore increasing salinities on the shelf.
OPTION 2: Comparing winter Tau_y between the Ctrl [c] and CM2atm simulations [d] we can see there is a small region on the eastern WS where TAU_y is reversed from Southward to northward. Perhaps this new Northward transport is continuously displacing sea ice to the North on the eastern WS, increasing polynya size and DSW formation locally.
Let me know if you see something I'm missing here.
important info: water flux from sea ice plot used the melt variable. I'm not 100% sure if that is the correct variable to use (can someone confirm?)
Full images, control fields and plots with 5mdz can be found here
adele-morrison
We have noticed that using the CM2atm forcing on OM2 increases the DSW formation. This issue is to discuss possible reasons why.
Overview of the issue below
Comparing the shelf SWMT between the control 21mbath , and CM2atm , we can see that CM2atm has increased transformation of waters across the 28 sigma0 value, i.e., we have more waters becoming denser than 28 kg/m3 when forcing OM2 with CM2atm.
This DSW signal gets even stronger when checking the volume transport across the 1km isobath. See the ~2 SV increase volume transport of waters with sigma0=>27.84 for example. For reference, waters below 4000m in OM2 in the Southern Ocean (i.e., AABW) seem to have sigma0=>27.84.