Testing with hybrid coordinates show spikes in surface pCO2 - problem in Bothnian Sea?

[TomasTorsvik]

There is a problem with the hybrid model run as seen in pCO2 time series. This can be traced down to the Bothnian Sea, where a couple of grid points have (annual mean) pCO2 that reaches several 10 000 mu atm. One observation that could be related is that the layer thickness in this region is very large: The top layer is >30 m as opposed to almost everywhere else where it is around 2.5 m. The 2-degree hybrid run is generally relatively noisy (given the normal year forcing) and there seem to be occasional convection(?) events in the Southern Ocean (best seen in CO2- and O2-fluxes).

Diagnostics: https://ns2345k.web.sigma2.no/datalake/diagnostics/noresm/jschwing/

Short explanation of runs:

NOINYOC_T62_tn21_27: CMIP6 version of the model NOINYOC_T62_tn21_37: Current master (sediment acceleration is switched on by default for 800 years) but increased sediment remineralization of POC (disso_poc = 5.0e-6) NOINYOC_T62_tn21_38: Same as 37, but increased sediment remineralization of POC (disso_poc = 5.0e-6) AND increased background diffusivity (BDMC2 = 1.2e-5) NOINYOC_T62_tn21_39: Same as 37, but with extendend N-cycle turned ON NOINYOC_T62_tn21_40: Same as 39, but with hybrid coordinates turned ON NOINYOC_T62_tn21_41: Same as 40, but with neutral diffusion turned off

All runs (except the CMIP6 one of course) are based on current master

[JorgSchwinger]

@matsbn @TomasTorsvik @jmaerz, I have now finished a 200 year hybrid-coordinate NOINYOC run (CORE normal year forcing) on the tnx1v4 grid for comparison with the same configuration on the tnx2v1 grid.

The 1-degree resolution shows in principle the same thing in the northern Baltic, just a bit weaker: There is elevated, noisy pCO2 in the northern Baltic sea, which can be even seen as large spikes in the global time-series (blue is the tnx1v4 grid):

Again, one observation is that layer structure in this region is characterized by relatively few layers: a top-layer that is 7-8 metres thick (versus 2.5 metres everywhere else) and a second layer down to the bottom (several 10s of metres). Notably, this is different from the isopycnic model, which had 3 layers in this region.

Another observation is that the surface salinity is less in hybrid (3.3 vs 3.6, the latter being closer observations (~3.7). The output (as well as the output of all 2-degree simulations mentioned above) is here:

/projects/NS2980K/schwinger/NOINYOC_T62_tn14_40