Hi, I’m using the CMEMS Mercator forecast/analysis product and was wondering if this is the right place to ask a question about the calculation of barotropic volume fluxes with the data that is available from there. If I understand the paper by Lellouche et al. ([3]), I think they use NEMO 3.1.

Currently I’m tyring to calculate the volume flux into the northern Bay of Bengal, northward of the zonal transect at 17 degrees north. I’m currently assuming that the mass/volume balance is:

dV/dt = \int_{transect} v dA + P - E + R

where

- dV/dt is the change of volume due to free surface movements (variable ‘zos’ in the CMEMS Mercator notation)
- v is the meridional velocity and the intergal is over the transect at 17 north.
- P and E are precipitation and evaporation, respectively (see Eq. 6.3 in the Nemo manual [1])
- R is the river runoff (which may be significant in the region, the runoff of the GBM delta alone has a climatological peak on the order of 0.1 Sv in late summer)

For velocity, I’m using daily averaged 3d meridional velocity (‘vo’ in the CMEMS Mercator notation). For ‘zos’ (free surface) I’m using daily averages too. The grid spacings are given by the e1t, e2t and e3t variables and the masks are available too from the static files ‘GLO-MFC_001_024_coordinates.nc’ and ‘GLO-MFC_001_024_mask_bathy.nc’.

For E and P, I’m using ERA5 data. For R, I’m using Dai and Trenberth 2002 data.

The problem is that the the volume flux through the boundary (first term on the RHS above, \int_{transect} v dA seems much too large. I’m getting volume fluxes on the order of 1 Sv throuhout the first three months of 2020. If i have done the calculation correclty, this would be equivalent to an absurd 5 m sea level rise in the domain within 3 months. Am I being naive about how to calculate this? Is it actually possible to do this type of calculation with daily averaged data? I’m aware that wave induced mean flow can in theory be a problem, but I guess this would be orders of magnitude smaller?

I have tried to stretch the vertical coordinate according to Eq. 2.24 in [2], although I have no clue which vertical coordinate the Mercator analysis/forecast product actually uses (the information is not in the user manual, unless I’ve missed it).

[1] https://www.nemo-ocean.eu/doc/node38.html

[2] Curvilinear generalised vertical coordinate System

[3] Lellouche, J. M., Greiner, E., Le Galloudec, O., Garric, G., Regnier, C., Drevillon, M., … & Le Traon, P. Y. (2018). Recent updates to the Copernicus Marine Service global ocean monitoring and forecasting real-time 1∕ 12 high-resolution system. Ocean Science, 14(5), 1093-1126.