Biological tracers coupling with currents

[chysun]

Hi,
I'm running ROMS coupling with an biological model(bio_Fennel.in) and meet some boundary problems which puzzled me for quite some time now.Hope I can get some help here.

The model domain is 1S~35N,99E~140E with resolution about 7km. HYCOM reanalysis is provided as hydrodynamic initial&boundary conditions and WOA09 data as biological initial&boundary conditions.

When I plot the monthly averaged surface NO3 field in comparison to WOA09 data, I can see obvious difference between them.
1.Seen at northern boundary of WOA09, the NO3 concentration at around 120E,35N is low but there are HYCOM currents coming from north for several days. That is to say ROMS boundary is providing inflows of low NO3 concentration at northern boundary which is not good for the result(as the red arrow shows).
2.Seen at eastern boundary of WOA09, the NO3 concentration below 20N should be low since there is the North Equatorial Current with low concentration of nutrient.However ROMS somehow gets high NO3 concentration at the eastern boundary which I cant tell.As a result of the NO3 abnormality,the chlorophyll elevates.

Fennel writes "Boundary artifacts, in particular, elevated chlorophyll levels and elevated primary production, are limited to the immediate vicinity of open boundaries and do not affect the continental shelf area of the MAB",but boundary artifacts can be nonnegligible if there are strong inflows.

I double-check my input file to find out what's wrong with the eastern boundary. My boundary conditions are RadNug for both active and passive tracers, the nudging time scales are 1 year(TNUDG=365,etc) and factor between out&in is 200(OBCFAC=200) which is sensible according to this paper.My application doesn't include sponge/nudging layer.It seems that I am stuck in here but I'll keep trying.

HYCOM reanalysis has a 1/12 degree resolution and is daily averaged while WOA09 are climatological fields with its spatial resolution of 1 degree.The data inconsistency of temporal&spatial resolutions might cause boundary problem like problem 1 above.

Biological variables can be set independently while we should be more careful in coupling these variables with flows.
It is appreciated to hear from you and your comments will be helpful to other ROMS users!

Thanks in advance,
Haiyun

[kfennel]

Hi Haiyun,

I would recommend you try a sponge/nudging zone along the open boundaries and nudge your NO3 variable to the WOA climatology in the nudging zone.

Best, Katja

[chysun]

Hi Katja,

Thanks for quick reply.
Your might be probably right,a nudging zone for NO3 variable plus a sponge zone for flows can be one way that helps with my boundary problems. I'll try it.

Best, Haiyun