Studies of the equatorial box with the UMD ocean model Jim Christian Fisheries and Oceans Canada Canadian Centre for Climate Modelling and Analysis University of Victoria

 a brief note on the sea-truth experiment  correlation of box mean concentrations with full domain EOF’s  carbon and nitrogen budgets

Since 2005 I have been testing diferent variants of the model against underway pCO2 data provided by Dick Feely (NOAA) and Masao Ishii (JMA)

Climatological forcing High friction Low friction High friction Low friction Original biology Modified biology W92 gas exchange Climatological forcing Friction = horizontal mixing or stirring (eddy diffusion) Orig biology = 2002 paper (Peter Frank’s grazing) Mod biology = 2005 has no multiplicative grazing

Most experiments show a fairly strong correlation over the full domain because of the mean spatial pattern, so I defined a ‘core ENSO’ region where the correlation of a climatology run with observed pCO2 anomalies would be ~0 (panel 8) Climatological winds or other physical properties does not do a good job

Time-mean section along equator is fairly good except in experiments 1, 4, and 5

 best results with original (Christian et al  best results with original (Christian et al. 2002) biology + low friction (Expt 2), or with the modified (Wang et al. 2005) biology + high friction (Expt 3)  we can suppress variability either within the biological model itself or in the mixing parameters  applying both of these smooths out a lot of the variability (Expts 1, 5), but applying neither generates too much variability, and extreme values that are not observed (Expt 4) Therefore I have selected Expts 2 and 3 for analyses of the equatorial box.

EOF analysis Q: what is the correlation between the mean concentration of e.g. DIC within the box, and the first principal component for the whole model domain?

Expt 2

Expt 3

EOF analysis A: if we can adequately sample what is happening in the box we have a pretty good index of what is happening across the whole domain First EOF accounts for around 30-40% of total variance for carbon system variables (DIC, nDIC, pCO2) and is strongly correlated with ENSO in all cases

C and N budgets (for the surface mixed layer)

For carbon, the primary mass balance is between upwelling and meridional advection, i.e., most of the carbon upwelled within the box is advected out of the box to the north and south. Zonal advection can be convergent or divergent, and is basically a mirror image of the net gain or loss from upwelling after correcting for the poleward advection.

The residual ‘excess’ upwelling of C into the box is ~1 The residual ‘excess’ upwelling of C into the box is ~1.4 Tmol/month and is lost to particle export (~80%) and CO2 outgassing (~20%)

For nitrogen, the primary mass balance is between upwelling and sedimentation, and meridional advection is negligible.

Conclusions  the box is a very good proxy for the first (ENSO) mode of variability over the full domain if the mean within the box is adequately sampled  most carbon upwelled within the box is advected poleward and outgasses in the subtropics, whereas nitrogen is almost entirely removed by sedimentation within the box  export of C by sedimentation within the box is ~4X larger than outgassing