Improved routines to model the ocean carbonate system: mocsy 2.0 (original) (raw)

Model description paper

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09 Mar 2015

Model description paper | | 09 Mar 2015

Abstract. Modelers compute ocean carbonate chemistry often based on code from the Ocean Carbon Cycle Model Intercomparison Project (OCMIP), last revised in 2005. Here we offer improved publicly available Fortran 95 routines to model the ocean carbonate system (mocsy 2.0). Both codes take as input dissolved inorganic carbon _C_T and total alkalinity _A_T, tracers that are conservative with respect to mixing and changes in temperature and salinity. Both use the same thermodynamic equilibria to compute surface-ocean _p_CO2 and simulate air–sea CO2 fluxes, but mocsy 2.0 uses a faster and safer algorithm (SolveSAPHE) to solve the alkalinity-pH equation, applicable even under extreme conditions. The OCMIP code computes only surface _p_CO2, while mocsy computes all other carbonate system variables throughout the water column. It also avoids three common model approximations: that density is constant, that modeled potential temperature is equal to in situ temperature, and that depth is equal to pressure. Errors from these approximations grow with depth, e.g., reaching 3% or more for _p_CO2, H+, and ΩA at 5000 m. The mocsy package uses the equilibrium constants recommended for best practices. It also offers two new options: (1) a recently reassessed total boron concentration BT that is 4% larger and (2) new _K_1 and _K_2 formulations designed to include low-salinity waters. Although these options enhance surface _p_CO2 by up to 7 μatm, individually, they should be avoided until (1) best-practice equations for _K_1 and _K_2 are reevaluated with the new BT and (2) formulations of _K_1 and _K_2 for low salinities are adjusted to be consistent among pH scales. The common modeling practice of neglecting alkalinity contributions from inorganic P and Si leads to substantial biases that could easily be avoided. With standard options for best practices, mocsy agrees with results from the CO2SYS package within 0.005% for the three inorganic carbon species (concentrations differ by less than 0.01 μmol kg−1). Yet by default, mocsy's deep-water _f_CO2 and _p_CO2 are many times larger than those from older packages, because they include pressure corrections for _K_0 and the fugacity coefficient.

Received: 24 Mar 2014

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Discussion started: 05 May 2014

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Revised: 18 Dec 2014

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Accepted: 19 Dec 2014

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Published: 09 Mar 2015