Evaluating global ocean carbon models: The importance of realistic physics

Abstract

[1] A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO 2 flux, chlorofluorocarbon and anthropogenic CO 2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25-40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO 2 . A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

Global ocean storage of anthropogenic carbon

“…The vertical penetration of C ant in the CCSM simulations is also noticeably weaker than the data-based estimates in intermediate and deep waters in the North Atlantic and intermediate waters of the South Atlantic, which likely reflects too-shallow and too-weak formation of North Atlantic Deep Water, a common problem in z-coordinate OGCMs (Doney et al, 2004). A low C ant bias is also found in the thermocline and intermediate depths in the Southern Ocean, contributing to the low column inventory relative to the data-based methods.…”

Evolution of carbon sinks in a changing climate

“…To begin with, the TCR is 1.4 K for NCAR CSM1 (12), at the low end of the range 1.1-3.1 K for climate models (17). The magnitude of the ocean carbon sink to carbon-climate feedbacks depends on the representation of ocean circulation in the physical climate model (22,23) and its response to changing climate, as well as on the sensitivity of marine ecosystem processes to the changing ocean climate (24,25). The NCAR carbon-CSM1.4 has a stronger fossil fuel CO 2 uptake (and a lower airborne CO 2 fraction) with the control climate than, for example, the Hadley model, and this sets the stage for weaker coupling between the carbon-climate systems.…”

A generalized model of pelagic biogeochemistry for the global ocean ecosystem. Part I: Theory

“…The aquatic chemistry of CO 2 and carbonates is a further extension to the original ERSEM formulation previously published in Blackford and Burkill (2002), and the theory of its chemical reactions is well understood (Zeebe and Wolf-Gladrow, 2001). Carbonate dynamics and surface exchange processes are currently being included in PELAGOS, particularly taking into account the works done in the Ocean Carbon Model Intercomparison Project (Doney et al, 2004). The biological production and consumption of CO 2 presently considered in the model can be easily derived by collecting the first 4 terms on the right hand side of eq.…”