A semi-analytical model of the ocean's overturning circulation with carbon cycle

We present a semi-analytical model of the deep overturning circulation and the ocean-atmosphere carbon cycle and use it to investigate millennial-scale climate and carbon cycle changes. The model includes the effects of wind, eddies and mixing, and predicts the two-cell deep overturning circulation. The carbon cycle is represented by the advection and diffusion of carbon and phosphate and includes the effects of biological uptake and CO2 exchange with the atmosphere. Despite its simplicity, the model reproduces the observed features of large-scale carbon and nutrient distributions remarkably well. The model sensitivity shows that the configuration of the meridional overturning dictates the lateral gradients of carbon and nutrients and their preformed concentrations. Switching from a two- to a one-cell configuration by perturbing the surface buoyancy boundary condition in the northern high latitude decreases atmospheric CO2 by 40 ppmv even though the global export production is significantly reduced. The drawdown of atmospheric CO2 is explained by the global reorganization of deep nutrient and the resulting reduction of the preformed nutrient. The implications of the results for past and future climates are briefly discussed.