|Title||Drake Passage oceanic pCO(2): Evaluating CMIP5 coupled carbon-climate models using in situ observations|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||Jiang C.L, Gille ST, Sprintall J, Sweeney C|
|Journal||Journal of Climate|
|Type of Article||Article|
|Keywords||air-sea interaction; carbon cycle; circulation; climate models; co2; cycle feedback; formulation; line simulation characteristics; model; Model evaluation; nutrients; Ocean circulation; part ii; performance; Southern Ocean; southern-ocean; system; temperature|
Surface water partial pressure of CO2 (pCO(2)) variations in Drake Passage are examined using decade-long underway shipboard measurements. North of the Polar Front (PF), the observed pCO(2) shows a seasonal cycle that peaks annually in August and dissolved inorganic carbon (DIC)-forced variations are significant. Just south of the PF, pCO(2) shows a small seasonal cycle that peaks annually in February, reflecting the opposing effects of changes in SST and DIC in the surface waters. At the PF, the wintertime pCO(2) is nearly in equilibrium with the atmosphere, leading to a small sea-to-air CO2 flux.These observations are used to evaluate eight available Coupled Model Intercomparison Project, phase 5 (CMIP5), Earth system models (ESMs). Six ESMs reproduce the observed annual-mean pCO(2) values averaged over the Drake Passage region. However, the model amplitude of the pCO(2) seasonal cycle exceeds the observed amplitude of the pCO(2) seasonal cycle because of the model biases in SST and surface DIC. North of the PF, deep winter mixed layers play a larger role in pCO(2) variations in the models than they do in observations. Four ESMs show elevated wintertime pCO(2) near the PF, causing a significant sea-to-air CO2 flux. Wintertime winds in these models are generally stronger than the satellite-derived winds. This not only magnifies the sea-to-air CO2 flux but also upwells DIC-rich water to the surface and drives strong equatorward Ekman currents. These strong model currents likely advect the upwelled DIC farther equatorward, as strong stratification in the models precludes subduction below the mixed layer.