Ocean Optics can Modulate the Cooling of the Southern Ocean under Doubled CO2 by Affecting Mixed Layer Dynamics

The pigment and particulate composition of the ocean’s surface water affects the dynamics of the mixed layer by changing the heating of these surface layers. We recently derived a new parameterization for the attenuation of light in the blue-green wavelength range that represents the absorption by chlorophyll and colored dissolved materials (CDM) independently. We examine the sensitivity of the response to doubled CO2 to this parameterization in an Earth System Model. Runs made with the new parameterization cool the surface air temperature in the Southern Ocean (SO) under doubled CO2 and expand the area over which sea surface temperatures cool, matching a surprising trend seen in observations but rarely captured by contemporary models. The simulations with the new parameterization are warmer in the control, but end up at roughly the same temperature in the SO under doubled CO2 thus resulting in a greater cooling . The differences in the control are due to the new parameterization having less sensitivity to chlorophyll, producing deeper penetration of light below the mixed layer within the low-CDM SO. This results in more heat being supplied from below to the surface in the fall, with deeper mixed layers and less sea ice formation in poorly stratified regions near the sea ice edge. Changes in winds also result in a complex pattern of changes in currents, resulting in greater advective supply of heat to these regions. Under doubled CO2 the convective regions near the sea ice edge become stratified in both parameterizations and the differences largely vanish.