Southern Ocean surface water pCO2 trends in CMIP5 ESMs

Abstract:

The growth rate of the Southern Ocean surface water pCO₂ (partial pressure of CO₂) significantly impacts the future atmospheric CO₂ levels. It is controlled by a complex interplay of physical, biological, and chemical processes. Examining the trends of the surface water pCO₂ and its drivers in CMIP5 ESMs can help identify the sources of the model bias of the Southern Ocean CO₂ sink rate from the atmosphere. In this study, we first examined the long-term growth rates of Southern Ocean surface pCO₂ and its four drivers (sea surface temperature, salinity, Dissolved Inorganic Carbon, and alkalinity) in all available CMIP5 ESMs during all seasons (annually), winter, and summer. We then tested the sensitivity of the trends to the temporal and spatial time spans.

Our preliminary results show that the existing CMIP5 ESMs show significantly different long-term trends from the observations, particularly in the Atlantic sector of the Southern Ocean. In the Pacific-Indian sectors (295E:0) of the Southern Ocean, only two of the existing nine ESMs reproduce the weakly decreasing sink from 1995 to 2008 found in the observational analysis, with other seven models showing equivalent or slightly smaller trends of oceanic pCO₂ than the atmospheric pCO₂. In the Atlantic sector (65W:0) of the Southern Ocean, none of the existing nine ESMs show little or no trend from 2001 to 2008 found in the observed oceanic pCO₂. In addition, our results show that the model trends in the Atlantic sector vary significantly with the selection latitudes and years, a finding also true in the observational analysis, owing to the aliasing of the internal variability in the ocean or the forced variability by the atmosphere. Possible processes that explain these biases in ESMs will also be addressed.