Heat Uptake and the Thermohaline Circulation in the Community Climate System Model, Version 2

Peter R. Gent, Gokhan Danabasoglu
2004 Journal of Climate  
Ocean heat uptake and the thermohaline circulation are analyzed in present-day control, 1% increasing CO 2 , and doubled CO 2 runs of the Community Climate System Model, version 2 (CCSM2). It is concluded that the observed 40-yr trend in the global heat content to 300 m, found by Levitus et al., is somewhat larger than the natural variability in the CCSM2 control run. The observed 40-yr trend in the global heat content down to a depth of 3 km is much closer to trends found in the control run
more » ... the control run and is not so clearly separated from the natural model variability. It is estimated that, in a 0.7% increasing CO 2 scenario that approximates the effect of increasing greenhouse gases between 1958 and 1998, the CCSM2 40-yr trend in the global heat content to 300 m is about the same as the observed value. This gives support for the CCSM2 climate sensitivity, which is 2.2ЊC. Both the maximum of the meridional overturning streamfunction and the vertical flow across 1-km depth between 60Њ and 65ЊN decrease monotonically during the 1% CO 2 run. However, the reductions are quite modest, being 3 and 2 Sv, respectively, when CO 2 has quadrupled. The reason for this is that the surface potential density in the northern North Atlantic decreases steadily throughout the 1% CO 2 run. In the latter part of the doubled CO 2 run, the meridional overturning streamfunction recovers in strength back toward its value in the control run, but the deep-water formation rate across 1-km depth between 60Њ and 65ЊN remains at 85% of the control run value. The maximum northward heat transport at 22ЊN is governed by the maximum of the overturning, but the transport poleward of 62ЊN appears to be independent of the deep-water formation rate.
doi:10.1175/1520-0442(2004)017<4058:huattc>2.0.co;2 fatcat:aoxx6gy7lfga3n7u4z6ublbi7q