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

摘要

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 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 300m is about the same as the observed value. This gives support for the CCSM2 climate sensitivity, which is 2.2deg C. Both the maximum of the meridional overturning streamfunction and the vertical flow across 1-km depth between 60deg and 65deg N decreasemonotonically 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 60 deg and 65 deg N remains at 85% of the control run value. The maximum northward heat transport at 22 deg N is governed by the maximum of the overturning, but the transport poleward of 62 deg N appears to be independent of the deep-water formation rate.