I suppose this would be the third part in my series, as I would like to revisit my post from a month and a half ago, "Sensitivity of the water vapor feedback to locations of SST trends". In that post I used the GFDL HiRAM180 AMIP runs to show that the effective water vapor feedback has likely been substantially less in the AMIP warming period (1981 – 2008) due to the bulk of the SST increase coming at higher latitudes rather than in the tropics. Amid the caveats, I noted that "it is quite possible that some of the lessening of the positive water vapor feedback may be counteracted by a decrease in the strength of the negative lapse rate feedback which could mitigate the effect on overall sensitivity."
As you can see from a figure like the one above from Soden et al. (2008, Journal of Climate), warming at higher latitudes will decrease the water vapor feedback, but so too will it decrease the outgoing OLR from the temperature response. Thus, I wanted to see what degree the lesser water vapor feedback would be offset by this effect, and thus processed the atmospheric and surface temperature values from GFDL CM 2.1 and HiRAM 180 in the same way I’d done for water vapor. Code and intermediate data available here.
First, here is the OLR anomaly due to temperature changes:
As you can see, the increase in OLR is not nearly as high in the AMIP runs bounded by SST observations as it is in the fully coupled, CM 2.1 runs. And when we combine the OLR anomalies from water vapor and temperature, we again see a lesser increase in the OLR anomaly (note that the y-axis units are smaller in this case than in the above chart):
However, again keep in mind that we haven’t seen as much warming over this period as is present in the CM2.1 runs, and the effective feedback is going to have that temperature increase as the denominator. So, here are the effective feedbacks for Water Vapor, Temperature, and then these two combined (CM2.1 comes first in each category):
Pretty interesting, at least to me. The decrease in the negative temperature feedback for our actual warming pattern seems to totally offset the decrease in the positive water vapor feedback from that same pattern, such that the combined feedback are extremely close between the CM2.1 and HiRAM AMIP runs.