Controls on the Activation and Strength of a High-Latitude Convective Cloud Feedback

• Published in: Journal of the Atmospheric Sciences
• Main Authors: Abbot, Dorian, Tziperman, Eli
• Language: English
• Published: American Meteorological Society 2009
• Subjects: Arctic; Global warming; polar night; Sea ice
• Online Access: http://nrs.harvard.edu/urn-3:HUL.InstRepos:2643029; https://doi.org/10.1175/2008JAS2840.1

Previous work has shown that a convective cloud feedback can greatly increase high-latitude surface temperature upon the removal of sea ice and can keep sea ice from forming throughout polar night. This feedback activates at increased greenhouse gas concentrations. It may help to explain the warm “equable climates” of the late Cretaceous and early Paleogene (~100-~35 million years ago) and may be relevant for future climate under global warming. Here the factors that determine the critical threshold CO2 concentration at which this feedback is active and the magnitude of the warming caused by the feedback are analyzed using both a highly idealized model and NCAR’s single column model (SCAM) run under Arctic-like conditions. The critical CO2 is particularly important because it helps to establish the relevance of the feedback for past and future climates. Both models agree that, in general, increased heat flux into the high latitudes at low altitudes decreases the critical CO2. Increases in oceanic heat transport and in solar radiation absorbed during the summer should cause a sharp decrease in the critical CO2, but the effect of increases in atmospheric heat transport depends on its vertical distribution. It is furthermore found that (1) if the onset of convection produces more clouds and moisture, the critical CO2 should decrease and the maximum temperature increase caused by the convective cloud feedback should increase; (2) reducing the depth of convection reduces the critical CO2 but has little effect on the maximum temperature increase caused by the convective cloud feedback. These results should help with interpretation of the strength and onset of the convective cloud feedback as found, for example, in IPCC coupled ocean-atmosphere models with different cloud and convection schemes. Earth and Planetary Sciences Engineering and Applied Sciences