AbstractWe force an Atmospheric General Circulation Model (AGCM) with patterns of observed sea-surface-temperature (SST) change and those output from Atmosphere-Ocean GCM (AOGCM) climate change simulations to demonstrate a strong dependence of climate feedback on the spatial structure of surface temperature change. Cloud and lapse-rate feedbacks are found to vary the most, depending strongly on the pattern of tropical Pacific SST change. When warming is focused in the southeast tropical Pacific – a region of climatological subsidence and extensive marine low cloud – warming reduces the lower tropospheric stability (LTS) and low cloud cover, but is largely trapped under an inversion and hence has little remote effects. The net result is a relatively weak negative lapse-rate feedback and a large positive cloud feedback. In contrast, when warming is weak in the southeast tropical Pacific and enhanced in the west tropical Pacific – a strong convective region – warming is efficiently transported throughout the free troposphere. The increased atmospheric stability results in a strong negative lapse-rate feedback and increases the LTS in low cloud regions, resulting in a low cloud feedback of weak magnitude. These mechanisms help explain why climate feedback and sensitivity change on multi-decadal timescales in AOGCM abrupt4xCO2 simulations and is different to those seen in AGCM experiments forced with observed historical SST changes. From the physical understanding developed here we should expect unusually negative radiative feedbacks and low effective climate sensitivities to be diagnosed from real world variations in radiative fluxes and temperature over decades in which the eastern Pacific has lacked warming.
Journal of Climate – American Meteorological Society
Published: Oct 12, 2017
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