The Polar Amplification of Global Warming in the Absence of the Ice Albedo Feedback Aaron Donohoe and David Battisti University of Washington

Annual Mean Temperature change in AGCM (CAM4) simulation of Last Glacial Maximum (LGM) and four times CO 2 (Quad) as compared to pre-industrial (PI) Latitude Delta Temperature (K) Delta Temperature (K) (Data : Camille Li)

Meridional anomaly of annual mean temperature change Latitude Surface Polar Amplification Indication of the Opposite Aloft K 5 0 K -5

Seasonal Cycle of Temperature Smaller amplitude surface seasonal cycle in warmer climate Opposite true aloft Latitude K K Seasonal Amplitude of Temperature – PI run Change in Seasonal Amplitude of Temp. K

How does the system respond to CO 2 only? GFDL 2.1 AGCM coupled to a 50 meter slab ocean NO LAND - aquaplanet NO ICE Ensemble of runs with 180 ppm (LGM), 350 ppm (P ), and 1300 ppm (QUAD) CO 2 (Thanks to Dargan Frierson)

Aquaplanet Temperature Changes Annual Mean Annual Mean, Meridional Anomaly QUAD - LGM K K K K Seasonal Amplitude

QUAD – LGM Change in global annual mean energy budget SURFACE ATMOSPHERE LW A ↓ LW S ↑ LW A ↑ -0.2 SW ABS LH SENS ALL TERMS ARE IN W/m 2 SW REF +0.2 SW TRANS

Water Vapor as a SW Absorber SW Heating by Water Vapor (Chou and Lee 1996) (Figure: Robert Rhode Global Warming Art Project) QUAD – LGM SW heating

SW absorption and the equator to pole gradient [S](1-α) β = +3.5% Absorption +β[S](1-α) -β[S](1-α) +8 W/m 2 -8 W/m 2 Global Annual Mean Meridional Anomaly of Annual Mean Tropics Extratropics -ΔS(1-α) +βΔS(1-α) -βΔS(1-α) +ΔS(1-α) +βΔS(1-α) -βΔS(1-α) +2.5 W/m W/m W/m 2 ΔS = Meridional Anomaly of Solar Insolation +70 W/m W/m 2

SW Absorption and the Seasonal Cycle SUMMER S E ’ = Seasonal Extratropical Insolation Anomaly Extratropics + S E ’(1-α)- S E ’(1-α) + βS E ’(1-α) - βS E ’(1-α) + βS E ’(1-α) - βS E ’(1-α) +140 W/m W/m W/m W/m W/m W/m 2 Enhanced Seasonal Cycle Reduced Seasonal Cycle WINTER

QUAD – LGM change in meridional anomaly of SW clearsky heating

(Quad – LGM) Change in seasonal amplitude of clearsky SW heating

Conclusions Warmer planets exhibit SURFACE polar amplification of temperature change and a reduction of the seasonal cycle independent of an ice albdeo feedback Enhanced SW absorption by water vapor in a warmer world explains both the polar amplification and the reduction of the seasonal cycle of temperature How much might this mechanism contribute to the real world? Percent Change in Seasonal Cycle – Aquaplanet Simulations %

Clearsky SW Heating in Aquaplanet runs Annual Mean Annual Mean, Meridional Anomaly k/day

Water Vapor as a SW Absorber SW Heating by Water Vapor (Chou and Lee 1996) (Figure: Robert Rhode Global Warming Art Project) QUAD – LGM SW heating

Annual Mean Heat Transport in Aquaplanet Simulations