ATMS 521: Fu on "Responses of Terrestrial-mean Aridity to Global Warming"

Start Date/Time: Thursday, January 23, 2014, 3:30 PM
Ending Date/Time: Thursday, January 23, 2014, 4:20 PM
Location: JHN 111

Speaker: Professor Qiang Fu, Department of Atmospheric Sciences, UW
Title: Responses of Terrestrial-mean Aridity to Global Warming

Abstract: The dryness of terrestrial climate can be measured by the ratio of annual precipitation (P) to potential evapotranspiration (PET), the latter represents the evaporative demand of the atmosphere which depends on the surface air temperature, relative humidity, wind speed, and available energy. This study examines how the terrestrial-mean aridity responds to global warming in terms of P/PET using the CMIP5 transient CO₂ increase to 2xCO₂ simulations. We show that the (percentage) increase (rate) in P averaged over land is ~1.6%/C (normalized by the ocean-mean surface air temperature increase) while the increase in PET is 5.3%/C, leading to a decrease in P/PET (i.e., a drier terrestrial climate) by ~3.4% C. Noting similar increases for P over land and evaporation (E) over ocean, we propose a framework to examine the change in P/PET by comparing the change in PET over land and E over ocean, both can be expressed with the Penman-Monteith formula. We show that a drier terrestrial climate is caused by (i) enhanced land warming relative to the ocean, (ii) a decrease in RH over land but an increase over ocean, and (iii) part of increase in net downward radiation going into the deep ocean. The relative contributions to the change in terrestrial-mean aridity from these three factors are 54%, 38%, and 8%, respectively. A slight slowdown of the surface wind over both land and ocean has little impact on the terrestrial-mean aridity.