Tuesday, 13 January 2009
Additive effect of two solar forcing mechanisms and influences on tropical Pacific climate
Hall 5 (Phoenix Convention Center)
Three climate model experiments are analyzed to study the relative contributions of two solar forcing mechanisms. One involves ozone in the stratosphere absorbing more UV radiation in peak solar years, ultimately resulting in enhanced vertical motion and greater precipitation in the tropical Pacific. A second mechanism involves coupled air-sea interaction at the surface in the tropical Pacific where greater solar forcing during peaks of the 11 year solar cycle produces increased evaporation in the relatively cloud-free Pacific subtropics, and additional moisture that is carried by the trade winds to strengthen precipitation in the ITCZ and SPCZ. Stronger trades in the intensified Hadley and Walker Cells produce greater upwelling, and a La Nina-like SST response in the eastern equatorial Pacific. A version of CCSM3 (coupled to a dynamical ocean but with no resolved stratosphere or ozone chemistry) captures the coupled air-sea mechanism and produces a weak La Nina-like response during peaks in solar forcing as seen in the observations. A version of WACCM (including a resolved stratosphere and ozone chemistry) run with climatological SSTs reproduces the stratospheric ozone mechanism with stronger precipitation in the western Pacific, ITCZ and SPCZ. Another version of WACCM coupled to the dynamical ocean of CCSM3 simulates both the stratospheric ozone and coupled air-sea mechanisms and shows a stronger response by about a factor of two compared to the CCSM3 with just the coupled air-sea mechanism. These results indicate that the two solar mechanisms act in the same sense and are additive to first order to produce a La Nina-like SST response in the tropical Pacific during peaks in solar forcing.