Investigating the role of terrestrial feedback and oceanic forcing in climate variability

Date of Completion

January 2012


Engineering, Environmental




This research focuses on studying the influences of the land and the ocean in the climate system based on the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 3-Community Land Model version 3 (CAM3 - CLM3). ^ The first part estimates the terrestrial and oceanic impacts over land in four seasons at the inter-annual time scale, through an experimental design similar to the Global Land-Atmosphere Coupling Experiment (GLACE). For the inter-annual variability of precipitation, oceanic impacts are mainly found in the tropics, soil moisture's influence is important during the rainy seasons of the semi-arid areas and the dry season of Amazon, while substantial impact of dynamic vegetation is only found in Amazon. For surface temperature, soil moisture's impact expands into the transitional seasons in semi-arid regions and dynamic vegetation's influence covers US Great Plains in addition to Amazon. ^ The potential physical processes leading to the enhanced climate variability are explored. ^ In the second part, the sensitivity of precipitation to soil moisture is quantified using a statistical method (the feedback parameter method) and is validated through numerical experiments. In tropics where the feedback parameter is high (therefore suggests strong impacts of soil moisture on precipitation), numerical experiments show little impact from soil moisture. This inconsistency is attributable to the external oceanic impacts. Regions where precipitation shows significant increase when soil moisture increases are located in semi-arid areas. ^ In the third part, in the climate system, the equilibrium state(s) of the coupled climate-vegetation system are diagnosed using a conceptual modeling approach and a physical process-based modeling approach. Precipitation demonstrates low sensitivity to vegetation cover changes in study regions. Land model CLM3 features excessively high soil evaporation and very low plant transpiration, which leads to high bare-ground evapotranspiration (ET) and low sensitivity of ET to vegetation changes. This land model deficiency leads to a high amount of precipitation in absence of vegetation cover and a low sensitivity of precipitation to vegetation changes in CAM3-CLM3, which work together to anchor the coupled biosphere-atmosphere model to a single climate equilibrium state. This statement also holds for the up-to-date version of the models using CLM4. ^