Attribution of Climate Variability During the Last 100 Years I
Presiding: M P Hoerling, NOAA Climate Diagnostics Center; A Kumar, NOAA Climate Prediction Center
A22A-01 10:30h
Simulation of 20th Century Climate Change in the GFDL Coupled Models
Ensembles of simulations for the 20th century have recently been conducted at GFDL as part of the 2007 IPCC process. The simulations were conducted with two new climate models recently developed at GFDL. The new models (CM2.0 and CM2.1) consist of recently developed atmospheric, oceanic, sea ice, and land component models, and do not use flux adjustments. The models simulate current climate in a realistic manner, including a substantially improved simulation of ENSO relative to previous models. Several aspects of 20th century climate change are examined. In the first we assess to what degree trends in surface air temperature simulated in the model are consistent with observations, and to what extent they are attributable to anthropogenic forcings. These assessments are done on both a global and regional basis. In addition, we examine simulations of the Sahelian drought in the latter half of the 20th century. Drought over the Sahel emerges in these coupled model simulations as a response to anthropogenic forcing, and is inconsistent with internal variability as estimated from the coupled model. The simulated drought has approximately half the amplitude of the observed drought. Diagnostics relevant to the simulated Sahelian drought are presented, including the relationship between the drought and global SST patterns.
A22A-02 11:00h
The great 20th Century drying of Africa
In concert with widespread African surface warming since 1950, a drying trend has been observed, most infamously over the Sahel during July-September, but also following the march of monsoon rains into southern Africa during December-April. The nature and causes for these 1950-1999 downward trends in both northern and southern African summer monsoon rainfall are diagnosed. They are found to be attributable to the atmosphere's response to observed global sea surface temperature variations of the last half-century. Every member of 80 atmospheric climate simulations, forced by the observed ocean history since 1950, yields an African drying trend. Analysis of coupled ocean-atmosphere climate simulations suggests that these drying trends are detectable and distinguishable from natural coupled ocean-atmospheric variations. Yet, neither northern nor southern African drying trends during 1950-1999 are found to occur in greenhouse gas forced coupled ocean-atmosphere simulations. An explanation for the observed 20th Century drying trends is offered, and the role of regional oceanic changes including their relation to greenhouse gas influences, is assessed.
http://www.cgd.ucar.edu/~jhurrell/press.html
A22A-03 11:30h
Attribution studies for understanding late 20th climate trends in the Sahel: Regional climate model simulation using NCEP Reanalysis (1960-2002)
Regional climate model simulations have been conducted in order to simulated the wet period during 1960s and shift toward drier conditions after the early 1970s. The regional model is driven by the National Center for Environmental Prediction (NCEP) reanalysis meteorological field as the lateral boundaries and observed sea surface temperatures from the Hadley Centre are used during this period (1960-2002). The regional climate model simulates West African Climate (1961-1990 mean) fairly well, but slightly underestimates precipitation rates during the summer season. There is also a cold bias over the guinea region and the Sahara Desert is too warm. The African Easterly Jet and Tropical Easterly Jet are captured in the regional climate model simulation. The regional model also captures the trend towards drier conditions beginning in the early 1970s. Given that the model is forced with unaltered vegetation and fixed anthropogenic forcing the results strongly suggests that the trend towards drier conditions was associated with large-scale forcing outside of the domain. In this presentation we also suggests what these results imply for 21st century climate change in West Africa.
A22A-04 11:45h
Variability of the Indian Ocean SST and Its Impacts on Asian-Australian Monsoon Climate
In this study, we investigate the relationship between the Indian Ocean (IO) sea surface temperature (SST) and the Asian and Australian monsoons on seasonal to interannaul timescales. We focus on the dominant features of IO SST, the impacts of IO SST on different monsoon components, and the relative importance of the northern and southern IO for the monsoons. The most dominant mode of IO SST is characterized by uniform warming or cooling, with maximum variance in the southern hemisphere. This warming or cooling exerts a larger impact on the monsoon climate than does the tropical IO dipole mode. The persistence of IO SST is large from the boreal fall to the next spring and small from summer to fall, a feature associated with a seasonal alternation of the dominance of ENSO-monsoon and IO-monsoon relationships. The IO SST is strongly linked to the Asian and Australian monsoons and has a notable predictive potential. The IO SST leads to opposite changes in the South Asian monsoon and the Southeast Asian monsoon, reinforcing the usually out-of-phase relationship between the two monsoon components. While an increase in IO SST strengthens the South Asian monsoon, a feature usually applied in the context of tropospheric biennial oscillation, it weakens the Southeast Asian monsoon. In spring, the southern IO SST is related to the Asian summer monsoon more closely than is the northern IO SST. In fall, the northern IO SST influences the following Australian monsoon more strongly than does the southern IO SST.