Journal of Climate 2013 ; e-View
Inter-model variability and mechanism attribution of central and southeastern U.S. anomalous cooling in the 20th century as simulated by CMIP5 models
Zaitao Pan,1 Xiaodong Liu,2 Sanjiv Kumar,3 Zhiqiu Gao,4 and James Kinter3
3 Center for Ocean-Land-Atmosphere Studies, 4041 Powder Mill Road, Suite 302, Calverton, MD, 20705, USA
Some parts of the U.S., especially the southeastern and central portion, cooled by up to 2°C during the 20th century, while the global mean temperature rose by 0.6 °C (0.76 °C from 1901-2006). Studies have suggested that the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) may be responsible for this cooling, termed “warming hole (WH)”, while other works reported that regional scale processes like the low-level jet and evapotranspiration contribute to the abnormality. In phase 3 of the Coupled Model Intercomparison Project (CMIP3), only a few of 53 simulations could reproduce the cooling. This study analyzes newly available simulations in CMIP5 (phase 5 of CMIP) experiments from 28 models, totaling 175 ensemble members. We found that (i) only 19 out of 100 all-forcing historical ensemble members simulated negative temperature trend (cooling) over the southeast U.S. with 99 members under-predicting the cooling rate in the region, (ii) the missing of cooling in the models is likely due to the poor performance in simulating the spatial pattern of the cooling rather than the temporal variation, as indicated by a larger temporal correlation coefficient than spatial one between the observation and simulations. (iii) the simulations with greenhouse gases (GHG) forcing only produced strong warming in the central U.S. that may have compensated the cooling, and (iv) the all-forcing historical experiment compared with the natural-forcing-only experiment showed a well-defined WH in the central U.S., suggesting that land surface processes, among others, could contributed to the cooling in the 20th century.