RAL Seminar Series
Wed 5 June 2013
Global “warming holes” – their possible mechanisms and simulation in CMIP5 models
Saint Louis University, St. Louis, Missouri
During the 20th century the south-central U.S. cooled 0.5-2.0 oC while global warming accelerated. This abnormal cooling, termed “warming hole (WH)”, is most evident during summer daytime. Examination of past observations further found more WHs: one in central China and the other in central South America. The common underlying features among these three mid-continent WHs are that they occur (1) on the eastern slope of major mountain ranges where the warming gradient exists, (2) near the low-level jet terminals where warm-moist air converges, and (3) in the intense agricultural regions where deep crop roots can extract soil moisture. Are these WHs related to internal variability of the atmosphere-ocean system such as the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO), or externally forced by land use changes and greenhouse gas (GHG) concentration increase in the atmosphere? To attribute the causal processes responsible for the WHs, we examined 175-member ensemble simulations in the 5th Coupled Model Intercomparison Project (CMIP5) experiments. The GHG forcing alone runs suggest that the atmospheric GHG concentration increase would cause more warming, rather than cooling, in the central continents, suggesting that GHG forcing is at least not directly responsible for the HWs. Only 19 out of 100 historical runs simulated negative temperature trend (cooling) over the south-central U.S. with 99 members under-predicting the cooling rate, implying the limited ability of CMIP5 models in capturing regional abnormality in climate change.