AR5 Inter-model variability and mechanism attribution of central and southeastern U.S. anomalous cooling in the 20th century

Many parts of the central and southeastern U.S. cooled by up to 2 °C during the 20th century, while global mean temperature rose by 0.6 oC (0.76 oC from 1901-2006). Although other regions such as central China and central South America also experienced a cooling trend, the so-called “warming hole (WH)”, the cooling is much weaker than in the U.S. WH. Studies have shown that the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) may be responsible for this cooling, while other works reported that regional scale processes like the low-level jet and evapotranspiration contribute to the abnormity. Only a few of 53 simulations by CMIP3 (phase 3 of the Coupled Model Intercomparison Project) models could reproduce the cooling. This study analyzes newly available simulations in CMIP5 (phase 5 of CMIP) experiments from 27 models, totaling 173 ensemble members. We found that (i) the observed cooling occurred largely in the southeastern U.S. in the 3rd quarter and central U.S. in the 4th quarter of the 20th century, (ii) while a large number of models have difficulty in reproducing the cooling, those with the highest resolutions tend to capture the WH-like summer cooling in the central U.S., (iii) the simulations with forcing only by greenhouse gases (GHG) 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., implying that land surface processes contributed to the cooling in the 20th century.