Long-term Behavior Of Surface Air Temperature And Its Impacts In CMIP5Models

Whether large trends exist in the pre-industrial control (PICTL) runs is critically important for simulations of present climate change, as their initial values are chosen from PICTL runs. This study has examined the long-term trend of PICTL surface air temperature (SAT) in the CMIP5(Coupled Model Inter-comparison Project Phase5) models and its impacts on the historical climate trends of surface air temperature (SAT), and the internal variability on different time scales.Small linear trends (<0.06℃/century) in the globally averaged SAT (GASAT) exist in most CMIP5models. The SAT linear trends in the middle to high latitudes are relatively large (>0.1℃/100a) with relatively large RMSD (>0.05℃/100a) than trends in the tropics. The largest PICTL trends are located in the Southern Ocean between55°S and75°S, south of40°S (including the Antarctic continent) and north of40°N (including the Bering Strait, the North Atlantic around Greenland and the Labrador Sea, and the Arctic Ocean). These regions with large linear trends have large model differences, which imply that large uncertainties in the PICTL linear trends exist in these models. The changed outgoing longwave radiation (OLR) and reflected shortwave radiation (RSW) have a combined effect on the changed SAT in most models. The integration time length and slow adjustments at the middle-high latitudes in the ocean may affect the long-term behaviors.The PICTL GASAT trends comprise less than10%of the historical trends, indicating that such trends are of negligible importance in the estimates of historical global warming in most models. Spatially, the trends comprise a nontrivial fraction (>20%) of the historical warming in the Southern Ocean between50°S and65°S and in the northern Atlantic and Pacific Oceans north of40°N, with large inter-model spreads.The PICTL GASAT trends cannot affect the internal variability on inter-annual time scales in most models. However, they can account for30%of the internal variability on decadal-interdecadal time scales in half of the CMIP5models. At regional scales, the local trends can contribute nontrivially (-20%) to the internal variability on decadal-interdecadal time scales and need to be considered in most parts of the Southern Hemisphere, in the Arctic Ocean, in the ocean between40°N～60°N, and in the tropical ocean.