Relative contribution of the anthropogenic forcing and natural variability to the interdecadal shift of climate during the late 1970s and 1990s

Global warming accelerated after the late1970 s and slowed down after the late 1990 s, accompanying the significant interdecadal changes in the regional climate.We hypothesized that the interdecadal changes linearly consisted of two independent components, anthropogenic forcing and natural decadal variability, which can be represented simply by the radiative forcing effect of carbon dioxide (RFCO_2) and the Pacific Decadal Oscillation(PDO), respectively. The combined effect of the RFCO_2 and PDO could explain the majority of the surface temperature changes during the late 1970 s and 1990 s, but the magnitudes of the relative contribution of the RFCO_2 and the PDO are inconsistent in different regions. For both the surface temperature and geopotential height, the RFCO_2 could induce significantly positive anomalies over almost the entire globe for these two shifts, exhibiting a larger magnitude in the mid–high latitudes and in the late 1990 s shift.The PDO could induce opposite anomalies for the two interdecadal shifts due to its phase transitions(negativepositive–negative). Furthermore, for the shift in the late 1970s, both the RFCO2(53.7 %–66.7 %) and the PDO(33.3 %–46.3 %) were important in regulating the tropical geopotential height, whereas the RFCO_2 dominated the changes in the mid-latitudes. For the western Pacific subtropical high, the RFCO2(PDO) could explain 52.3 %–62.1 %(37.9 %–47.7 %) of the change. The negative effect of the PDO counteracted most of the RFCO_2 effects for the late 1990 s shift.