Abstract
Since the slowing of the trend of increasing surface air temperature (SAT) in the late 1990 s, intense interest and debate have arisen concerning the contribution of human activities to the warming observed in previous decades. Although several explanations have been proposed for the warming-trend slowdown (WTS), none has been generally accepted. We investigate the WTS using a recently developed methodology that can successfully identify and separate the dynamically induced and radiatively forced SAT changes from raw SAT data. The dynamically induced SAT changes exhibited an obvious cooling effect relative to the warming effect of the adjusted SAT in the hiatus process. A correlation analysis suggests that the changes are dominated primarily by the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Our results confirm that dynamically induced variability caused the WTS. The radiatively forced SAT changes are determined mainly by anthropogenic forcing, indicating the warming influence of greenhouse gases (GHGs), which reached levels of 400 ppm during the hiatus period. Therefore, the global SAT will not remain permanently neutral. The increased radiatively forced SAT will be amplified by increased dynamically induced SAT when the natural mode returns to a warming phase in the next period.
Highlights
sea level pressure (SLP) over the North Pacific, and the AMO12–15 is a measure of SST in the North Atlantic relative to the global mean
The raw surface air temperature (SAT) is divided into two parts: one part is associated with dynamically induced forcing which is called dynamically induced SAT, the other part is associated with the build-up of greenhouse gases (GHGs) and other various radiative forcings called adjusted SAT
We conclude that for approximately 10 years after 2000, the decreasing SAT in the cold season led to the annual mean SAT slowdown that occurred over the Northern Hemisphere
Summary
It exhibits a positive pattern over the mid- to high-latitude regions of Eurasia, northwestern North America and nearby the coast of Pacific Ocean. Because the interdecadal NAO time series is negative during the recent hiatus period, the positive correlation coefficient represents the cooling effect on the dynamically induced SAT changes over the mid- to high-latitude areas of Eurasia, northwestern North America and the nearby coast of the Pacific Ocean. Besides NAO and PDO, AMO is an efficiency index in measuring the dynamical activities[27] It illustrates that the AMO has been relatively constant since 2000 (Fig. 7a) and exhibits a negative relationship over the mid- to high-latitude regions of Eurasia, western North America and the nearby coast of the Pacific Ocean (Fig. 7b). The AMO, which represents a comprehensive dynamical factor, does not exhibit any pronounced local maxima
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