Origin of Pacific Multidecadal Variability in Community Climate System Model Version 3 (CCSM3)

Observations indicate Pacific Multidecadal Variability (PMV) is a basin-wide phenomenon with robust tropical- extratropical linkage, though its genesis remains the topic of much debate. In this study, the Community Climate System Model version 3 (CCSM3) is used to investigate the PMV. In favorable comparison to the observations, the modeled North Pacific climate system undergoes coherent multidecadal atmospheric and oceanic variability with preferred 30-65-year timescale. These extratropical signals exhibit apparent connections to the tropical Indo-Pacific, which is in agreement with the recent observational findings. Two sensitivity experiments are performed to identify the origin of the CCSM3 PMV. With the atmosphere detached from the tropical ocean, North Pacific climate system retains its preference for multidecadal variations. In contrast, without midlatitude oceanic feedback to atmosphere, the experiment shows a deficit of multidecadal power in both extratropical atmosphere and surface ocean; moreover, the tropical multidecadal variability seen in the CCSM3 control run is dramatically reduced. The experiment results support a midlatitude coupled origin for the PMV, which can be described as follows: extratropical large-scale air-sea interactions give rise to multidecadal variability in the North Pacific region; these extratropical signals then imprint themselves in the tropical Indo-Pacific climate system, and establish a robust tropical-extratropical linkage. This study highlights a midlatitude origin of multidecadal tropical-extratropical linkage in the Pacific.