Dynamics of the stationary anomalies associated with the interannual variability of the midwinter pacific storm track - The roles of tropical heating and remote eddy forcing

摘要

The leading mode of interannual variability of the midwinter Pacific storm track is such that the storm track is weaker during the winters when the Pacific jet is strong, and stronger when the jet is weak. In this paper, experiments are conducted using a stationary wave model as well as an idealized global circulation model to explore the roles of anomalous tropical heating and eddy fluxes in forcing the observed Pacific jet anomalies. It is found that enhanced tropical heating over the region 60 degrees E to the date line, 25 degrees S to 25 degrees N, acts to force a stronger and narrower Pacific jet. On average, tropical heating may account for about one-third of the strong jet anomaly, but there is significant year-to-year variability. Moreover, tropical heating does not appear to contribute to the weak jet anomaly. Much of the Pacific jet anomalies are forced by anomalous eddy fluxes. By examining the regional contributions from the Pacific, the Atlantic, and Asia, it is found that local eddy feedback over the Pacific only acts to force part of the stationary anomaly, while much of the signal is forced by remote eddy forcings from the Atlantic and Asia. Since significant parts of the jet anomalies are forced by anomalous tropical heating and remote eddy fluxes, it is concluded that the observed Pacific jet/storm-track variability is not a pure local wave-mean flow interaction mode internal to the Pacific basin. Both stationary wave model diagnostics and idealized global circulation model experiments suggest that stronger eddy activity over the Atlantic may force a weaker Pacific jet and stronger Pacific eddies. On the other hand, changes in eddy activity over the Pacific may also act to force changes in the Atlantic storm track. There are also indications that tropical heating anomalies may force a simultaneous weakening of both storm tracks. These possibilities may be some of the factors behind the observed significant correlation between the Pacific and Atlantic storm tracks and should be further explored in more realistic GCM experiments.