The Possible Physical Mechanism for the Synoptic-Scale Eddy over the North Pacific in Winter

Based on NOAA (National Oceanic and Atmospheric Administration) sea surface temperature (SST) dataset and NCEP (National Center for Environmental Prediction) reanalysis dataset during 1981-2013, the synoptic-scale transient geopotential height at 850 hPa is decomposed by using EOF (empirical orthogonal function). The results show that the first two leading EOF modes represent the propagation characteristics of the same developing synoptic-scale eddy. The synoptic-scale eddy forms to the west of Japan and develops while it moves along the northeast path. It reaches the strongest near the dateline and then weakens rapidly along its northeastward moving and eventually disappears. This eddy is defined as WSE (western synoptic-scale eddy) in the present study. The composite results show that the intensity of the WSE is closely linked with the SST and subpolar oceanic front anomalies in the North Pacific. When the WSE is strong, the SST is significantly colder in the northern and central North Pacific and warmer in the subtropical zone, and the subpolar oceanic front is robust. At the same time, the variation of the WSE is significantly coordinated with the atmospheric circulation anomaly. Corresponding to the strong WSE, the Aleutian-Low strengthens and shifts eastward, and the zonal westerly wind in the mid-latitude zone enhances. The above variations of oceanic and atmospheric circulations increase the atmospheric baroclinicity over the northwestern Pacific, leading to enhanced conversion from available potential energy to perturbation kinetic energy that is favorable for the WSE development.