The asymmetric eddy–background flow interaction in the North Pacific storm track

Abstract

Using a recently developed methodology, namely the multiscale window transform (MWT) and the MWT‐based theory of canonical transfer and localized multiscale energetics analysis, we investigate in an eddy‐following way the nonlinear eddy–background flow interaction in the North Pacific storm track, based on the ERA‐40 reanalysis data from the European Centre for Medium‐Range Weather Forecasts. It is found that more than 50% of the storms occur on the northern flank of the jet stream, about 40% are around the jet centre, and very few (less than 5%) happen on the southern flank. For storms near or to the north of the jet centre, their interaction with the background flow is asymmetric in latitude. In higher latitudes, strong downscale canonical available potential energy transfer happens, especially in the mid‐troposphere, which reduces the background baroclinicity and decelerates the jet; in lower latitudes, upscale canonical kinetic energy transfer intensifies at the jet centre, accelerating the jet and enhancing the middle‐level baroclinicity. The resultant effect is that the jet strengthens but narrows, leading to an anomalous dipolar pattern in the fields of background wind and baroclinicity. For the storms on the southern side of the jet, the baroclinic canonical transfer is rather weak. On average, the local interaction begins about 3 days before a storm arrives at the site of observation, achieves its maximum as the storm arrives, and then weakens.