Eddy‐shedding is a highly nonlinear process that presents a major challenge in geophysical fluid dynamics. Using the newly developed localized multiscale energy and vorticity analysis (MS‐EVA), this study investigates an observed typical warm eddy‐shedding event as the Kuroshio passes the Luzon Strait, in order to gain insight into the underlying internal dynamics. Through multiscale window transform (MWT), it is found that the loop‐form Kuroshio intrusion into the South China Sea (SCS) is not a transient feature, but a quasi‐equilibrium state of the system. A mesoscale reconstruction reveals that the eddy does not have its origin at the intrusion path, but comes from the Northwest Pacific. It propagates westward, preceded by a cyclonic (cold) eddy, through the Kuroshio into the SCS. As the eddy pair runs across the main current, the cold one weakens and the warm one intensifies through a mixed instability. In its development, another cold eddy is generated to its southeast, which also experiences a mixed instability. It develops rapidly and cuts the warm eddy off the stream. Both the warm and cold eddies then propagate westward in the form of a Rossby wave (first baroclinic mode). As the eddies approach the Dongsha Islands, they experience another baroclinic instability, accompanied by a sudden accumulation of eddy available potential energy. This part of potential energy is converted to eddy kinetic energy through buoyancy conversion, and is afterward transferred back to the large‐scale field through inverse cascading, greatly reducing the intensity of the eddy and eventually leading to its demise.

Abstract. State-of-the-art climate models suffer from significant sea surface temperature (SST) biases in the tropical Indian Ocean (TIO), greatly damaging the climate prediction and projection. In this study, we investigate the multidecadal atmospheric bias teleconnections caused by the TIO SST bias and their impacts on the simulated atmospheric variability. A set of century long simulations forced with idealized SST perturbations, resembling various persistent TIO SST biases in coupled climate models, are conducted with an intermediate complexity atmospheric model. Bias analysis is performed using the normal-mode function decomposition which can differentiate between balanced and unbalanced flow regimes across spatial scales. The results show that the atmospheric circulation biases caused by the TIO SST bias have the Gill-Matsuno-type pattern in the tropics and Rossby wave-train distribution in the extratropics, similar to the steady state response to tropical heating. The teleconnection between the tropical and extratropical biases is set up by the Rossby wave-train emanating from the subtropics. Over 90 % of the bias variance is contained in planetary scales (zonal wavenumber k ≤ 5). These biases have great impacts on the simulated energy and interannual variance (IAV). The zonal-mean-flow energy and the extratropical (balanced) wave-flow energy responses are closely related to bias phase (i.e., the covariance between the bias and reference state). In contrast, the tropical (both unbalanced and balanced) wave-flow energy responses are primarily associated with bias amplitude. The response of the IAV is contingent upon the sign of the SST bias. A positive SST bias reduces the IAV, whereas a negative SST bias increases it, regardless of dynamical regimes. Geographically, strong IAV responses are observed in the tropical Indo-west Pacific region, Australia, south and northeast Asia, the Pacific-North America region and Europe, where the background IAVs are strong.

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Previous studies show that in boreal winters when the Pacific jet is extremely strong, the Pacific storm track is, however, unexpectedly weak. Using a recently developed technique, namely, the multiscale window transform (MWT), and the MWT-based localized multiscale energetics analysis, we investigate in this study the underlying mechanism of this counterintuitive phenomenon, based on ERA-40 data. It is found that most of the synoptic storms are generated at latitudes far north of the jet core, which lowers the relevance of the jet strength to the storm-track intensity, and the inverse relationship between the Pacific jet strength and storm-track intensity is mainly attributed to the internal dynamics. In the strong jet state, on one hand, the jet is narrow, and thus the jet winds at high latitudes are weak, resulting in weak baroclinic instabilities and hence reduced eddy growth rate; on the other hand, although baroclinic instabilities are strong at the jet core, inverse kinetic energy (KE) cascades are even stronger (by 43%). The resultant effect is that more eddy energy is transferred back to the background flow, leaving an overall weak storm track in a strong Pacific jet. In addition, diabatic processes are found to account for the inverse relationship: it is greatly weakened (by 25%) in the strong-core jet state. Apart from these, we also find that the role that barotropic canonical transfer plays in the inverse relationship is opposite to that in the formation of the midwinter minimum (MWM), another counterintuitive phenomenon in the Pacific storm track.

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. KEYWORDS canonical energy transfer, eddy-mean flow interaction, extratropical cyclone, feature tracking, multiscale energetics