The sensitivity of the North Pacific storm track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the storm track activity and meridional SST gradient; i.e., the storm track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the storm track intensification due to increased SST gradient, velocity‐temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic‐scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front‐induced enhancement of storm track activity. In order to estimate the impact of the oceanic front on the maintenance of the near‐surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near‐surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross‐frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

In this study, potential impacts of the North Pacific subarctic frontal zone (SAFZ) variation, including its intensity variation and meridional shift, upon the subseasonally varying North Pacific storm track are investigated by using the 100‐year reanalysis data sets. Regression analysis indicates that the changes in the SAFZ intensity and meridional position have significant influence on the North Pacific storm track, which intensifies with the strengthening of the SAFZ and moves northwards following the northwards shift of the SAFZ. However, the storm‐track response pattern exhibits distinct differences from one calendar month to another. Specifically, the storm‐track response to the SAFZ intensity variation is strongest in February and March; while its response to the SAFZ meridional shift is most pronounced in November and December. However, the storm‐track response is relatively weak in January. Further analysis shows that the intensified (or northwards shifted) SAFZ would result in changes in the near‐surface baroclinicity and hence affects the storm track, while the weak storm‐track response in January is not the result of the anomalous near‐surface baroclinicity. The investigation of the local energetics reveals that changes in the baroclinic energy conversion (BCEC) associated with the SAFZ variation are consistent with the storm‐track anomalies, indicating that the BCEC plays a crucial role in modulating the subseasonal changes in the storm‐track response. In January, the weakened BCEC contributes to the reduced storm‐track response to the SAFZ variation.

A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific storm‐track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced storm track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of storm‐track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the storm‐track‐induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with storm‐track‐induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific storm‐track activity and midlatitude oceanic frontal intensity.

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