The Relation Between the Seasonal Changes in Jet Characteristics and the Pacific Midwinter Minimum in Eddy Activity

This study relates the occurrence of the midwinter minimum in eddy activity over the North Pacific with the seasonality in jet characteristics. During winter, the Pacific jet core is typically around latitude 32°N and has features of a merged subtropical eddy-driven jet. On the other hand, during transition seasons, the jet is at higher latitudes (≈40°N) and resembles more an eddy-driven jet. We find that these differences in jet characteristics play a role in the occurrence of the midwinter minimum. It is found that a midwinter minimum-like behavior in eddy activity, as observed, is obtained in idealized simulations where zonally symmetric temperature profiles are adjusted to mimic the seasonality of longitudinally averaged temperature observed across the North Pacific. Furthermore, we find in both reanalysis data and the idealized simulations that a poleward shift of the January jet leads to an increase in eddy kinetic energy.