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 degrees N and has features of a merged subtropical eddy-driven jet. On the other hand, during transition seasons, the jet is at higher latitudes (approximate to 40 degrees 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. Plain Language Summary The Pacific jet transitions from a more northward (approximately latitude 40 degrees N) jet during spring and fall to a stronger, more subtropical, jet (approximately latitude 32 degrees N) during midwinter. It has been well known that the Pacific storm track intensity undergoes a minimum during midwinter. The fact that the Pacific jet is strongest in midwinter while the eddies (storm track) are at a temporal minimum has been a major puzzle. Here we propose that the jet's natural seasonal transition from a more northern eddy-driven jet state to a more subtropical state results in less eddies during midwinter. Our results imply that when the northern Pacific jet shifts poleward and has an eddy-driven jet characteristic, eddy activity strengthens. Conversely, when the jet resembles a subtropical jet, eddy activity weakens. We show that the equatorward shift of the north Pacific jet during midwinter contributes to the presence Pacific midwinter minimum in eddy activity.