A new look at the jet-storm track relationship in the North Pacific and North Atlantic

Abstract. The western ocean boundaries of the North Pacific (NP) and North Atlantic (NA) set favourable conditions for upper-level jets and baroclinic weather systems that propagate downstream and form the storm tracks. Despite these similarities between the two ocean basins, distinct forcing mechanisms during the winter season give rise to differences in the jet intensity, structure, and variability, as well as in the storm track activity. In particular, the phenomenon of the NP midwinter suppression of the monthly averaged storm track activity sparked ongoing discussions about fundamental differences between jet-storm track interactions in the NP and NA. This study introduces an alternative method, which avoids monthly averaging, to study the relationship between the background jet core strength (U) and, as a measure of storm track activity, the eddy kinetic energy (EKE), both evaluated in the upper troposphere. With our approach, we find that the U-EKE relationship is remarkably consistent across the NP and NA, with previously observed differences largely attributable to the differing timescales of jet variability in the two basins. For our interpretation, the separate consideration of two distinct timescales is important: On seasonal timescales, baroclinic instability results in an increase of EKE with increasing U from summer to winter. In contrast, on sub-monthly timescales, particularly during winter, EKE decreases with increasing U, reflecting the effect of baroclinic conversion. Periods of enhanced baroclinic conversion lead to reduced baroclinicity (quantified by U) and high EKE, whereas periods of low baroclinic conversion are followed by high U and low EKE. In this framework, the NP midwinter suppression of monthly averaged EKE reflects that, in midwinter, U remains persistently high in the NP (because baroclinic conversion is suppressed) while EKE is reduced. In other words, in the NP, jet strength varies predominantly from month to month, whereas in the NA, it varies more within individual months such that the midwinter suppression of the monthly averaged storm track activity is less obvious in the NA. The observed U-EKE relationship implies that the jet core strength U alone cannot explain the EKE variability across seasons, and we reveal the additional importance of the jet width, which affects eddy characteristics. A reduced jet width likely plays a role in deforming and meridionally confining eddies, thereby reducing their baroclinic growth. For jets with comparable weak to moderate core strengths in summer and winter, the summertime jets tend to be narrower, and therefore EKE smaller. Similarly, very strong jets in winter are particularly narrow, which implies reduced EKE, supporting the observed U-EKE relationship in winter. Finally, cyclone composites show that the reduced EKE during strong jet episodes in winter is manifested by a reduction in the amplitude of the cyclones' surface pressure anomalies, and, in particular of their associated troughs and ridges. Therefore, the reduction of EKE with increasing U is not related to a decrease in cyclone frequency, but rather to a reduction in cyclone intensity and the associated upper-level wave pattern.