A 45‐year climatology of extratropical cyclone locations relative to upper‐level jet streak determined by an automatic procedure

Abstract

AbstractThe main purpose of the analysis is to characterize the location of extratropical lows relative to the jet streaks over Europe and the North Atlantic. Algorithms are presented for jet streak detection and cyclone positioning relative to jet streaks. All time steps in the tracked cyclone lifetime together as well as cyclogenesis, maximum deepening and cyclolysis stages separately are examined. The analysis includes strong jet streaks (>50 ms−1) detected in 300 hPa wind fields in the winter period (1958–2002). The location of cyclones in the jet streak‐associated coordinate system is determined. The cyclone positions, situated near jet streak (<1,300 km), are depicted in the circular distribution diagrams and counted within 15° intervals and jet streak quadrants. The distribution of low‐pressure systems at the cyclogenesis and maximum deepening stages resembles the four‐quadrant model of upper‐level divergence: modal intervals are situated in the Left Front (LF) (primary mode: 45–60°) and the Right Rear (RR) (secondary mode: 195–210°) quadrants. Significant changes in cyclone counts at the LF‐LR and LR‐RR boundaries are likely to be a response to the divergence field aloft. Bimodality is distinctly pronounced in the case of cyclogenesis events and maximum deepening phases. The cyclolysis sites are unimodally distributed with maximum frequency in the Left Rear (LR) quadrant. The modal intervals of cyclogenesis and maximum deepening samples deviate towards the LF‐LR and RR‐LR boundary lines from the bisectors of the LF and RR quadrants, respectively. Such angular displacement is congruent with the shift of divergence‐convergence meridional dipoles in the entrance and exit region towards the cyclonic side of the jet stream. The incipient and maximum deepening stages of cyclones are located within the divergent quadrants in almost 80% of cases (of those near jet streaks). Nearly 60% of the cyclolysis events are situated in the convergent quadrants.