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Downstream development and Kona low genesis

A composite analysis of 43 Kona lows in conjunction with a case study of a particularly damaging Kona low indicate that downstream development is dynamically important to the subtropical cyclogenesis. It takes the form of eastward propagating, statistically significant upstream potential vorticity (PV) anomalies with accompanying meridional wind anomalies at the tropopause level prior to the formation of a Kona low. The downstream development culminates in the formation of a PV streamer, a meridionally‐elongated stratospheric intrusion of high PV air into the troposphere, associated with a breaking wave on the dynamical tropopause. Subsequently, the streamer ‘cuts off’ from the stratospheric reservoir of high PV and translates equatorward, thereby providing a necessary dynamical forcing for the subtropical surface cyclogenesis.