Explosive and binary cyclogenesis over a mid-latitude hotspot and its Rossby number dependence in an idealized general circulation model

Abstract

This study presents the optimal conditions for the formation of explosive and binary cyclones over a mid-latitude hotspot in an idealized Earth-like general circulation model of variable planetary size. Multiple solutions of cyclogenesis were obtained for the same Rossby number over the hotspot (RoH), whereas general circulation is uniquely determined with respect to the Rossby number of the zonal-mean westerly jet (RoJ). For low RoJ values, the zonal jet core and its related area of active eddies are longitudinally narrow at 300 hPa over the hotspot. However, the latitudinal extension of the active area over the hotspot is not small. Binary cyclones (pairs of northern and southern lows) are thus frequently formed. For intermediate RoJ values, the zonal jet core and the area of active eddies at 300 hPa are extended from the northern hotspot toward the east. At 900 hPa, the active eddy region is bifurcated along the jet axis and over the hotspot. This may correspond to the bifurcation of storm tracks in winter in the North Pacific hotspot, where both explosive and binary cyclones occur frequently, along with a meridionally elongated trough. For high RoJ values, the zonal jet core and active eddy area at 300 hPa occur north of the hotspot, and the active eddy area at 900 hPa occurs over the hotspot. The upper-level active area is separated from the lower-level area, so cyclones are not explosively developed and binary cyclones are not formed.