Synoptic climatology of subtropical cyclogenesis

During the Hawaiian cool season (October - April), upper-level lows sometimes become cut off from the polar westerlies south of 30°N latitude in central Pacific. Seventy such lows formed during the years 1980-2002. There is strong inter-annual variability in their frequency, with an average of ~3 lows per season. The number of lows decreased during the 3 strongest EI Nino seasons and increased during the 3 strongest La Nina seasons, with statistically significant excursions. Low formation is greatest during October and November, when storm genesis is concentrated to the west-northwest of the Hawaiian Islands. Low genesis shifts eastward through the cool season, favoring the area to the east-northeast of Hawaii during February and March, consistent with the shift in the climatological position of trough aloft during the cool season. Out of a total of 70 upper-level lows, 43 were accompanied by surface cyclogenesis and were classified as kona lows. Kona-Iow formation is concentrated to the west-northwest of Hawaii, whereas lows without surface development are concentrated in the area to the east-northeast of Hawaii. Analysis reveals several distinctions between the kona lows and the lows without surface development. Surface deepening correlates strongly with positive vorticity advection by the thermal wind. Surface deepening also correlates with potential vorticity on the 340-K isentropic surface and the maximum v-component of the wind aloft. Static stability and advection of low level moisture are less strongly correlated to surface deepening. These results confirm that kona-Iow formation, to first order, is driven be upper-level forcing that originates in the midlatitudes, and that convection and latent heat release playa secondary role.