Surface and upper-air analyses from the ECMWF-TOGA data set for 10 Northern Hemisphere cool seasons (October-March) were used to construct a synoptic-climatology of subtropical cyclones in the central and eastern Pacific Ocean. Inspection of a daily time series of subtropical cyclone occurrence revealed substantial intraseasonal variability concentrated into distinct, prolonged periods characterized by either numerous or zero subtropical cyclones. Such periods were deemed "active" and "inactive" periods, respectively.

Daily outgoing longwave radiation data were used to explore the potential link between anomalous convection in the tropical Pacific and the occurrence of active and inactive periods. Based on prior theoretical results, it was inferred that active (inactive) periods might be associated with suppressed (enhanced) tropical convection in the central Pacific due to the generation of an anomalous subtropical trough (ridge) by Rossby wave dispersion away from the anomalous convection. A detailed analysis of each period, however, revealed that only 55% of the periods were characterized by the theoretically-expected distribution of anomalous tropical convection (deemed "correct") while an additional 30% of the periods were actually characterized by the exact opposite distribution (deemed "incorrect"). Inspection of the composite 300 hPa height and zonal wind anomalies for these correct and incorrect periods revealed that Rossby wave dispersion away from suppressed (enhanced) tropical convection in the central Pacific during correct active (inactive) periods was associated with an extratropical response resembling the negative (positive) phase of the Pacific/North America pattern. Further analysis revealed that the lack of subtropical cyclones during incorrect inactive periods was primarily associated with the westward expansion of the subtropical high in the eastern Pacific. The observed broadening and weakening of the Asian jet during the transition to incorrect active periods suggests that barotropic energy conversions may play a significant role in fostering a large-scale environment that is conducive to the frequent development of subtropical cyclones during incorrect active periods.