The Large-Scale Modulation of Subtropical Cyclogenesis in the Central and Eastern Pacific Ocean

Abstract

An analysis of the composite large-scale circulation associated with periods of enhanced (active) or diminished (inactive) cyclogenesis in the subtropical central and eastern Pacific Ocean is presented. Composites were constructed using surface and tropospheric analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset for 10 Northern Hemisphere cool seasons (1986?96). Active periods of subtropical cyclogenesis were defined to be periods in which two or more cyclones developed in close succession to each other, while inactive periods were defined to be periods of at least 10-days duration during which no cyclones with a subtropical origin were present in the Pacific basin. The analysis revealed that the occurrence of subtropical cyclones in the central and eastern Pacific Ocean is strongly linked to the strength and location of the Asian jet, with active periods characterized by a weaker, zonally retracted Asian jet while inactive periods are characterized by a stronger, zonally elongated Asian jet. Consideration of the stationary wavenumber, Ks, showed that the strong, zonally elongated jet characterizing inactive periods produced a continuous waveguide across the Pacific basin that severely limited the equatorward propagation of upper-level cyclones into the subtropical Pacific. However, the zonally retracted jet during active periods was associated with a poorly organized, or ?leakier,? waveguide across the Pacific, which produced a decidedly more favorable situation for the equatorward propagation of upper-level cyclones leaving the exit region of the Asian jet. 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. A detailed analysis of each active and inactive period revealed that only 55% of the periods were characterized by the theoretically expected distribution of anomalous convection across the tropical Pacific (deemed ?correct?) and that 30% of the periods were actually characterized by the exact opposite distribution (deemed ?incorrect?). During correct active and correct inactive periods, Rossby wave dispersion away from anomalous tropical convection in the central Pacific is associated with an extratropical response resembling the Pacific?North American pattern. Further analysis revealed that the lack of subtropical cyclones during most incorrect inactive periods was associated with a strengthened and zonally elongated Asian jet. The observed broadening and weakening of the Asian jet that occurs during the transition to incorrect active periods suggests that barotropic energy conversions may play an important role in fostering a large-scale environment conducive to the frequent development of subtropical cyclones during incorrect active periods.