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

Abstract

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 elon...