Interannual variability of tropical cyclone activity over the eastern North Pacific

Abstract

During 1966-2003, the annual tropical cyclone (tropical storm and hurricane) genesis frequency over the eastern North Pacific (ENP) ranges from 8 to 27, with a standard deviation above 4. This study examines the interannual variability of tropical cyclone (TC) activity and the associated variation of the seasonal mean environment utilizing NCEP-NCAR reanalysis data. Both accumulated cyclone energy (ACE) and TC genesis frequency of each peak hurricane season (July, August, and September) are employed to represent TC activity. Environmental conditions are compared between 1992 and 1977, which experience the most active and inactive TC season respectively. Sea surface temperatures in both 1992 and 1977 are warm and favorable for TC formation. Other environmental parameters that are favorable for cyclogenesis in 1992 include: weaker vertical wind shear, larger low-level relative vorticity, stronger midtropospheric ascending motion, stronger upper-level divergence, and larger midtropospheric moisture content. In addition, a number of features associated with high TC activity are also observed in 1992. A well-established monsoon trough is observed over the genesis region. In 1992, convective disturbances with 4-10-day period propagating consistently from the east bring stronger convection, as compared to the case in 1977. An unstable upstream state with negative meridional potential vorticity (PV) gradient is observed over the Caribbean. In both years, anomalous zonal wind over the cyclogenesis region oscillates with the MJO timescale. However, TCs do not necessarily form during the westerly phases in either year in the MJO timescale.