Characteristics of tropical cyclone activity over the eastern North Pacific: the extremely active 1992 and the inactive 1977

The eastern North Pacific experiences large variability in tropical cyclone (tropical storm and hurricane) frequency from year to year. Large-scale environmental conditions during the peak hurricane season (July, August and September) are contrasted for two extreme years; 1992 is the most active year and 1977 the most inactive year.

Sea surface temperatures in both 1992 and 1977 arewarm and favourable for tropical cyclone (TC) formation, whereas other environmental factors undergo pronounced changes over the major development area (MDA) for the two extreme years. For instance, the 1992 hurricane season features weaker vertical wind shear, larger low-level relative vorticity, stronger mid-tropospheric ascending motion, stronger upper-level divergence, and larger mid-tropospheric moisture content than the 1977 hurricane season. These changes correspond well to the variation of the TC activity during the two extreme years.

In addition, a monsoon trough is only present over the MDA in 1992. Convective disturbances within a 4–10-d period propagating consistently from the east bring stronger convection in 1992 than in 1977. In both years, anomalous zonal wind over the MDA oscillates with the intraseasonal timescale. However, TCs do not necessarily form during the westerly phases in either year in the intraseasonal timescale.