Abstract :
This study attempts to determine the possible causes of the interannual variations of intense tropical cyclones (TCs, or
typhoons) in the western North Pacific (WNP, defined here as the region 0–40◦N, 120–180◦E). It is found that such
variations cannot be explained by those of sea-surface temperature averaged over the same region. Rather, in years with
a high frequency of occurrence of intense typhoons (inferred from high values of accumulated cyclone energy, or ACE),
both the dynamic and thermodynamic conditions in the atmosphere, especially in the southeastern part of the WNP, are
favourable for the formation of TCs. On the other hand, these conditions are not conducive for TC formation in years
with small number of intense typhoons (low values of ACE). The temporal coefficients of the empirical orthogonal
functions of the relative vorticity anomalies in the lower troposphere, the vertical zonal shear and the moist static energy
correlate very well with ACE. The ACE is also significantly correlated with the Nino3.4 SST anomalies. It is concluded
that the interannual variations of intense typhoons in the WNP are likely caused to a large extent by changes in the
planetary-scale atmospheric circulation and thermodynamic structure associated with the El Ni˜no phenomenon
