Tropical Waves and Tropical Cyclogenesis over the Western North Pacific

The present study investigates the relationship between the tropical waves and the tropical cyclone (TC) genesis over the western North Pacific (WNP) for the period 1979–2011. It is shown that the TC genesis is strongly related to enhanced low-level vorticity and convection of tropical waves and significant difference are detected in the TC modulation by dynamic and thermodynamic components of the waves. More TCs tend to form in regions of waves with overlapping cyclonic vorticity and active convection. About 83.2% of TCs form within active phase of tropical waves, mainly in a single wave and two coexisting waves. The activity of westward-propagating tropical waves (WTW) may play an important role in modulating TC genesis over the WNP. The interannual variation in the TC genesis is well reproduced by the tropical wave-related TC genesis, especially in the region east of 150°E. An eastward extension of the enhanced monsoon trough (MT) coincides with increased tropical wave activity by accelerated wave-mean flow interaction.

Furthermore, this study evaluates different wave structures and dynamics of two types of WTWs, equatorial Rossby (ER) waves and mixed Rossby–gravity (MRG)–tropical depression (TD)-type waves, in strong MT (S-MT) and weak MT (W-MT) years over the WNP. There is a significant change in the three-dimensional structure as those waves propagate westward to the east of the MT. For the TD–MRG waves, an apparent transition from MRG waves to off-equatorial TD disturbances is identified in the region of the MT. For the ER waves, their amplitudes have a faster growth, but their structures and propagation characters have no marked change. The barotropic conversion associated with the MT is the most important mechanism for the growth of eddy energy in both TD–MRG and ER waves. The large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower-tropospheric waves through a positive feedback between the wave growth and horizontal structure. The baroclinic conversion process associated with the MT contributes a smaller part for TD–MRG waves, but is of importance comparable to barotropic conversion for ER waves as it can produce the tilted vertical structure. The growth rates of the waves are much larger during strong MT years than during weak MT years. Numerical experiments are conducted for an idealized MRG or ER wave using a linear shallow-water model. The results confirm that the monsoon background flow can lead to an MRG-to-TD transition and the ER wave amplifies along the axis of the MT and is more active in the strong MT state. This indicates that the mean flow of the MT provides a favorable background condition for the development of the waves and acts as a key energy source. Differences in the location of the MT may lead to an east–west contrast in the tropical waves, which provide an important source for TC genesis.