Influence of the Monsoon Trough on Tropical Cyclones over the Western North Pacific: Role of Westward-Propagating Tropical Waves

Tropical cyclone (TC) activity over the western North Pacific (WNP) is closely related to the monsoon trough (MT) location. Such a relationship can be explained by the changes in large-scale environmental factors and synoptic-scale disturbances associated with the movement of the MT. The activity of westward-propagating tropical waves (WTW) may play an important role in modulating TC genesis over the WNP. The present study investigates the possible linkage between the MT and the interannual variability of the activity of WTW over the WNP during July–November for the period 1979–2007. It is shown that the interannual variability of WTW activity is closely related to the location of the MT. During the years when the enhanced (weakened) MT extends eastward (retreats westward), the lower-tropospheric WTW activity is above (below) normal within the southeastern quadrant of the WNP. 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 monsoon trough may lead to an east–west contrast in the WTWs, which provide an important source for TC genesis.