Western Pacific Tropical Cyclogenesis Precursors

The basic ingredients for tropical cyclogenesis have been well established: a sufficiently deep warm ocean layer, low-level cyclonic vorticity, weak vertical wind shear, and deep moist convection (e. et al. 2008) have attributed cyclogenesis to a variety of low-level precursor systems that enhance one or more of these ingredients. These studies have traditionally focused on features such as the monsoon trough, monsoon gyres, Rossby dispersion from preexisting tropical cyclones, and tropical depression (TD)-type disturbances, sometimes referred to as easterly waves. Recently, significant research has been focused on convectively coupled equatorial waves (Kiladis et al. 2009). These waves include mixed Rossby–gravity (MRG) waves, equatorial Rossby (ER) waves, Kelvin waves, and the Madden–Julian oscillation (MJO; Zhang 2005). For simplicity, the MJO will also be referred to as an equatorial wave because it consists of forced equatorial waves (Gill 1980). Equatorial waves appear to play a major role in modulating tropical weather, including influencing fraction of western Pacific tropical cyclones that may be attributed to equatorial waves has not been established. These traditional and equatorial wave approaches to western Pacific tropical cyclogenesis have largely ignored one another. For example, studies using the traditional approach indicated that the monsoon trough is responsible for roughly 70% of western but equatorial wave studies rarely discussed the monsoon trough. On the other hand, studies using the traditional approach barely mentioned the MJO, which was found to significantly modulate cyclogenesis in the equatorial wave studies (e.g., Liebmann et al. 1994). The proposed research will systematically determine which storms may be attributed to equatorial waves and then compare these results to traditional cyclogenesis attribution studies (e.g., Ritchie and Holland 1999; Fu et al. 2007; Lee et al. 2008). The 2 ultimate goal is to develop a more complete understanding of tropical cyclogenesis in the western Pacific by comparing both approaches and searching for common ground. a. Traditional precursors Gray (1968) identified the western Pacific monsoon trough as a climatologically favorable location for cyclogenesis because it provides low-level cyclonic vorticity and weak vertical shear. The eastern end of the monsoon trough is particularly favorable for cyclogenesis because the confluence of monsoon westerlies with trade easterlies can induce wave accumulation (Holland 1995; Sobel and Bretherton 1999). Westward propagating disturbances from the central Pacific can grow in the confluence region to form tropical cyclones. Tropical cyclogenesis in the monsoon confluence region may be modulated by easterly waves (Ritchie and Holland 1999), and …