Application of the Marsupial Paradigm to tropical cyclone formation from northwestward propagating disturbances

A wave-tracking algorithm is developed for northwestward propagating waves which, on occasion, play a role in tropical cyclogenesis over the western oceans. To obtain the Lagrangian flow structure, the frame of reference is translated obliquely with the precursor disturbance. Trajectory analysis suggests that streamlines in the obliquely translated frame can be used to approximate flow trajectories. The algorithm was used to examine the formation of Super-Typhoon Nakri (2008) and Tropical Cyclone Erika (2009). Diagnoses of meteorological analyses and satellite-derived moisture and precipitation fields show that the marsupial paradigm (Dunkerton et al. 2009) for tropical cyclogenesis in tropical easterly waves is also relevant for northwestward propagating disturbances as are commonly observed in the tropical western Atlantic, Gulf of Mexico and western North Pacific. It is also shown that a convective divergence profile drives a positive vorticity tendency at low levels and precedes the most rapid increase of vorticity. Although analyzed divergence is less reliable than vorticity, its spatio-temporal average near the center of the wave's “gyre-pouch” may be used as an indicator for tropical cyclone formation. Proper analysis of the global model data and satellite observations can provide useful guidance on early tropical cyclone advisories.