Tropical cyclogenesis in tropical waves

The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a " surface low along the wave " as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This 5 structure, equatorward of the easterly jet axis, resembles the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convec-tion. The critical layer represents a sweet spot for tropical cyclogenesis in which a 10 proto-vortex may form and grow within its parent wave. A common location for storm development within the critical layer is given by the intersection of the wave's critical latitude and trough axis, with analyzed vorticity centroid nearby. The wave and vor-tex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical de-15 pression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally this " marsupial paradigm " one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. This translation requires an appropriate " gauge " that renders translating streamlines and isopleths of translating 20 stream function approximately equivalent to flow trajectories. In the translating frame, the closed circulation is stationary, and a dividing streamline effectively separates air within the critical layer from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because it provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of mois-25 ture entrained by the gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile , and (v) maintenance or enhancement of the parent wave until the vortex becomes Abstract Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion a self-sustaining entity and emerges from the wave as a tropical depression. These ideas are formulated in three …