Tropical cyclogenesis in a tropical wave critical layer: easterly waves

Abstract

Abstract. 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 structure, equatorward of the easterly jet axis, is identified herein as 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 convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex 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 depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by Correspondence to: T. J. Dunkerton (tim@nwra.com) the resolved flow, (ii) containment of moisture entrained by the developing 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 a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother's pouch. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the "marsupial paradigm". A survey of 55 named tropical storms in 1998-2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the co-moving frame. It is shown that the kinematics of isolated protovortices carried by the wave also can be visualized in a fra...

Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics

“…More precisely, genesis occurs near the intersection of the disturbance's 700-hPa trough axis and the zero-relative-flow isopleth. This behavior is consistent with the ''marsupial'' paradigm of TC genesis from easterly waves, recently proposed by Dunkerton et al (2009). As its name suggests, the marsupial paradigm envisions the closed gyre circulation of an easterly wave critical layer as being especially favorable for TC genesis, by protecting the incipient storm from the detrimental effects of horizontal mixing and straining, analogous to a joey inside a mother kangaroo's pouch.…”

Properties of strong off‐shore Borneo vortices: a composite analysis of flow pattern and composition as captured by ERA‐Interim

“…Using the RGB and averaging all cases contributing to the number of occurrences in it, we construct a composite of strong off-shore BVs. This approach is complementary in philosophy to a moving framework of reference as suggested by Dunkerton et al (2009). Instead of tracking one vortex in a flow-following coordinate system, we select all vortices with similar properties that have occurred over a certain period of time at a given location.…”

Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

“…Raymond and López Carrillo (2011) found that vertically overlapping closed circulations in the boundary layer and at 5-km elevation provided a deep region of protection from negative environmental influences during the formation of TY Nuri. Montgomery et al (2010) suggested that despite the detrimental influences of strong vertical wind shear and dry air during the early stages of TY Nuri, the presence of a quasi-closed circulation in the system-relative frame (wave pouch) characterized by recirculating flow (Dunkerton et al 2009) protected the incipient vortex until it could reach a more favorable environment. They further hypothesized that an increase in the depth of the wave pouch is favorable for storm formation and may have occurred prior to the formation of TY Nuri.…”