OBSERVATIONAL EVIDENCE FOR ALTERNATE MODES OF TRACK-ALTERING BINARY TROPICAL CYCLONE SCENARIOS

An observational study of western North Pacific tropical cyclones (TC) revealed many cases of two TCs whose tracks were altered by processes that were quite different from the mutual advection (Fujiwhara-type) processes. Thus, four conceptual models are proposed to describe these track alterations. A conceptual model called direct interaction is pr oposed that is a modification of one by Lander and Holland and has thr ee modes: 1) a one-way influence in which the track of a smaller TC th at is embedded in the circulation of a larger TC has a cyclonic orbiti ng motion, but no significant track alteration of the larger TC is app arent; 2) a similar case in which a mutual advection occurs with the t racks of both the smaller and larger TCs being altered; and 3) a subse t of 2) in which the mutual advection includes an attraction component such that the two similarly sized TC circulations eventually merge in to a larger circulation with a single center. During the 7-yr period ( 1989-95), the one-way influence, mutual interaction, and merger modes were detected seven, three, and two times.A semidirect interaction con ceptual model is proposed in which the two TCs have a relative cycloni c rotation as in the Lander and Holland model, but the TCs are separat ed by 10 degrees-20 degrees longitude so that a direct (advective-type ) interaction is excluded. Rather, the track alteration is attributed to an environmental flow established by the juxtaposition of a TC on o ne side and a subtropical anticyclone cell on the opposite side. In an east-west orientation of the two TCs and a subtropical anticyclone ce ll to the east (west), the height gradient between the western (easter n) TC and the eastern (western) subtropical anticyclone establishes a poleward (equatorward) environmental steering flow across the eastern (western) TC. In the 1989-95 sample, a semidirect interaction that alt ered the tracks of the eastern or the western TC occurred 18 and 14 ti mes, respectively. An indirect interaction conceptual model is propose d in which the distinguishing feature is the Rossby wave dispersion-in duced anticyclone to the east and equatorward of the western TC. This anticyclone imposes an equatorward (poleward) steering how across the eastern (western) TC. Several variations of the indirect interaction a re possible depending on the separation distance, sizes of each TC, an d their relative orientation. During the 7-yr period, an indirect inte raction affecting the western TC or the eastern TC occurred 36 and 22 times, respectively. A fourth conceptual model of track alterations in volving two TCs is proposed in conjunction with a reverse-oriented mon soon trough formation. The distinguishing feature of this conceptual m odel is the combination of the peripheral anticyclones of both TCs as the eastern TC moves into an east-west orientation and has a separatio n of 10 degrees-20 degrees longitude. In the 1989-95 sample, a reverse -oriented trough formation involving two TCs occurred seven times. The frequency of track alterations whenever two TCs are present emphasize s that forecasters must give special attention to such situations. The four conceptual models proposed here emphasize that the physical mech anisms are complex and in the vast majority of cases cannot be attribu ted to the mutual advection (Fujiwhara-type) process implied in the La nder and Holland model.