The 2004 merger of tropical storm Lisa and an african eastery wave: an example of a Fujiwhara interaction in the central tropical Atlantic

Analysis of global tropical cyclone best track data from 1946 through 1981 reveals that binary interactions between tropical cyclones occur more frequently in the western North Pacific (1.5 times/year) than in the Atlantic (0.33 times/year). The higher frequency of binary interactions in the western North Pacific has resulted in many documented cases that have tested the classic Fujiwhara effect, in which the two spatially proximate vortices begin orbiting about a center point between them; are subsequently attracted to each other; and eventually spiral in towards the center point and merge. However, few cases of Atlantic interactions have been documented.

A case of a binary interaction between Tropical Storm Lisa (2004) and a well organized African Easterly Wave that approached Lisa from the southeast is presented. Various data and analysis tools, including Met7 satellite imagery; easterly wave diagnostics computed from the NCEP Global Forecast System potential vorticity; and Quikscat data; were used to examine the direct interaction between these two tropical systems. The period of interaction included approach and capture, followed by a 120 degree mutual cyclonic orbit, eventual merger, then convective reorganization and reintensification. Satellite imagery and Quikscat data suggest the approaching easterly wave vortex eventually became the dominant circulation, which gradually entrained Lisa's convection. This behavior is consistent with documented western North Pacific tropical cyclone mergers. In contrast, GFS PV analysis shows the merger yielding a new vortex at the mutual center point of rotation, behavior more consistent with a classic Fujiwhara interaction.