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B. Ziv
P. Alpert


In contrast to earlier studies, where only binary tropical storms were explored, the rotation at midlatitude and the subtropics is studied here. The point vortex theory applied to two neighboring cyclonic vortices isolated from external forcing predicts the following features: rotation in a cyclonic sense at a rate directly proportional to the sum of the cyclones' intensities and inversely to the square of their separation, with the weaker cyclone rotating faster than the more intense one. This interaction, noticed in the Tropics, was entitled the Fujiwhara effect or binary interaction. Objective analysis of 17 313 cyclone pairs using ECMWF initialized datasets was done to examine the existence and behavior of binary interaction between extratropical cyclones. The impact of anticyclones is studied through the moments of distribution for the relative vorticity. The anticyclonic shear of the background flow and the prevalence of anticyclones in the subtropics are suggested to explain the absence of binary rotation there. Midlatitude cyclone pairs with separations of up to 2000 km are indeed found to rotate cyclonically about each other at a rate proportional to their combined intensities, in agreement with theory. The binary rotation rate decreases with the square of the separation distance, as in the point vortex theory, up to 1400 km. But a pronounced unexpected peak was found near 1800 km. No significant correlation was found between the individual rotation speeds ratios and that of the intensities of the interacting cyclones. Only partial agreement between the observed rotation of midlatitude surface binary cyclones and the barotropic predictions indicates the need to adopt a more elaborate model. Indeed, the 500-hPa binary interaction study reveals a much better agreement with theory. In a companion study, the authors propose a two-level conceptual model that employs the PV ideas for exploring the binary surface cyclone behavior.

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