ROTATION OF BINARY CYCLONES - A DATA-ANALYSIS STUDY

In contrast to earlier studies, where only binary tropical storms were explored, the rotation at midlatitude and the subtropics is studied h ere. The point vortex theory applied to two neighboring cyclonic vorti ces isolated from external forcing predicts the following features: ro tation in a cyclonic sense at a rate directly proportional to the sum of the cyclones' intensities and inversely to the square of their sepa ration, with the weaker cyclone rotating faster than the more intense one. This interaction noticed in the Tropics, was entitled the Fujiwha ra effect or binary interaction. Objective analysis of 17 313 cyclone pairs using ECMWF initialized datasets was done to examine the existen ce and behavior of binary interaction between extratropical cyclones. The impact of anticyclones is studied through the moments of distribut ion for the relative vorticity. The anticyclonic shear of the backgrou nd flow and the prevalence of anticyclones in the subtropics are sugge sted to explain the absence of binary rotation there. Midlatitude cycl one pairs with separations of up to 2000 km are indeed found to rotate cyclonically about each other at a rate proportional to their combine d intensities, in agreement with theory. The binary rotation rate decr eases with the square of the separation distance, as in the point vort ex theory, up to 1400 km. But a pronounced unexpected peak was found n ear 1800 km. No significant correlation was found between the individu al rotation speeds ratios and that of the intensities of the interacti ng cyclones. Only partial agreement between the observed rotation of m idlatitude surface binary cyclones and the barotropic predictions indi cates the need to adopt a more elaborate model. Indeed, the 500-hPa bi nary interaction study reveals a much better agreement with theory. In a companion study, the authors propose a two-level conceptual model t hat employs the PV ideas for exploring the binary surface cyclone beha vior.