A study on the relationship between the rotation of binary typhoons and steering current

Many studies show that, within a certain distance (ca. 700–800 n mi), two typhoons forming a binary typhoon (BT) system would rotate as a whole and attract each other, which is known as “Fuji-whara Effect” (FE). This paper indicates that only 30.3% of BTs has experienced remarkable cyclonic rotation with a 12-hr angle ≥+10° when two components are less than 20° lat apart, and that the probability is much higher with the eastern component in the NE than in the SE quadrant for the western one, implying the steering effect of the environmental flow field (EFF) on them. 47 observations from 13 BTs are separately used for calculating the angular velocity due to FE and EFF and the results are compared. The conclusion can be stated as follows: FE is dominant with the centers of two elements below 7° lat apart; the EFF steering current plays a major role when they are in the range of 7–15° and for a distance above 15° the principle of FE holds no longer.