Abstract:Typhoons Chanchu (0601) and Megi (1013) make similar abrupt northward recurving movements after they enter the South China Sea from the Philippine Sea. This analysis is performed to probe the main environmental factors responsible for TCs suddenly turning. The results show that both typhoons experience similar environmental adjustment periods, which can explain TCs movement: In the first stage, a westerly trough moves eastward, forcing the subtropical high to weaken and recede from South China, making the TC moving slow down. In the second stage, the subtropical high reinforces and extends southwestward, passing by the south area to TC; when it is connected with the equatorial high, the cross equatorial flow joins the southerly wind west of the subtropical high, leading TC to move northward. Meanwhile, at higher latitudes, rather weak cold air flow intrudes southward. The further calculation confirms that steering flows change abruptly with the environmental situation, from easterly to southerly, which is principal for both Chanchu and Megi making northward turns. For autumn typhoon, Megi, the best steering flow is at lower levels than that of early summer typhoon Chanchu. In operational practice, there are many kinds of forecast information provided by different methods, models and super ensemble models, the real time modifications are necessary in accordance with environmental circulation changes.