May to November is generally the tropical cyclone season of Hong Kong. But why do some tropical cyclones that formed over the western North Pacific come close to Hong Kong while some turn northwards?

The track of a tropical cyclone is affected by many different factors. Whether it moves close to Hong Kong depends on the coordination of different weather systems. Among them, the steering flow is an important factor in determining how a tropical cyclone moves. The steering flow can generally be represented by the wind field at a certain height or the mean wind field of a number of layers.

Major weather systems that affect the track of tropical cyclone over the western North Pacific include:
(1)   Subtropical ridge (abbr. STR): Located over the subtropical region with the largest area of coverage.
(2)   Westerly trough: A disturbance in the mid-latitude westerlies covering a smaller area.
(3)  Monsoon trough: Extending from the intertropical convergence zone. The monsoon trough over the southeast Asia covers an area ranging between a subtropical ridge and a westerly trough.

Generally speaking, the subtropical ridge steers a tropical cyclone along its periphery. If the subtropical ridge is covering an area towards the east, the tropical cyclone may recurve over the western North Pacific towards the vicinity of Japan. On the contrary, if the subtropical ridge covers East China, there is a chance that the tropical cyclone enters the South China Sea (abbr. SCS) (Conceptual model 1 in Figure 1). However, the strength, position and orientation of the subtropical ridge will be affected by other weather systems. If there is a deep westerly trough moving towards the northwestern side of the subtropical ridge, the latter may split into two cells and the tropical cyclone will be steered northwards^[1](Conceptual model 2 in Figure 1). On the other hand, the monsoon trough also affects the track of a tropical cyclone. If the monsoon trough is relatively strong and extends eastwards forming a large low pressure gyre with the tropical cyclone, the subtropical ridge will be north-south oriented^[2], and the tropical cyclone may move northwards at a position farther to the east (Conceptual model 3 in Figure 3).

It is worth noting that tropical cyclones can exhibit a variety of tracks, sometimes making a loop or a sharp turn (for example, Bebinca and Son-Tinh in 2018, Vongfong in 2020). The aforementioned conceptual models serve only as a reference. Forecasters have to combine the observations, experience and outputs from numerical weather prediction to predict the challenging tropical cyclone track.