Entering July this year, tropical cyclone activities over the western North Pacific have been increasing. The satellite imagery in Figure 1 shows that there are a total of three tropical cyclones over the northeastern part of the South China Sea and the western North Pacific. You may wonder: If Chan-hom continues to edge closer to Linfa, what will be the impacts on the latter? Concurrent occurrences of three tropical cyclones happened in the past, but were not many. Some examples in recent years can be found in the HKO's educational material.

Dating back to the 20's to 30's of the last century, Dr. Sakuhei Fujiwhara (1884 - 1950) already discovered that when two tropical cyclones approach each other, they tend to rotate anti-clockwise about a point between them. This phenomenon is usually known as the Fujiwhara effect. Research studies showed that tropical cyclones begin to interact more prominently when their centers come within around 1200 km of each other. The degree of interaction increases as the separation distance decreases^[1], while the separation distance where interaction commences depends on the sizes of the tropical cyclones^[2]. Other research studies also pointed out that the interaction between two tropical cyclones depends on the sizes and intensities of the tropical cyclones as well as the environmental steering flow; whereas tropical cyclones of unequal sizes likely to have greater interaction than two of similar sizes^[3]

When two tropical cyclones come into proximity, it may bring about the following situations:

1. The two tropical cyclones rotate in a stable orbit (Fujiwhara effect), followed by a release and escape (Figure 2). For example in 2009, tropical cyclone Parma near the Philippines interacted with another tropical cyclone Melor, and Parma underwent a looping motion during 5-7 October (Figure 3 and 4).

2. One tropical cyclone is captured and "swallowed up" by another tropical cyclone, or the two tropical cyclones undergo a merger (Figure 5). This scenario is most likely to occur when one tropical cyclone is much larger and stronger than the other[4]. Examples are Zeb and Alex in 1998 (Figure 6); Namtheun in 2010 near the Taiwan Strait interacting with Lionrock over the northeastern part of the South China Sea, weakening and dissipating afterwards (Figure 7 and 8).

3. The two tropical cyclones only exhibit "semi-direct" interaction, and their tracks are largely determined by the steering flow associated with other synoptic weather systems^[4][5] (Figure 9).

Whenever there are interactions between two or more tropical cyclones, they will drag each other, rotate, cause one of them weakening, merge together or escape from each other, etc. These are superimposed on the steering flow of the synoptic environment. The tracks of the tropical cyclones will then become rather complex, making forecasts more difficult. Nowadays, we have a grasp of the basic conceptual models, and computer numerical models can also generally capture the interaction processes between tropical cyclones. However, as many factors come into play (including changes in intensities, sizes and relative positions, etc. of the tropical cyclones), there will be discrepancies between different model forecasts, posing great challenges to forecasters. In any case, members of the public are advised to pay close attention to the latest tropical cyclone information and weather forecasts issued by the Observatory.