1. Why do typhoons first move west, away from Japan, because of the trade winds?
Typhoons are born in the warm, low-latitude seas far south of Japan. In this region, easterly winds known as trade winds blow, and typhoons are carried westward by these winds [2]. For example, a typhoon born in low latitudes in early spring may move west toward the Philippines [1].
A typhoon does not decide its own direction. It moves because it is carried by the large-scale flow of air around it. The Japan Meteorological Agency explains that a typhoon’s path is influenced by the wind flow around the Pacific High and the westerlies flowing through middle latitudes [3]. The National Research Institute for Earth Science and Disaster Resilience also states that the path is determined by the balance between low-latitude trade winds, middle-latitude westerlies, and the Pacific High in between [9].
Movement in low latitudes is not always straightforward. In summer, upper-level winds often weaken, causing typhoons to move slowly or even loop back south, making their movements difficult to predict [3]. When the winds carrying them are weak, the typhoon’s movement can become complex. So, where and why does a typhoon that was only heading west change its direction?
2. Why do typhoons often turn north near Okinawa as they circle the Pacific High?
Near Japan, the Pacific High expands in summer. Typhoons move northward from the southern seas, circling around this high-pressure system [1].
This change in direction is called a "turn," and Okinawa is a common place for it to occur. According to the Japan Meteorological Agency’s climate column for Okinawa, typhoons slow down and are prone to turning near Okinawa, which lies on the edge of the high-pressure system [4].
Imagine a relay race in a sports festival. A runner slows down slightly before the baton zone and turns their body toward the direction the next runner will start. Similarly, a typhoon slows down and changes direction at the boundary where it is handed over from trade winds to the flow along the edge of the high pressure, and then to the next wind. However, this is just an analogy to help understand the mechanism; no actual baton is passed.
A turn is a sign that the typhoon is at a "corner." Which wind will carry the typhoon next after it turns?
3. Why do typhoons speed up when they are carried by the westerlies?
Westerlies are winds that blow from west to east across middle latitudes throughout the year [7]. It is said that high-pressure and low-pressure systems move from west to east across Japan because they are carried by these winds [7].
When a typhoon moves near Honshu, it is influenced by these westerlies, causing it to speed up and move northeast [4]. Dictionaries state that before the turn in low latitudes, the speed is about 20 km/h (12 mph), while after the turn, the average is about 40 km/h (25 mph), and they become faster as autumn deepens [6].
Let’s convert these numbers into daily travel distances. At 20 km/h, a typhoon travels 480 km in one day. At 40 km/h, it travels 960 km. These are simple calculations assuming continuous movement for 24 hours. An average doubling of speed means the distance traveled in the same day doubles.
However, these are averages for all typhoons. The speed of individual typhoons varies greatly depending on the year and their specific path. The figure of 40 km/h is not a prediction that a specific typhoon will move at that speed. So, is the path riding the westerlies the same for every typhoon and season?
4. How do typhoon paths differ between July and September?
Looking at the Japan Meteorological Agency’s monthly typhoon tracks, we can see that the paths change with the seasons. In early spring, typhoons born in low latitudes move west toward the Philippines. In summer, the Pacific High covers Japan from the south, and typhoons often approach the Japanese archipelago when they move northeast along its northern edge [1][2].
In August and September, the westward extension of the Pacific High is weaker than in July. As a result, typhoons often pass near Okinawa [4]. From September onwards, more typhoons pass near Japan, tracing a parabolic curve from the southern seas [1].
According to the 30-year average from 1991 to 2020, about 25 typhoons are born each year. Of these, about 12 come within 300 km (186 miles) of Japan, and about 3 make landfall. Formation, approach, and landfall are most frequent from July to October [1]. With 25 formations and about 12 approaches, a little less than one in two typhoons comes near Japan.
Paths change with the seasons because the arrangement of the winds that carry typhoons shifts with the seasons. After approaching Japan, how does the typhoon itself change?
5. How does a typhoon turn into an extratropical cyclone after moving north?
When a typhoon moves north from the tropics to temperate zones, cold air flows into the upper atmosphere, and it gradually changes into an extratropical cyclone [5]. Sea surface temperatures may fall as a typhoon moves north, which can weaken the storm [5]. Typhoons that move near Honshu speed up due to westerlies and move northeast while entering a decay phase and transforming into extratropical cyclones [4].
Dictionaries note that for turning typhoons, the peak intensity often occurs before the turn, and the lowest pressure is frequently achieved before the turn [6]. Since lower pressure indicates a stronger typhoon, moving fast is a separate issue from being strong.
The view that typhoons are carried by winds connects here. If the destination leaves the warm sea and enters cold air, the energy gained from the warm sea decreases.
6. How can you find the westerlies' "handover" in the Japan Meteorological Agency's typhoon path maps?
The Japan Meteorological Agency’s website has a page on "Typhoon Formation, Approach, and Landfall" with monthly typhoon track maps [1]. Try to verify the following with your own eyes:
Compare the maps for July and September onwards to describe how the line shapes differ. Check how many typhoons pass near Okinawa. Consider how the spacing of the lines changes before and after the curve (if lines are marked at equal time intervals, wider spacing indicates higher speed).
There are limits to what can be read from the map. Track maps show the position of the lines, not the speed itself. However, you can find the location of the corner and differences in shape by season yourself. While looking at the lines, remember that the winds carrying the typhoon are in the upper atmosphere.