Typhoons

In Japan, August and September are the most active typhoon months of the year, and many typhoons approach or make landfall. Recently, the number of local heavy rains or rainstorms has been increasing, and typhoons or intense rainfalls preceding typhoons have frequently caused life-threatening natural disasters such as river floods, landslides, and mudslides. Typhoons remind us of the power of nature, but what about their influence on other living things? Please explain to us the relationship between typhoons and biodiversity.
Prof. Michio Kumagai of Ritsumeikan University who has conducted research on this topic answered our questions.

Questions answered by: Michio Kumagai: Chair Professor, Ritsumeikan University
Principal Researcher, Underwater Technology Center, the National Maritime Research Institute

How do typhoons occur? Is there a relationship between the occurrence of typhoons and the rotation of the earth? What is the typhoon eye and what is the inside like?

Typhoons are a kind of geostrophic eddy caused by the development of tropical cyclones which are formed in areas located north of the equator, between 100 and 180 degrees of east longitude of the Western Pacific and the South China Sea. In other words, typhoons are rotating gyres created by a balance of a barometric gradient, Coriolis force (force due to the earth’s rotation) and a centrifugal force. When the sea surface is warmed by the sun, convection, including a great volume of vapor, is developed. Then, heat is generated when the air is cooled in the sky and the vapor is precipitated. Due to this, the air is warmed more and ascends higher. The bigger the atmospheric convection becomes, the stronger the winds blow. When the convection becomes large-scale, the gyre becomes a large anti-clockwise gyre under the influence of the earth’s rotation. This is a tropical cyclone. When the wind speed is over 34 knots (17.2 meters/second), the gyre is called a typhoon. There is a chimney-like convection around the center of the typhoon, where the air travels downward. According to pictures taken from space satellites, we can see a transparent hole without clouds in the center of the gyre. This is the typhoon eye. In the typhoon eye, a calm environment is maintained as the direction and strength of the wind cancel each other out.

How do typhoons affect biodiversity? Is biodiversity lost due to floods or landslides caused by typhoons? Or, are there any other reasons that biodiversity will be lost?

Direct damage caused by typhoons includes the destruction of biological species. For example, precious plants and animals are washed away or buried due to floods or landslides, and the original forms are no longer retained. However, such natural disasters have been repeated over a long history. We can also consider that the present biodiversity has been made as a result of such outer turbulences. Therefore, biodiversity might sometimes be diminished due to typhoons in the short term, but typhoons might also be effective in enhancing biodiversity in the long term. Rather than a direct influence of typhoons, changes in water environments which generate typhoons are more serious. As I answered in Question 1, the temperature increase of the sea surface is a factor generating a lot of typhoons. In fact, such a temperature increase diminishes biodiversity in oceans and lakes. Temperature increases affect living things, but the more important factor diminishing biodiversity is that nutrients in deep water are not transported upward when the temperature gradient increases in a vertical direction. Biodiversity around the water surface, such as in the Pacific Ocean or Lake Biwa, is being lost in this way.

Typhoons sometimes inflict immense damage on human lives. Are typhoons necessary for ecosystems?

Not just with typhoons but in general, low pressures play an important role in water circulation on earth. If there is no low pressure, vapor will not be transported upward. As a result, it will not rain, and desertification will be accelerated in desert areas. Most notably, the East Asian monsoonal region receives a great amount of fluid replacement from the Tropics thanks to the occurrence of seasonal typhoons. Also, strong wind events strongly mix sediments as well as coastal areas or lakes, increasing oxygen availability. Anoxic sediments and deep waters are oxidized by storm events and new habitats will be created for living things. In addition, biological species living in upper reaches of streams are transported downstream, or seeds are transported to remote places by strong winds. These lead to enlarging the reproduction areas of living things. Typhoons bring us disasters, but they can be an external force for creating new living environments for other living things. Strong external forces such as typhoons are sometimes necessary to maintain healthy ecosystems.

Thank you very much, Prof. Kumagai.