Caution urged as strong typhoons expected to hit Japan before seasonal peak: expert

Strong Typhoon Mawar brought heavy rains to parts of Japan as it approached Okinawa on June 1. According to the Japan Meteorological Agency (JMA), it was the third most powerful typhoon to develop in May since records began in 1951. Why did such a strong storm occur this early, and what kind of typhoon season awaits this year?

Mawar developed on May 20 (Japan time) near the Caroline Islands in the western Pacific Ocean. It quickly gained intensity as it moved northwest, attaining maximum wind speeds of 60 meters per second (216 kilometers per hour) on May 26, momentarily making it a "Violent Typhoon" under JMA classification and causing massive damage to the U.S. territory of Guam. At one point, it also fell under the U.S. National Weather Service's highest level of tropical cyclone classification as a "super typhoon." Even after weakening as it approached Japanese territory, Mawar remained a "large tropical storm" under the JMA classification as of 3 p.m. on June 2.

Ocean surface temperatures have a large effect on the formation of typhoons. According to the JMA, the sea surface temperature near Guam was about 1 degree Celsius higher than normal. The conditions were also right for Mawar to intensify as it continued on its way westward without ever landing on large islands and therefore not losing its strength.

Tomoe Nasuno, lead researcher for the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), pointed out that the Madden-Julian Oscillation (MJO), in which large cloud formations slowly move eastward from the Indian to Pacific oceans, may be related to the development of Mawar. Since its discovery in the 1970s, the large-scale weather phenomenon is thought to play a major role in the activity of typhoons and monsoons.

During summer in the western Pacific Ocean, tropical westerlies flowing from south of Eurasia toward the South China Sea create a low-pressure vortex when coming into contact with the Pacific high-pressure ridge of subtropical easterlies. This is called the "monsoon trough," considered a hotbed for the formation of typhoons.

Although May is during the season westerlies usually remain weak, this year the MJO became quite active between around May 20 and May 25. Nasuno suspects the monsoon trough developed from westerlies strengthened by the MJO, setting the right conditions for the easier formation and development of typhoons.

This year, it is expected with an 80% probability that the El Nino effect will raise the seas' surface temperatures by summer after its opposite La Nina effect, which lowers ocean surface temperatures between the equatorial Pacific area and the coast of South America, ends. It is known that more typhoons than usual tend to develop in the southeast tropical western Pacific in years in which El Ninos form, and a greater number of these become strong typhoons due to the extended time they take passing over the warm tropical waters.

"At present, water temperatures in the western Pacific Ocean have not yet fallen due to remnants of the La Nina effect. It's likely this was one of the factors that allowed Typhoon Mawar to retain its intensity, and that a higher than usual level of caution for strong typhoons will be needed this year," Nasuno said.

(Japanese original by Tomoko Mimata, Lifestyle, Science and Environment News Department)