Monsoon Gyre joins Tokyo Olympic Games

The summer Olympics are underway in Tokyo, Japan, and are scheduled to last through August 8th. The Games have already been heavily impacted by the COVID-19 Pandemic, which has resulted in the games being pushed back a year with a number of health and safety protocols implemented to limit the spread of COVID in the Olympic village. COVID isn’t the only phenomenon that could impact the Games. Here at FleetWeather, we are also monitoring the potential for one or more tropical cyclones impacting Japan and the surrounding seas over the next few  weeks. Tokyo has already dodged one bullet, with Tropical Storm (TS) Nepartak making landfall well north of the city in northern Honshu over the last couple of days. But more systems may be on the way. So why?

Tracks for Tropical Storm Nepartak and Typhoon In-fa from 27 July 21 (Source: Japan Meteorological Agency)

A monsoon gyre has become established over the Philippine Sea. A monsoon gyre is a very large low-level cyclonic vortex which is episodic in nature and once formed can last up to two weeks and generate multiple disturbances that seed tropical cyclones. The gyre in the Philippine Sea has now spawned two tropical cyclones in rapid succession: Typhoon In-fa, which recently made landfall just south of Shanghai, China, and TS Nepartak. Global model data indicates that this gyre may continue to produce new systems over the next couple weeks which will make things more complex and challenging not just for ship routing but also for the Tokyo Olympics.

ECWMF 26 July 00Z : Tropical Storm Strike Probability for 1-8 August (Source: European Centre for Medium-Range Weather Forecasts)

This potential for more systems is highlighted in the ECWMF Tropical Storm Strike probability images with a 40-50% chance of development of another system for the period 1-8 August just south of mainland Japan.  The next system associated with the monsoon gyre may already be in the early stages of development as the latest satellite imagery indicates that an area of low pressure is developing within the gyre just east of Okinawa. This system (designated Invest 95W by the Joint Typhoon Warning Centre) will initially track to the east-northeast ahead of a trough over the Sea of Japan. However, there is good model agreement that a building ridge located several hundred miles east of Tokyo is expected to then turn this system northward towards Japan.  The Japanese Meteorological Agency(JMA) forecast this system to have developed into a tropical depression by 31 July.

JMA Surface analysis forecast valid 00Z July 31 showing Invest 95W to be a tropical depression SE of Japan. (Source: JMA)

ECWMF and GFS ensemble plots below currently show the system moving east to northeast then turning more northerly and strengthening to Tropical Depression status only.

Ensemble tracks for Invest 95W from the 00Z July 29 run of the ECMWF ensembles (left) and GFS ensembles (right) (Source:

It is too soon to determine how many additional tropical cyclones (if any) develop from this monsoon gyre. However, the blocking high east of Japan is expected to persist for several days, so any system that forms in the eastern portion of the gyre may tend to be directed towards Japan, potentially bringing impacts to the major shipping lanes off Japan and to the Olympics.  Meantime, FleetWeather will continue to monitor this portion of the world closely for potential further tropical cyclone development.

Stay connected and safe.

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