The extratropical transition of Typhoon Merbok
Typhoon Merbok became a very strong extratropical storm as it moved through the Bering Sea on 15 – 16 September. The animation above shows the development (in the eastern third of the domain) of the tropical cyclone and, starting later in the day on 14 September, its subsequent interaction and merger with a mid-latitude system that moves out over the Pacific Ocean from Asia. The deep red/orange region in the Himawari-8 airmass RGB is associated with strong descent in association with an intrusion of stratospheric air with higher potential vorticity. The potential vorticity structure of this system is discussed in more detail by Prof. Jon Martin (UW Department of Atmospheric and Oceanic Sciences) here at that Department’s weekly Weather Watch (starting at 44 minutes).
Himawari-8 data in this animation is courtesy JMA. Note in the animation the persistence of Typhoon Nanmadol. That typhoon has generated significant swell and the National Weather Service in Guam issued High Surf Advisories from Guam to Saipan. Also, the daily appearance of Keep-Out Zones (see a previous blog post on this subject; here is a NESDIS/OSPO site on the topic) is apparent on the western and eastern limbs of the globe.
Back in 2014, Typhoon Nuri also evolved into a very strong Bering Sea extratropical cyclone — albeit much later in the year! (Link)