How Does a Hurricane Become "Annular"? (Credit: Japan Meteorological Agency)

​During the lifespan of a tropical cyclone, the system will go through what is known as an eyewall replacement cycle (ERC). They naturally occur when a tropical cyclone reaches major hurricane strength with winds of 115 mph or greater. When this happens, the rainbands outside of the main eyewall may strengthen and organize into a ring of thunderstorms. This is known as the outer eyewall. This newly formed eyewall moves slowly inward towards the inner eyewall and begins to take a toll on the tropical cyclones dynamics. It robs the inner eyewall of its moisture and angular momentum.

In intense tropical cyclones, the strongest winds are located inside the main eyewall. As the outer eyewall takes control, it will “choke” out the inner eyewall and eventually replace it. This tends to weaken the tropical cyclone as it organizes with its newly formed eyewall. Once the ERC is complete, the storm will have a new smaller eyewall and may re-intensify depending on the environment it is propagating towards.

However, sometimes an ERC can lead to a hurricane forming a large annular eyewall. This is when an eyewall is surrounded by a thick and organized ring of intense thunderstorm activity, but the storm lacks a few discrete convective features outside of the eyewall such as rainbands. This tends to give the cyclone a truck tire or donut shaped look as the eye expands, becomes larger and more asymmetric. Unlike typical intense tropical cyclones, this type of storm is not prone to the fluctuations in strength that comes with an eyewall replacement cycle. Annular tropical cyclones tend to hold their peak intensities for a longer period than do most intense tropical cyclones. 

Typhoon Soulik for example, a tropical cyclone in the western Pacific basin, just recently underwent this process before tracking through the southern Japanese islands. A tropical cyclone on average has an eyewall that is approximately 20-40 miles wide across. According to phys.org, Typhoon Souliks eye was between 40-50 nautical miles wide. Another example of an annular tropical cyclone was hurricane Isabel back in the 2002 Atlantic hurricane season. Hurricane Isabel displayed annular characteristics and had an eye that spanned about 40 nautical miles. 

The most recent update on Typhoon Soulik states that it is a category 3 typhoon with a centralized pressure of 950 mb and maximum sustained winds of 110 mph. The Japanese Meteorological Agency has Soulik continuing its northwest track before making a northeast turn towards the Korean Peninsula. Soulik is expected to maintain category 3 strength through Wednesday, but then start to weaken rapidly as it enters an unfavorable atmospheric environment. With that said, even when it makes landfall as a tropical storm, it will still be capable of producing flooding rains, damaging wind gusts and dangerous coastal conditions across the peninsula through Friday. Below is the projected path of Typhoon Soulik provided by the Japan Meteorological Agency.

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© 2018 Meteorologist Joseph Marino