The motion of Typhoon In-fa or Fabian made a hard 90 degree left turn and started heading towards Taiwan, slowed to 5 km/hr before reaching it, then took a hard 90 degree right turn and proceeded along its original direction.

Question: What is it that causes these apparently abrupt 90 degree turns?

Below captured from Central Weather Bureau. The original image dated 2021-07-26 06:00 (UTC I assume).

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My first observation upon looking at the picture was that Typhoon In-fa (the southernmost one in the picture) nearly stalled prior to making the right angled turns in its path. There is a clustering of very closely spaced location markers for the dates 19 and 20 July, prior to the first 90 degree turn and then again for the dates 22 to 24 July at the time of the second 90 degree turn.

The the first paragraph of the Wikipedia article you refer to states,

... on July 17, being assigned the name In-fa by the Japan Meteorological Agency. Located in a week steering environment, the system struggled to organize under dry air and moderate wind shear before organizing further. It continued to move mostly westward, strengthening into a typhoon and deepening quickly. The storm struggled to organize itself significantly due to continuous dry air intrusions.

The emphasis is mine.

Tropical cyclones (typhoons, hurricanes) thrive when surface sea temperatures are very warm and the air is humid. Encountering dry air will disrupt a typhoon, which is what happened with In-fa.

... The large size of In-fa, with the continuous dry air entrainment, together restrained the system's rapid development over the warm waters of the Philippine Sea; however, as the storm continued its trajectory towards the Ryukyu Islands, the curved banding of the system further consolidated ...

... At the same time, the system began to take a northwest track before moving west-northwestwards under a subtropical ridge over Japan by the early hours of July 19. ...

... The system, decelerates again as it started to impact the Ryukyu's, with In-fa's convection or thunderstorms deepening and an eye further appearing. ...

As it passed south of Daito Islands, a 35 nautical mile (65 km; 40 mi) eye was further evident on satellite imageries, which later shrinked; however, In-fa remained a high-end Category 1 system as it started to move westwards and later, a west-southwest track. ...

... on July 21 with winds of 175 km/h (110 mph); this made the system's peak intensity according to the JTWC's estimates.

... another satellite imagery revealed an eyewall that was disrupted due to continuous dry air intrusions while moving westwards. Later, as it shifted its motion west-southwestwards, In-fa's eye became ragged due to another pack of dry air. Furthermore, dry northerlies suppressed the core of the system as it dived southwards. ...

... In-fa started an eyewall replacement cycle while its northern quadrant started to weaken as it changed its motion yet again to the south-southwest. In addition, the system is being steered by the southern periphery of a high-amplitude ridge over the Sea of Japan. Six hours later, the typhoon started to weaken with its eye expanding and weakening banding on the system while tracking westward and further, northwestward at an slow motion rate. Its steering movements were caused by a subtropical ridge to the north and further, another one over the east coast of North Korea. As it continued to approach the Ryukyus while traveling from the northwest, In-fa further weakened to a high-level Category 1 system due to continuous up welling, its suppressed outflow and slowly cooling sea surface temperatures.

In summary, the formation of the typhoon was periodically interrupted by the entrainment of dry air. This coupled with the subtropical ridge initially over Japan then to its north and another to the east of North Korea.


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