https://phys.org/news/2018-09-nasa-noaa-satellite-barijat-gulf-tonkin.html

In general, wind shear is a measure of how the speed and direction of winds change with altitude. Winds at different levels of the atmosphere pushed against the cylindrical circulation center and skewed it, weakening the rotation.

Barijat weakened from a tropical storm to a tropical depression today, Sept. 13.