Most common in 呼啸 (to whistle past, used of wind, bullets, vehicles) and 咆哮-like roaring contexts. 长啸 is a literary 'long howl', often a hero or beast venting emotion.
Left mouth radical, marking this as a sound made through the mouth - a sharp whistle or piercing cry. It puts 啸 in the vocal-sound family alongside 叫 and 喊.
Right 肃 supplies the sound - sù drifting to xiào, a wide shift typical of older phonetic series. It contributes no meaning here; the simplified shape contracts the fuller traditional form on the right of 嘯.
