Wave
A tsunami hits your skin and creates smaller waves. Those hit you too!
If you’ve ever been to the ocean or a large lake, you know what kind of forces even small waves can hit with. Now imagine the force with which a tsunami wave would strike you. You’d have the wind knocked out of you for sure.
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When each wave hits your body, you shout, "Ha (は)!" It's not like it's on purpose. When the wave crashes into you, the wind gets knocked right out of your mouth, making a HA sound.
The waves are basically punching you in the gut, forcing the air out of your lungs, and making you shout HA!