Tsunami is Japanese, from tsu, "harbour," plus nami, "waves" — a harbour wave, named for where the effect is felt rather than for what causes it. English took the word in 1896, in reference to the wave that struck Japan on 15 June of that year.
The initial ts- is the interesting part for English spelling. Japanese has that cluster; English does not use it at the start of a word, and most English speakers simply say "sunami," dropping the t exactly as they drop the p in psalm and the m in mnemonic. It is the same accommodation English has always made to borrowed clusters it cannot manage: keep the letters, lose the sound. Otherwise the word shows the usual clean Japanese syllable structure that lets such borrowings enter English unaltered.
tsunami (n.) "large wave which rolls over and inundates the land," 1896, in reference to the one that struck Japan that year on June 15, from Japanese tsunami, from tsu "harbor" + nami "waves." also from 1896
tsunami came in through Later and wider borrowings, and its spelling follows that family's conventions.
How that family writes these sounds
| no consonant clusters, every syllable open | Japanese: write it exactly as it soundstsunami, sushi, karaoke, futon |
What gives it away on the page
| no consonant clusters, every syllable open | Japanese.tsunami, sushi, karaoke, futon, tycoon |
Later and wider borrowings in the spelling guide · guess another word's family