A word for currency's ups and downs (5)

I believe the answer is:

tidal

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If this is the answer I cannot explain how the clue works.

(Other definitions for tidal that I've seen before include "Concerning lunar-driven ocean movements" , "Affected by changes in sea level" , "Relating to sea rise/fall" , "Going back and forth" , "(Waters) subject to moon/sun" .)

I've seen this clue in The Globe and Mail.

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