In a manner relating to decades or occurring once every ten years.
From 'decadal' + '-ly' (adverbial suffix). This adverb form takes the adjective 'decadal' and adds the standard English ending to create an adverbial form describing how something happens on a decadal timescale.
While 'decadally' sounds formal, it's actually useful in scientific writing when discussing things that happen roughly every ten years rather than annually or monthly—it's the Goldilocks adverb for describing medium-long-term patterns.
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