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A cold wind blows fiercely every winter.
Literal
Cold wind [subject-が] every-winter [at-に] fiercely blows.
冬ごとに ('every winter') uses the ごとに suffix attached to a noun to mean 'every X, at each X.' A productive pattern: 日ごとに (every day), 週ごとに (every week). 激しく is the く-adverbial form of 激しい ('fierce, intense, violent'). The sentence reads as a description of recurring seasonal phenomenon.