The answer to your question would be that the group of words which is the adjective clause in the sentence is the following one: Which is very severe in the Arctic.
An adjective clause, also known as relative clause, is a dependent clause that modifies a noun or a pronoun. They can either be essential or nonessential depending on the type of the information they provide. The former ones provide information which is considered necessary to identify the word it modifies. The latter ones, just as the one in the example, add information which is not necessary to identify said noun. What is more, the meaning of the main sentence would not be altered if they were removed.