Westerly is both an adjective and an adverb, that makes the noun or verb in relation to West; the precise meaning as an adverb depends upon the verb. West is usually a noun, can also be an adjective. The context is essential in this case.

West describes place, but westerly describes motion. Compare: Oregon is a state in the US's northwest. vs. The tropical storm moved in a westerly direction toward the peninsula.

West is a location, a noun. Westerly is a movement in the direction of the West.

Westerlies = winds from the west

West as a adjective we can use before a noun (for example west London), as a adverb we use to ot towards the west ( for example to travel west, we live west of the city), as we use as a noun it is a direction you look towards in order to see the sun of down, one of the four pionts of the compass (There's a road to the west of here), the part of aby country, city etc. I I live in the west of Germany) Westerly is a adjective and we use only before a noun; to, towards or in the west (in a westerly direction. We can also use it about winds which comes from the west.