From AMS Glossary
(Rare.) The categorizing of one year's weather at a given locality according to one of the climatic classifications.
Thus, if the weather of a given year at a certain station was unusually mild and rainy, falling within (for example) Köppen's definition of a tropical climate, that station is said to have had a "tropical year," whatever its normal classification.
Köppen, W. P., and R. Geiger 1930–1939. Handbuch der Klimatologie. Berlin: Gebruder Borntraeger, 6 vols.