Final answer:
El Niño is the ocean oscillation process that causes an unusually warm season in the Northern United States.
Step-by-step explanation:
If the Northern United States experiences an unusually warm season, the ocean oscillation process that is likely taking place is El Niño. El Niño is a climate pattern that occurs in the Pacific Ocean and affects weather patterns across the globe. During El Niño, warm waters from the equatorial Pacific flow towards the western coast of the Americas, leading to changes in atmospheric circulation and temperature patterns.
Learn more about El Niño