Explain how surface currents are driven by wind and Earth's rotation and how deep currents are driven by density differences, and describe their effect on regional climate.
Ocean water is never still. Near the surface, steady winds drag the water along with them, building broad rivers of moving water called surface currents. Earth's rotation bends the path of these currents, called the Coriolis effect, curving them clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere instead of letting them flow in a straight line.
Deep below the surface, a different force drives water: density. Colder, saltier water is denser and sinks, while warmer, less salty water is less dense and stays higher, and this density difference drives a slow, deep circulation called thermohaline circulation that connects every ocean basin on Earth. Currents matter far beyond the ocean itself: the warm Gulf Stream keeps Western Europe milder than its latitude would suggest, and the cool California Current keeps the California coast noticeably cooler than places inland at the same latitude.
Mastery looks like: They can explain the difference between wind-and-rotation-driven surface currents and density-driven deep currents, and can name a real climate effect of at least one current.
Common stumbles: Assuming all currents are driven by wind alone; forgetting that density depends on both temperature and saltiness, not just one or the other.