In the first year of my PhD, I took a Physical Oceanography class that introduced me to the idea of Global Overturning Circulation. Up until this point, my mental image of the ocean was that it was mixed like a soup. But as it turns out, winds and storms only reach to about 500 m, only scraping the surface of the ~4000 m deep ocean! Below the mixed layer, water is divided into “water masses” that stack on top of each other like iced tea and lemonade in an Arnold Palmer drink. These water masses circulate based on their densities but very, very slowly. Some of the “oldest” water in the ocean hasn’t seen the surface in over 1000 years.
