The modern ocean is comprised of four units: an equatorial belt shared by the two hemispheres, tropical-subtropical anticyclonic gyres, mid-latitude belts of water with steep meridional temperature gradients, and polar oceans characterized by cyclonic gyres. These units are separated by lines of convergence, or fronts: subequatorial, subtropical, and polar. Convergence and divergence of the ocean waters are forced beneath zonal (latitude-parallel) winds. The Early Cretaceous ocean closely resembled the modern ocean. The developing Atlantic was analogous to the modern Mediterranean and served as an Intermediate Water source for the Pacific. Because sea-ice formed seasonally in the Early Cretaceous polar seas, deep water formation probably took place largely in the polar region. In the Late Cretaceous, the high latitudes were warm and deep waters moved from the equatorial region toward the poles, enhancing the ocean's capacity to transport heat poleward. The contrast between surface gyre waters and intermediate waters was less, making them easier to upwell, but because their residence time in the oxygen minimum was less and they contained less nutrients. By analogy to the modern ocean, ”Tethyan” refers to the oceans between the subtropical convergences and ”Boreal” refers to the ocean poleward of the subtropical convergences.