The Ross Gyre and Weddell Gyre are two gyres that exist within the Southern Ocean. Other smaller gyres are found in lesser seas and a single gyre flows around Antarctica. North Atlantic Deep Water flows southward below the thermocline of the subtropical gyre. The South Atlantic is dominated by the anti-cyclonic southern subtropical gyre. The NPC forms the northward portion of the North Pacific Subtropical Gyre. It is the southern side of a clockwise subtropical gyre. The southern subtropical gyre is partly masked by a wind-induced Ekman layer. In the subtropical regions, flow is weak where influences from the Subtropical Gyre are minimal. These cause frictional surface currents towards the latitude at the center of the gyre. The center of a subtropical gyre is a high pressure zone. It is a strong western boundary current and part of the North Pacific ocean gyre. The optical properties of the South Pacific Gyre remain largely unexplored. They specifically indicated the North Pacific Gyre. In the center of the South Pacific Gyre, the sedimentation rate is per million years. This water becomes the Agulhas Return Current, rejoining the Indian Ocean Gyre.