Subtropical circulation in the North Pacific Like all subtropical gyre systems, the North Pacific Subtropical Gyre is an
anticyclone; since it is in the
Northern Hemisphere this means its circulation is in a clockwise direction around its high pressure at the center. This circulation is also associated with equatorward
Sverdrup transport and
Ekman downwelling. The
Kuroshio Current is the narrow, strong westward boundary current of the subtropical circulation. This current influences the water column all the way to the bottom. The Kuroshio current flows in a northerly direction, then eventually flows further from the westward boundary where it then takes an eastward direction into the North Pacific. This eastward flowing current is then called the Kuroshio Extension. The
North Pacific Current is located just north of the Subtropical Gyre and flows in an easterly direction. Also, known as the West Wind Drift or the Subarctic Current, the North Pacific Current also includes the westward flow of the southern boundary of the North Pacific Subpolar Gyre. The
North Equatorial Current borders the North Pacific Subtropical Gyre on the south and flows in a westerly direction. The westward flow within the elongated tropical
cyclonic circulation is also included in the North Equatorial current. The
California Current System comprises the eastern boundary of the North Pacific Subtropical Gyre and flows south along the coast of California. Here
coastal upwelling drives the eastern boundary current and an undercurrent that flows poleward. In the western region of the North Pacific, the surface of the Subtropical Gyre generally has a "C-shape". The Kuroshio current and Kuroshio Extension roughly from the outside of this "C-shape" where it then turns westwards into recirculation, where it then flows south parallel to the Kuroshio Current. From here the "C-shape" then flows eastward comprising the
Subtropical Countercurrent at roughly 20–25°N, then finally the "C" wraps back towards the west forming the North Equatorial Current just south of 20°N. It is common for subtropical gyres to have this "C-shape" surface flow. The Subtropical Countercurrent is a shallow area of this "C"; at only about 250 dbar under the surface, circulation is a simpler closed, anticyclonic gyre. Narrow east-west frontal zones that cross the Pacific are less than 100 km wide. The Subarctic Frontal Zone or Subarctic Boundary, about 42°N, is fixed in the North Pacific Current. At the surface, the boundary that separates the westward and eastward flows from south of 20°N to about 25–30°N at 200 m. The "C-shape" in the western region of the Subtropical Gyre, including the Subtropical Countercurrent, generally does not exist below 200 m. At about 1000–1500 m, the Subtropical Gyre is located entirely in the western region of the North Pacific near the Kuroshio Current and Kuroshio Extension. In the subtropical regions, flow is weak where influences from the Subtropical Gyre are minimal. Differences in steric heights over distances of 1000 km are on the order of 1 cm, rather than the differences of 10 cm within the area of the North Pacific Subtropical Gyre. == Garbage patch ==