North Pacific Subtropical Convergence Zone

The North Pacific Gyre (NPG) or North Pacific Subtropical Gyre (NPSG), located in the northern

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

, is one of the five major

oceanic gyre In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determi ...

s. This gyre covers most of the northern

. It is the largest ecosystem on Earth, located between the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

and 50° N latitude, and comprising 20 million square kilometers. The gyre has a clockwise circular pattern and is formed by four prevailing

ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth conto ...

s: the North Pacific Current to the north, the

California Current The California Current is a cold water Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. It is considered an Eastern bound ...

to the east, the North Equatorial Current to the south, and the

Kuroshio Current The , also known as the Black or or the is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...

to the west. It is the site of an unusually intense collection of human-created

marine debris Marine debris, also known as marine litter, is human-created waste that has deliberately or accidentally been released in a sea or ocean. Floating oceanic debris tends to accumulate at the center of gyres and on coastlines, frequently washing ...

, known as the Great Pacific Garbage Patch. The North Pacific Subtropical Gyre and the much smaller North Pacific Subpolar Gyre make up the two major gyre systems in the mid-latitudes of the Northern Pacific Ocean. This two-gyre circulation in the North Pacific is driven by the

trade Trade involves the transfer of goods and services from one person or entity to another, often in exchange for money. Economists refer to a system or network that allows trade as a market. An early form of trade, barter, saw the direct exc ...

and westerly winds. This is one of the best examples of all of Earth’s oceans where these winds drive a two-gyre circulation. Physical characteristics like weak

thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...

in the North Pacific and the fact that it is mostly blocked by land in the north, also help facilitate this circulation. As depth increases, these gyres in the North Pacific grow smaller and weaker, and the

high pressure In science and engineering the study of high pressure examines its effects on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure. By ''high pressure'' is usually meant pressures of th ...

at the center of the Subtropical Gyre will migrate poleward and westward.

Physical oceanography

Subtropical circulation in the North Pacific

Like all subtropical gyre systems, the North Pacific Subtropical Gyre is an

anticyclone An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from ...

meaning the circulation is in a clockwise direction around its high pressure at the center because of its placement in the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

. This circulation is also associated with equatorward

Sverdrup In oceanography, the sverdrup (symbol: Sv) is a non- SI metric unit of volumetric flow rate, with equal to . It is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1): 1 Sv is equal to 1 hm3/s. It is u ...

transport and Ekman downwelling. Ekman transport causes water to flow toward the center of the gyre, creating a sloped sea-surface, and initiating

geostrophic flow A geostrophic current is an oceanic current in which the pressure gradient force is balanced by the Coriolis effect. The direction of geostrophic flow is parallel to the isobars, with the high pressure to the right of the flow in the Norther ...

. Harald Sverdrup applied Ekman transport while including pressure gradient forces to develop a theory for Sverdrup transport. The

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 Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nutr ...

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 The subtropical countercurrent (STCC) is a narrow eastward ocean current in the central North Pacific Ocean (20–30°N) where the Sverdrup theory predicts a broad westward flow. It originates in the western North Pacific around 20°N, and flows ...

at roughly 20–25⁰N, then finally the "C" wraps back towards the west forming the North Equatorial Current just south of 20⁰N. 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 42⁰N, is fixed in the North Pacific Current. The Subarctic Frontal Zone, slightly south of the maximum westerly wind speeds, separates the North Pacific Subpolar Gyre from the Subtropical Gyre. In the central and eastern Pacific at roughly 32⁰N is the Subtropical Frontal Zone. Sometimes referred to as the Subtropical Convergence Zone, this frontal zone serves as the boundary between the west flowing North Equatorial Current from the North Pacific Current. With increasing depth in the North Pacific Subtropical Gyre, it gets smaller in the western region near Japan and it also loses strength. The Subtropical Gyre does not exist below 1500 m below the surface with the exception of the Kuroshio Current and Extension regions.

Circulation dependence on depth

The North Pacific Subtropical Gyre diminishes spatially with increasing depth. Similar to all subtropical gyre systems, the North Pacific Subtropical Gyre shrinks towards its most energetic surface flows, in a northwestern direction between the Kuroshio Current and the Kuroshio Extension. This is drastic shrinkage from the surface to about 200 m below. At the surface, the boundary that separates the westward and eastward flows from south of 20⁰N to about 25–30⁰N 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

References

External links

Ocean currents
{{Ocean Pacific Ocean Oceanic gyres ru:Северное Тихоокеанское течение sv:Nordpacifiska strömmen