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The North Equatorial Current (NEC) is a westward wind-driven current mostly located near the equator, but the location varies from different oceans. The NEC in the
Pacific 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 contine ...
and the
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
is about 5°-20°N, while the NEC in the
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
is very close to the equator. It ranges from the sea surface down to 400 m in the western Pacific. The NEC is driven by the north-hemisphere easterly trade wind. In couple with NEC, there is another current called
South Equatorial Current The South Equatorial Current are ocean currents in the Pacific, Atlantic, and Indian Ocean that flow east-to-west between the equator and about 20 degrees south. In the Pacific and Atlantic Oceans, it extends across the equator to about 5 degr ...
( SEC), generated by the easterly trade wind in the southern hemisphere. Despite the well-coupled name of the two equatorial currents, the distribution of the NEC and the SEC is not in symmetry at the equator, but slightly northward to the equator. This asymmetric distribution is aligned to the location of the Intertropical Convergence Zone (ITCZ), which is the area that the northeast and the southeast trade wind converge.


Related processes


The Equatorial Counter Current

The NEC and the SEC will generate an
Equatorial Counter Current The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at ...
( ECC), named as the North Equatorial Counter Current (NECC) in both Pacific and Atlantic and the South Equatorial Counter Current (SECC) in the Indian Ocean. The NEC and SEC continuously flow westward. However, the seawater does not just pile up at the west basin surface. The advent water must have gone back to the east by some means. The Sverdrup balance can partly explain where the water ends up at. When the NEC and the SEC reached the west end of a basin, some of the water travels poleward to join the low-latitude circulations, while some travels equatorward to join the
Equatorial Counter Current The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at ...
.


The Ekman transport

The Ekman transport is a wind-driven transport. It occurs due to the rotation of the globe. A transport is found to the right of the flow direction in the northern hemisphere, while to the left of the flow in the southern hemisphere. It is noteworthy that in the tropical regions, where NEC and SEC both flow to the west, a northward Ekman transport in the NEC and a southward Ekman transport in the SEC take place. Due to the fact that the Ekman transport is perpendicular to the flow itself, these Ekman transports contribute to the meridional branch of the NEC and SEC. However, the magnitude of the meridional component is of no comparison to the current itself. Another subsequent result of the Ekman transport is the
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 nut ...
, which occurs in between the NEC and the SEC, where a massive water divergence at the sea surface takes place.


Interaction with climate

The NEC, the SEC and the ECC play an important role in the climate system causing various of climate patterns, such as El Niño–Southern Oscillation (ENSO), the
Atlantic Meridional Mode The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe a ...
(AMM), the Atlantic Multidecadal Oscillation (AMO) and the seasonal monsoon in the Indian ocean. Reversely, the climate motion also affects the behavior of the equatorial current itself.


In different oceans


Pacific NEC

The NEC is evident around 10°-18°N across the entire Pacific basin, from the Philippines to Nicaragua. Its typical zonal velocity is 30 cm\cdot s^. The NEC shows little seasonal variability, but an interannual instability. The interannual instability of the NEC is strongly linked to ENSO. The NEC strengthens in La Niña years and weakens in
El Niño El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date ...
years. The meridional component of the NEC, also known as the Ekman transport, is evident northward at any location all the way along with itself. When the current reaches the west end, the Philippines, it splits into two western boundary flows. One of the branches flows poleward feeding 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 ...
, another one flows equatorward feeding the Mindanao Current. This North Equatorial Current Bifurcation (NECB) plays an important role in the south Asian climate system. As currently, climate change is more and more evident, thus, leading to a more amplified migration of the NECB. As a result, this amplification of the migration may lead to redistribution of the water mass and heat transport along the western boundary, and thus warm pool and monsoon climate.


Atlantic NEC

The NEC in the Atlantic is evident around 10°-20°N, spanning the longitude from 16°-60°W. The typical flow velocity is about 10 cm\cdot s^, lower than in the Pacific. Instead of interannual variability, the NEC shows a strong seasonality, in which NECC is stronger from July to December, weaker from January to June. Besides, the NEC is more equatorward from January to June. The NEC splits into two after reaching the north of South America, joining the North Brazil Current (NBC) and the NECC respectively. The northward meridional Ekman transport dominates the tropical Atlantic Ocean, playing a very important role in the northward heat transport. This strong northward surface transport is well known as the upper component of the Atlantic Meridional Overturning Circulation (AMOC). On a seasonal time scale, variability of the heat transport is responsible for the tropical sea temperature anomaly. The temperature anomaly at the sea surface is a possible cause that leads to Atlantic hurricane season. On the interannual and longer timescales, the equatorial and tropical Atlantic ocean has a strong interaction with the dynamics of several patterns of variabilities, the Atlantic Niño, the Atlantic Meridional Mode (AMM) and the Atlantic Multidecadal Oscillation (AMO).


The Indian Ocean NEC

The NEC in the Indian ocean is strongly affected by the continent to the north. The NEC is more southward than the other two oceans, which drives the
Equatorial Counter Current The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at ...
to the southern hemisphere. So, the counter current is called South Equatorial Counter Current (SECC) here. The NEC sits right on the equator, across a longitude from 45°-100°E. The typical speed in the winter can reach up to 50 cm\cdot s^, thanks to the northeast seasonal wind from the continent. In the Indian Ocean, the NEC is faster than the SEC. Multiple reasons are considered. The equator-located NEC receives more solar heat than the more poleward-located SEC, which leads to a much denser but thinner upper layer for the NEC. The current flows faster in the thinner layer. Another reason is due to the smaller Coriolis effect at the equator. The NEC, thus, is more aligned to the westward wind on the equator. The NEC shows a very strong seasonal pattern. During January and February, thanks to the prevailing northeast wind, the NEC travels all the way to the east coast of Somalia and joins the
Somali Current The Somali Current is a warm ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean. This current is heavily influenced by the monsoons and is th ...
flowing towards the southwest to feed the SECC. As a result, SECC is strong during the winter. And at this time, the NEC carries surface waters from the southern Bay of Bengal to the southern
Arabian Sea The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channel ...
. While during July and August, the location of the NEC moves southward and the Somali Current reverses. As a result, the NEC and the SEC feed Somali Current instead of the SECC. So, the SECC becomes very weak. Due to the prevailing southwest wind in the summer, the surface waters move from the southern
Arabian Sea The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channel ...
to the southern Bay of Bengal. During the transition of these two phases, specifically around May and November, the NEC becomes very weak, almost invisible in Figure 3. Instead of the NEC, a strong eastward current is found near the equator, known as the Wyrtki jets.


References


See also

* Ocean current *
Oceanic gyres 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 determine ...
* Physical oceanography *
Equatorial Counter Current The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at ...
{{Marine-current-stub Currents of the Atlantic Ocean Currents of the Pacific Ocean