Equatorial Counter Current
The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of 100–150 metres (330–490 ft) in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at about 3-10°N in the
In the Indian Ocean, circulation is dominated by the impact of the reversing Asian monsoon winds. As such, the current tends to reverse hemispheres seasonally in that basin. [1] The NECC has a pronounced seasonal cycle in the Atlantic and Pacific, reaching maximum strength in late boreal summer and fall and minimum strength in late boreal winter and spring. Furthermore, the NECC in the Atlantic disappears in late winter and early spring.[2]
The NECC is an interesting case because while it results from wind-driven circulation, it transports water against the mean westward wind stress in the tropics. This apparent paradox is concisely explained by Sverdrup theory, which shows that the east-west transport is governed by the north-south change in the curl of the wind stress.[3]
The Pacific NECC is also known to be stronger during warm episodes of the
There is also a South Equatorial Countercurrent (SECC) that transports water from west to east in the Pacific and Atlantic basins between 2°S and 5°S in the western basin and farther south toward the east.
Theoretical background
The NECC is a direct response to the meridional changes in the
Sverdrup theory succinctly summarizes this phenomenon mathematically by defining a geostrophic mass transport per unit latitude, M, as the east-west integral of the meridional derivative of wind stress curl, minus any Ekman transport. The Ekman transport into the current is typically negligible, at least in the Pacific NECC. The total NECC is found by simply integrating M over the relevant latitudes.[8]
Atlantic North Equatorial Countercurrent
The Atlantic NECC consists of the eastward zonal transport of water between 3°N and 9°N, with typical widths on the order of 300 km. The Atlantic NECC is unique among the equatorial currents in that basin because of its extreme seasonality. The maximum eastward flow is attained in late boreal summer and fall while the countercurrent is replaced by westward flow in late winter and spring. The NECC has maximum transport of approximately 40 Sv (10^6 m3/s) at 38°W. Transport reaches 30 Sv two months per year at 44°W, while farther east at 38°W the transport reaches that level five months per year. The magnitude of the NECC weakens substantially east of 38°W due to water being absorbed by the westward equatorial current south of 3°N.[9]
While the variability of the Atlantic NECC is dominated by the annual cycle (weak late winter, strong late summer), there is also interannual variability as well. The strength of the Atlantic NECC is notably stronger in years following El Niño in the tropical Pacific, with 1983 and 1987 being notable examples.[10] Physically, this implies that the altered convection in the Pacific Ocean due to El Niño drives changes in the meridional gradient of wind stress curl over the equatorial Atlantic.
Pacific North Equatorial Countercurrent
The Pacific NECC is a major eastward moving surface current that transports more than 20 Sv from the West Pacific warm pool to the cooler east Pacific. In the western Pacific the countercurrent is centred near 5°N while in the central Pacific it is located near 7°N.[11]
At the surface, the current is located on the southern slope of the North Equatorial Trough, a region of low sea level which extends from east to west across the Pacific. The low sea level is a result of Ekman suction caused by the increased easterly winds found just to the north of the Intertropical Convergence Zone (ITCZ). In the western basin, the NECC may merge with the Equatorial Undercurrent (EUC) below the surface. Generally, the current weakens to the east in the basin, with estimated flows of 21 Sv, 14.2 Sv, and 12 Sv in the western, central, and eastern Pacific, respectively.[12]
Like the Atlantic NECC, the Pacific NECC undergoes an annual cycle. This is a result of the annual Rossby wave. [13] Early each year increased winds in the eastern Pacific generate a region of lower sea level. Over the following months this propagates westward as an oceanic Rossby wave. Its fastest component, near 6°N, reaches the western Pacific around mid-summer. At higher latitudes the wave travels more slowly. As a result in the western Pacific the NECC tends to be weaker than normal in the boral winter and spring, and stronger than normal in the summer and autumn. [14]
Fluctuations of the Pacific NECC with El Niño
The Pacific NECC is known to be stronger during classic El Niño events, when there is an anomalous warming of the eastern and central Pacific that peaks in the boreal winter. Klaus Wyrtki was the first to report the connection, in the early 1970s, based on analysis of tide-gauge measurements at Pacific island stations on either side of the current. On the basis of this analysis, Wyrtki hypothesized that such an unusually strong NECC in the western Pacific would lead to an anomalous accumulation of warm water of the coast of Central America and thus an El Niño. [4]
See also
- Coral bleaching
- Ocean current
- Oceanic gyres
- Physical oceanography
- Walker circulation
- Humboldt Current
Notes
- ^
Wyrtki, Klaus (1973). "An Equatorial Jet in the Indian Ocean". Science. 181 (4096): 262–264. S2CID 2931890.
- ^ Carton and Katz, 1990
- ^ Yu et al., 2000
- ^ a b
Wyrtki, Klaus (1973). "Teleconnections in the equatorial Pacific Ocean". Science. 180 (4081): 66–68. S2CID 187575.
- ^ Reid, Jun., 1959
- ^ Stramma, 1991
- ^ Wyrtki, Klaus (1974). "Equatorial Currents in the Pacific 1950 to 1970 and their relations to the trade winds". Journal of Physical Oceanography. 4 (3): 372–380. .
- ^ Yu et al., 2000
- ^ Carton and Katz, 1990
- ^ Katz, 1992
- ^ Yu et al., 2000
- ^ Yu et al., 2000
- ^ Myers, G. (1979). "On the annual Rossby wave in the Tropical North Pacific Ocean". Journal of Physical Oceanography. 9 (4): 663–674. .
- ^ Wyrtki, Klaus (1974). "Sea level and the seasonal fluctuations of the equatorial currents in the western Pacific Ocean". Journal of Physical Oceanography. 4 (1): 91–103. .
References
- Carton, J. and E. Katz, 1990. "Estimates of the Zonal Slope and Seasonal Transport of the Atlantic North Equatorial Countercurrent[permanent dead link]." Journal of Geophysical Research, Vol. 95, 3091-3100.
- Katz, E., 1992. "An Interannual Study of the Atlantic North Equatorial Countercurrent Archived 2017-08-21 at the Wayback Machine." Journal of Physical Oceanography, Vol. 23, 116-123.
- Reid, Jun.,J., 1959. "Evidence of a South Equatorial Countercurrent in the Pacific Ocean." Nature, Vol. 184, 209-210.
- Stramma, L., 1991. "Geostrophic transport of the South Equatorial Current in the Atlantic." Journal of Marine Research, Vol. 49, 281-294.
- Wyrtki, K., 1974. "Equatorial Currents in the Pacific 1950 to 1970 and Their Relations to the Trade Winds." J. Phys. Oceanography, Vol. 4, 372-380.
- Wyrtki, K., 1973. "Teleconnections in the Equatorial Pacific Ocean." Science, Vol. 180, 66-68.
- Wyrtki, K., 1973. "An Equatorial Jet in the Indian Ocean." Science, Vol. 181, 262-264.
- Yu,et al., 2000. "Influence of Equatorial Dynamics on the Pacific North Equatorial Countercurrent." J. Phys. Oceanography, Vol. 30, 3179-3190.