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42-479: In comparison to other major oceanic fronts created by the western boundary currents , the ABF is confined to a relatively narrow band of latitudes and is characterized by strong horizontal gradients in sea surface temperature and salinity. The ABF has a variable morphology, geographic location, and thermal characteristics.  It plays an important role for the southern African continent due to its close proximity to

84-1116: A Stream function ψ {\displaystyle \psi } and linearize by assuming that D >> h {\displaystyle D>>h} , equation (4) reduces to ∇ 2 ψ + α ( ∂ ψ ∂ x ) = γ sin ⁡ ( π y b ) ( 5 ) {\displaystyle \nabla ^{2}\psi +\alpha \left({\frac {\partial \psi }{\partial x}}\right)=\gamma \sin \left({\frac {\pi y}{b}}\right)\qquad (5)} Here α = ( D R ) ( ∂ f ∂ y ) {\displaystyle \alpha =\left({\frac {D}{R}}\right)\left({\frac {\partial f}{\partial y}}\right)} and γ = π F R b {\displaystyle \gamma ={\frac {\pi F}{Rb}}} The solutions of (5) with boundary condition that ψ {\displaystyle \psi } be constant on

126-465: A Benguela Niño, the Angola-Benguela front is abnormally displaced to a southern position, causing a reduced upwelling intensity at the coast and the advection of warm, highly saline water as far as 25°S. Two main forcing mechanisms responsible for this interannual variability of the Angola-Benguela frontal zone are considered but are still under debate. These are the local atmospheric forcing and

168-417: A closed circulation for an entire ocean basin and to counteract the wind-driven flow. Sverdrup introduced a potential vorticity argument to connect the net, interior flow of the oceans to the surface wind stress and the incited planetary vorticity perturbations. For instance, Ekman convergence in the sub-tropics (related to the existence of the trade winds in the tropics and the westerlies in the mid-latitudes)

210-594: A combination of factors. These include coastal orientation, bathymetry , movements of the South Atlantic Anticyclone, interaction between the south-flowing warm water of the Angola current and the north-flowing cold water of the Benguela current and the associated surface wind stress. However, Meeuwis and Lutjeharms (1990) concluded that the position of the front seems to be almost exclusively due to

252-1331: A linearized, frictional term to account for the dissipative effects that prevent the real ocean from accelerating. He starts, thus, from the steady-state momentum and continuity equations: f ( D + h ) v − F cos ⁡ ( π y b ) − R u − g ( D + h ) ∂ h ∂ x = 0 ( 1 ) {\displaystyle f(D+h)v-F\cos \left({\frac {\pi y}{b}}\right)-Ru-g(D+h){\frac {\partial h}{\partial x}}=0\qquad (1)} − f ( D + h ) u − R v − g ( D + h ) ∂ h ∂ y = 0 ( 2 ) {\displaystyle \quad -f(D+h)u-Rv-g(D+h){\frac {\partial h}{\partial y}}=0\qquad \qquad (2)} ∂ [ ( D + h ) u ] ∂ x + ∂ [ ( D + h ) v ] ∂ y = 0 ( 3 ) {\displaystyle \qquad \qquad {\frac {\partial [(D+h)u]}{\partial x}}+{\frac {\partial [(D+h)v]}{\partial y}}=0\qquad \qquad \qquad (3)} Here f {\displaystyle f}

294-465: A more realistic frictional term, while emphasizing "the lateral dissipation of eddy energy". In this way, not only did he reproduce Stommel's results, recreating thus the circulation of a western boundary current of an ocean gyre resembling the Gulf stream, but he also showed that sub-polar gyres should develop northward of the subtropical ones, spinning in the opposite direction. Observations indicate that

336-560: A simple, homogeneous, rectangular ocean model to examine the streamlines and surface height contours for an ocean at a non-rotating frame, an ocean characterized by a constant Coriolis parameter and finally, a real-case ocean basin with a latitudinally-varying Coriolis parameter. In this simple modeling the principal factors that were accounted for influencing the oceanic circulation were: In this, Stommel assumed an ocean of constant density and depth D + h {\displaystyle D+h} seeing ocean currents; he also introduced

378-498: Is a constant, ocean circulation has no preference toward intensification/acceleration near the western boundary. The streamlines exhibit a symmetric behavior in all directions, with the height contours demonstrating a nearly parallel relation to the streamlines, in a homogeneously rotating ocean. Finally, on a rotating sphere - the case where the Coriolis force is latitudinally variant, a distinct tendency for asymmetrical streamlines

420-458: Is found, with an intense clustering along the western coasts. Mathematically elegant figures within models of the distribution of streamlines and height contours in such an ocean if currents uniformly rotate can be found in the paper. The physics of western intensification can be understood through a mechanism that helps maintain the vortex balance along an ocean gyre. Harald Sverdrup was the first one, preceding Henry Stommel, to attempt to explain

462-497: Is most distinct, widest and has steeper meridional sea surface temperature (SST) gradients in austral summer (summer in the Southern Hemisphere), when it reaches its southernmost position. Whereas in austral winter it is less intense, and it reaches its northernmost position. The core of the ABF, which is considered as the region of steepest temperature gradients within the frontal zone, remains very steady throughout

SECTION 10

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504-461: Is the strength of the Coriolis force, R {\displaystyle R} is the bottom-friction coefficient, g {\displaystyle g\,\,} is gravity, and − F cos ⁡ ( π y b ) {\displaystyle -F\cos \left({\frac {\pi y}{b}}\right)} is the wind forcing. The wind is blowing towards

546-605: The Kuroshio Current . Low-latitude western boundary currents are similar to sub-tropical western boundary currents but carry cool water from the subtropics equatorward. Examples include the Mindanao Current and the North Brazil Current . Western intensification applies to the western arm of an oceanic current , particularly a large gyre in such a basin . The trade winds blow westward in

588-434: The ocean warming over the subtropical western boundary currents is two-to-three times stronger than the global mean surface ocean warming. A study finds that the enhanced warming may be attributed to an intensification and poleward shift of the western boundary currents as a side-effect of the widening Hadley circulation under global warming. These warming hotspots cause severe environmental and economic problems, such as

630-473: The ABF. The sharpest temperature gradients are found within 250 km of the coast. Multiple sharp fronts can also occur, especially when the Angola Current is strongest in austral summer. There are several assumptions about the most significant processes and driving forces controlling the development of the ABF. Many past studies suggest that the thermal characteristics of the front are influenced by

672-781: The area like the Angola Dome , and began to document the seasonal cycle of the ABF position. During the past 20 years the cooperation between Angola, Namibia and other countries in Europe and Africa has been greatly improved through different projects and collaborations like the Enhancing Prediction of Tropical Atlantic Climate and its Impacts (PREFACE) (November 2013 - April 2018) and the Benguela Current Commission (BCC). The objective of these joint research projects has been to investigate and monitor

714-537: The coast of Namibia, with positive SST anomalies reaching up to 4 °C. However, Benguela Niño events are less intense and less frequent than Pacific El Niños. They are observed with an interval of 7 to 11 years and are associated with a southward intrusion of warm and saline Angolan water into the northern Benguela. Benguela Niño tend to reach their maximum in late austral summer mainly during March–April. There have been major, well-documented Benguela Niño events in 1934, 1950, 1964, 1974, 1984, 1995, 1999 and 2010. During

756-410: The coast, having a significant impact on the local marine ecosystem and regional climate. Variability in position and intensity of the ABF has been suggested to affect local biology and thus fish stocks, as well as rainfall variability. The ABF was first named and described by Janke (1920) based on ship log data. However, consistent research on the front itself has only been conducted since the 1960s. It

798-415: The coastlines, and for different values of α {\displaystyle \alpha } , emphasize the role of the variation of the Coriolis parameter with latitude in inciting the strengthening of western boundary currents. Such currents are observed to be much faster, deeper, narrower and warmer than their eastern counterparts. For a non-rotating state (zero Coriolis parameter) and where that

840-474: The connection with the equatorial variability. On the one hand, some studies have shown that temperature and upwelling anomalies are caused by local wind changes related to the magnitude and location of South Atlantic Anticyclone . On the other hand, past studies indicated that, rather than being triggered by variation in local wind-stress, the Benguela Niño is associated with large-scale remote changes in

882-706: The eastern side of oceanic basins (adjacent to the western coasts of continents). Subtropical eastern boundary currents flow equatorward, transporting cold water from higher latitudes to lower latitudes; examples include the Benguela Current , the Canary Current , the Humboldt (Peru) Current , and the California Current . Coastal upwelling often brings nutrient-rich water into eastern boundary current regions, making them productive areas of

SECTION 20

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924-409: The front is oriented normal to the coast and stretches offshore in a west to north-westerly direction between 15 and 17°S. The front has an average width of about 200 km, but it can be much narrower at certain times, with steeper temperature gradients. The average distance the front penetrates seawards from the coast is 250 km, but traces can be found up to 1000 km offshore. The region of

966-454: The frontal zone appear to fluctuate out of phase: as the northern boundary moves southwards in the austral winter, the southern boundary is displaced northwards. The converse is true for austral summer. Apart from seasonal and mesoscale features, interannual fluctuations of the ABF are also significant and cause great temporal and spatial variability in the frontal zone.  Minor warm and cold interannual anomalies have been observed throughout

1008-404: The frontal zone was previously defined by a characteristic temperature gradient of between 1 °C per 28 km and 1 °C per 90 km. A more recent study calculated a meridional sea surface temperature gradient of 1 °C per 34 km (or 3 °C per 100 km) across the ABF in austral summer, whereas Colberg and Reason (2006) estimated ~4 °C per 100 km in the middle of

1050-406: The intensity of the ABF is tied to the strength of the meridional wind field which determines the coastal upwelling. However, even though the ABF is influenced by the intensity and location of the trade winds, the effect is not linear. The ABF is characterized by a typical seasonal cycle with meridional frontal movements and changes in the cross-thermal gradient. Previous studies found that the front

1092-497: The mid-ocean vorticity balance by looking at the relationship between surface wind forcings and the mass transport within the upper ocean layer. He assumed a geostrophic interior flow, while neglecting any frictional or viscosity effects and presuming that the circulation vanishes at some depth in the ocean. This prohibited the application of his theory to the western boundary currents, since some form of dissipative effect (bottom Ekman layer) would be later shown to be necessary to predict

1134-530: The ocean. Western boundary currents may themselves be divided into sub-tropical or low-latitude western boundary currents . Sub-tropical western boundary currents are warm, deep, narrow, and fast-flowing currents that form on the west side of ocean basins due to western intensification . They carry warm water from the tropics poleward. Examples include the Gulf Stream , the Agulhas Current , and

1176-418: The opposing flows of the Angola Current and Benguela system. An alternate hypothesis, proposed by Shannon and Nelson (1996), suggests that wind stress is the most important mechanism for the maintenance of the front. Kostianoy and Lutjeharms (1999) found that short term changes in the ABF are correlated to variations of the pressure gradient driven by the South Atlantic Anticyclone. In order to better understand

1218-496: The overlying atmospheric circulation. The strong anticyclonic wind stress curl of the region determines the motion of the South Equatorial Counter Current which causes the southward flow of the Angola Current. At the same time, the alongshore wind stress further to the south causes coastal upwelling resulting in the northward flow of the Benguela current. In the same study, Colberg and Reason found that

1260-420: The productivity and oceanographic processes and interactions within the region surrounding the ABF, aiming at improving the management of the fisheries and water resources. The physical properties of the ABF have been studied by historic hydrographic data, satellite-derived sea surface temperature observations, in situ measurements and by model-based studies. All of the findings are in general agreement that

1302-478: The record and appear to develop regularly in the ABF. However, a particular phenomenon and the most significant interannual signal that can be encountered in the frontal region is the Benguela Niño event. The inverse of a Benguela Niño is called Benguela Niña. Like the well-known El Niño phenomenon in the South Pacific, these events are characterised by an intense and unusual warming of the surface layer at

Angola–Benguela Front - Misplaced Pages Continue

1344-402: The sensitivity of the position and intensity of the ABF to atmospheric forcing, Colberg and Reason (2006) were the first to attempt to model the front.  They showed that the frontal position may be determined by the confluence of the northward and southward opposing flows, similar to what has been proposed by Meeuwis and Lutjeharms (1990). However, this confluence zone is primarily affected by

1386-404: The subtropical gyre. The opposite is applicable when Ekman divergence is induced, leading to Ekman absorption (suction) and a subsequent, water column stretching and poleward return flow, a characteristic of sub-polar gyres. This return flow, as shown by Stommel, occurs in a meridional current, concentrated near the western boundary of an ocean basin. To balance the vorticity source induced by

1428-447: The tropics. The westerlies blow eastward at mid-latitudes. This applies a stress to the ocean surface with a curl in north and south hemispheres, causing Sverdrup transport equatorward (toward the tropics). Because of conservation of mass and of potential vorticity , that transport is balanced by a narrow, intense poleward current, which flows along the western coast, allowing the vorticity introduced by coastal friction to balance

1470-869: The vorticity input of the wind. The reverse effect applies to the polar gyres – the sign of the wind stress curl and the direction of the resulting currents are reversed. The principal west-side currents (such as the Gulf Stream of the North Atlantic Ocean ) are stronger than those opposite (such as the California Current of the North Pacific Ocean ). The mechanics were made clear by the American oceanographer Henry Stommel . In 1948, Stommel published his key paper in Transactions, American Geophysical Union : "The Westward Intensification of Wind-Driven Ocean Currents", in which he used

1512-476: The west at y = 0 {\displaystyle y=0} and towards the east at y = b {\displaystyle y=b} . Acting on (1) with ∂ ∂ y {\displaystyle {\frac {\partial }{\partial y}}} and on (2) with ∂ ∂ x {\displaystyle {\frac {\partial }{\partial x}}} , subtracting, and then using (3), gives If we introduce

1554-424: The west coast of Africa as coastal trapped waves influencing the temperature variability. Western boundary currents Boundary currents are ocean currents with dynamics determined by the presence of a coastline , and fall into two distinct categories: western boundary currents and eastern boundary currents . Eastern boundary currents are relatively shallow, broad and slow-flowing. They are found on

1596-531: The wind patterns. More specifically, remote forcing is caused by a sudden relaxation of the trade winds in the western or central equatorial Atlantic. This generates equatorial Kelvin waves which propagate eastward along the Atlantic equator until the African coast where one part of their energy is reflected back to the west as equatorial Rossby waves . Another part of their energy is transmitted poleward along

1638-558: The wind stress forcing, Stommel introduced a linear frictional term in the Sverdrup equation, functioning as the vorticity sink. This bottom ocean, frictional drag on the horizontal flow allowed Stommel to theoretically predict a closed, basin-wide circulation, while demonstrating the west-ward intensification of wind-driven gyres and its attribution to the Coriolis variation with latitude (beta effect). Walter Munk (1950) further implemented Stommel's theory of western intensification by using

1680-479: The year and always lies between 17 and 15°S (mean location 16.4°S). Mean temperatures at the core of the frontal zone are 20.7 °C in austral summer and 18.0 °C in austral winter. The front exists between 15.5 and 17°S in the austral summer with more intense temperature gradients (~1 °C per 34 km), while in the austral winter it lies between 15.5 and 17°S with weaker temperature gradients (1 °C per 40 km). The northern and southern boundaries of

1722-613: Was Hart and Currie (1960) who first documented the existence of the Angola-Benguela front when the RRS William Scoresby sailed southwards surveying the Benguela Current off the west coast of Africa during the autumn and spring of 1950. They reported a sharp decrease in sea surface temperature from 27 to 20.5 °C within the span of one hour. In the early 1970s research increased steadily in this region with subsequent cruises revealing ocean circulation features of

Angola–Benguela Front - Misplaced Pages Continue

1764-437: Was suggested to lead to a downward vertical velocity and therefore, a squashing of the water columns, which subsequently forces the ocean gyre to spin more slowly (via angular momentum conservation). This is accomplished via a decrease in planetary vorticity (since relative vorticity variations are not significant in large ocean circulations), a phenomenon attainable through an equatorially directed, interior flow that characterizes

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