Atlantic Equatorial Undercurrent Research Articles

Impacts of Climate Change on the Ascension Island Marine Protected Area and Its Ecosystem Services

AbstractThis is the first projection of marine circulation and biogeochemistry for the Ascension Island Marine Protected Area (AIMPA). Marine Protected Areas are a key management tool used to safeguard biodiversity, but their efficacy is increasingly threatened by climate change. To assess an MPA's vulnerability to climate change and predict biological responses, we must first project how the local marine environment will change. We present the projections of an ensemble from the Sixth Coupled Model Intercomparision Project. Relative to the recent past (2000–2010), the multi‐model means of the mid‐century (2040–2050) project that the AIMPA will become warmer (+0.9 to +1.2°C), more saline (+0.01 to +0.10), with a shallower mixed layer depth (−1.3 to −0.8 m), a weaker Atlantic Equatorial Undercurrent (AEU) (−1.5 to −0.4 Sv), more acidic (−0.10 to −0.07), with lower surface nutrient concentrations (−0.023 to −0.0141 mmol N m−3 and −0.013 to −0.009 mmol P m−3), less chlorophyll (−6 to −3 µg m−3) and less primary production (−0.31 to −0.20 mol m−2 yr−1). These changes are often more extreme in the scenarios with higher greenhouse gases emissions and more significant climate change. Using the multi‐model mean for two scenarios in the years 2090–2100, we assessed how five key ecosystem services in both the shallow subtidal and the pelagic zone were likely to be impacted by climate change. Both low and high emission scenarios project significant changes to the AIMPA, and it is likely that the provision of several ecosystem services will be negatively impacted.

Influence of the Gulf of Guinea Islands on the Atlantic Equatorial Undercurrent Circulation

AbstractIn the easternmost portion of the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE–SW to the Equator, where São Tomé Island sits in the path of the Equatorial Undercurrent (EUC). In this study, we explore the meso‐to‐large‐scale effects of local flow‐topography interactions that escalate from the EUC encounter with the Gulf of Guinea islands. A mean shipboard Acoustic Doppler Current Profiler section captures the EUC as a strong subsurface jet that ultimately hits São Tomé. Motivated by these observations, we ran two ocean general circulation model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (PV) from these simulations show that the EUC bifurcates at 6°E, triggering mesoscale activity that spreads the EUC waters. On both sides of São Tomé, the EUC branches roll up into eddies that propagate westward. These low‐PV anticyclones carry high salinity through the tropical Atlantic, introducing PV anomalies along eddy corridors mirrored by the Equator. The formation of such eddies is affected by diapycnal and isopycnal mixing and friction, and their distribution between hemispheres is intrinsically related to the location of São Tomé. The eddies are modulated at both seasonal and interannual scales. A strong EUC generates numerous and stronger eddies, while seasonal equatorial upwelling indirectly hinders their formation. Convergence of the EUC upstream of São Tomé reveals intense downwelling and freshening, whereas divergence and upwelling are associated with the EUC negotiating the island.

Equatorial Atlantic Ocean dynamics in a coupled ocean–atmosphere model simulation

The ocean temperatures and zonal currents at the equatorial Atlantic simulated by an improved version of the Brazilian earth system model (BESM), with changes in the cloud cover scheme and optical properties of the atmospheric component, are analyzed and compared to those obtained from a previous version of BESM and also from other seven selected CMIP5 models. It is shown that this updated version of BESM, despite some persistent biases, more accurately represents the surface temperature variation at the Equator and the equatorial thermocline east–west slope. These improvements are associated to a more realistic seasonal cycle achieved for the Atlantic equatorial undercurrent, as well as sea surface temperatures and zonal wind stress. The better simulation of the equatorial undercurrent is, in its turn, credited to a more realistic representation of the surface wind position and strength at the tropical Atlantic by the coupled model. With many of the systematic errors noticed in the previous version of the model alleviated, this version of BESM can be considered as a useful tool for modelers involved in Atlantic variability studies.

Zonal structure and seasonal variability of the Atlantic Equatorial Undercurrent

Simultaneous mooring arrays were maintained along the path of the Equatorial Undercurrent (EUC) at three longitudes (23°W, 10°W, and 0°E), from October 2007 to June 2011, as part of the CLIVAR Tropical Atlantic Climate Experiment. The measurements allow for the first time a description of the seasonal cycle and interannual variability of the EUC across the Atlantic basin. The mean transport of the EUC at 23°W is 14.3 ± 0.6 Sv, decreasing to 12.1 ± 0.9 and 9.4 ± 0.6 Sv at 10°W and 0°E, respectively. The EUC shows a changing seasonal cycle across the basin: at 23°W, the strongest EUC transport occurs in boreal fall in association with maximum easterly wind stress, at 10°W the EUC transport shows a semiannual cycle with a maximum in boreal spring and fall, while at 0°E the EUC has a single spring maximum. At all locations the EUC core exhibits a similar seasonal vertical migration, with shallowest core depths occurring in boreal spring and deepest core depths in boreal fall. The maximum core intensity occurs in boreal spring all across the basin, when the EUC is shallow, during the annual wind relaxation. The weakest EUC core intensity occurs during the boreal summer cold tongue phase, especially in the eastern part of the basin. At both 23°W and 10°W, a deep extension of the EUC occurs in boreal summer, which increases the transport in the lower thermocline and partially offsets the weaker upper EUC transport during boreal summer. No clear linkage could be established between the interannual variability of the EUC in the eastern part of the basin and the intensity of the summer cold tongue, despite evidence for such a linkage in the western part of the basin.

Salinity intrusion and convective mixing in the Atlantic Equatorial Undercurrent

This study investigates the advection of positive-salinity anomalies by the Equatorial Undercurrent (EUC) and their potential importance in inducing vertical convective mixing. For this purpose we use hydrographic and velocity observations taken in April 2010 along the western Atlantic equatorial ocean (32 to 43°W). The high-salinity EUC core is a few tens of metres thick and occupies the base of the surface mixed layer and the upper portion of the surface thermocline. It leads to high positive values of the vertical salinity gradient, which in many instances cause statically unstable conditions in otherwise well-stratified regions. The unstable regions result in vertical convection, hence favouring the occurrence of step-like features. We propose that this combination of horizontal advection and vertical-instability leads to a sequence of downward-convective events. As a result the EUC salinity is diffused down to a potential density of 26.43, or about 200 m deep. This mechanism is responsible for water-mass and salt downwelling in the equatorial Atlantic Ocean, with a potentially large influence on the tropical and subtropical cells.

Atlantic Equatorial Undercurrent and associated cold tongue variability

The Atlantic Equatorial Undercurrent (EUC) is studied using a simulation for the period 1990–2002 with a high‐resolution ocean general circulation model. Simulated transports of the EUC that supplies the annual mean upwelling in the central and eastern equatorial Atlantic are in good agreement with new transport estimates derived from ship observations, i.e., 19.9 and 14.0 Sv at 35°W and 23°W, respectively. Although the observations are not conclusive concerning the seasonal cycle of EUC transports, the simulated seasonal cycles fit largely in the observed range. The analysis of the EUC variability associated with interannual boreal summer variability of the equatorial cold tongue showed that cold tongue indices, defined either by near‐surface temperature or steric height anomalies, are anticorrelated with thermocline EUC transport anomalies: A strong EUC corresponds to low near‐surface temperatures and steric heights. The importance of equatorial waves for the cold tongue region is shown: Surface layer transport anomalies at 23°W and 10°W are significantly correlated with both near‐surface temperature and steric height anomalies in the equatorial and coastal upwelling regions, indicating an associated eastward phase propagation along the equator toward the African coast where the signal bifurcates into two poleward branches along the coast and is reflected into a westward propagating wave.

The Atlantic Equatorial Undercurrent: PIRATA observations and simulations with GFDL Modular Ocean Model at CPTEC

This paper shows the Atlantic Equatorial Undercurrent (EUC) revealed by a PIRATA ADCP during 2002 at 0°N, 23°W and simulated by an eddy‐resolving ocean general circulation model (MOM) forced by NCEP/NCAR reanalysis wind stresses. The PIRATA data revealed the shallowing of the EUC between January and May, concurrently with the reversal of the easterly trades to westerly, and the deepening of the EUC from May to December. Empiric Orthogonal Function (EOF) analysis shows two main components; the first mode, explaining 63.7% of the total variance, represents the main seasonal variation of the undercurrent during 2002, while the second mode, explaining 19.9% of the variance, represents higher frequency variability occurring in the EUC core, from 60 to 80 m. All model experiments were able to reproduce this variability, and the best one was chosen objectively using EOF and joint EOF analysis techniques.

Patch diffusion computed from Lagrangian data, with application to the atlantic equatorial undercurrent

Abstract Radar‐tracked drifter trajectories reported by Fahrbach et al. (1986) are reanalysed. The data are obtained from 4 drifter clusters that were deployed in the Atlantic Equatorial Undercurrent for approximately 48 h each. The ratio of Lagrangian integral length scale to the standard deviation of drifter positions ranged from C = 0.02 to 0.36. However, on average it was 0.12 in the meridional direction, thus confirming Fahrbach et al.’s (1986) calculations of the meridional diffusion of salt from the Atlantic Equatorial Undercurrent. Making a quasi‐stationary assumption we find that relative velocities had Lagrangian integral time‐scales of about 1 h when relative velocities were large O(0.1 m s−1) and about 3 h when they were smaller O(0.05 ms−1 ). The Eulerian integral time‐scale for relative velocities is only slightly greater than the Lagrangian integral time‐scale. Joint space‐time correlations indicated that the eddy velocity changes rapidly compared with time‐scales required to advect a particle over the eddy length scale. In the case of D1, D2 and D3 experiments (which had large relative velocities) the relative velocity became spatially decorrelated at scales small compared with cluster dimensions. The spatial scales of the relative velocity were somewhat larger for the CIPREA experiment, which had weaker relative velocities than D1, D2 and D3.

On the Difference in Strength between Atlantic and Pacific Undercurrents

Abstract From an earlier study of the longitudinal evolution of the equatorial zonal momentum balance in a nonlinear stratified simulation of the Atlantic equatorial undercurrent it was shown that the annual mean simulated undercurrent reaches its maximum zonal velocity as it emerges from an accelerated deep inertial regime extending over about 15 degrees of longitude off the western boundary. Further east, frictional forces overcome the accelerating effect of the zonal pressure gradient form and the undercurrent keeps losing eastward momentum as it flows eastward. If the accelerating deep western regime extended over a wider range of longitudes the maximum emerging velocity would be larger. This is shown to occur in a nonlinear stratified simulation of the Pacific equatorial undercurrent because the western Pacific thermocline is thicker due to the basin size and to the longitudinal structure of the zonal winds. This is offered as an explanation for the larger observed zonal velocities of the Pacific und...

The fractal dimension of relative lagrangian motion

14 clusters of drifters are studied to obtain the fractal dimension of relative trajectories between pairs of drifters. Averaging over all experiments, the relative trajectories have a fractal dimension of 1.3 over separation scales ranging from 10 m to 4000 m. The clusters were deployed in Lake Erie, the Atlantic Equatorial Undercurrent, and in coastal waters off the south shore of Long Island. The average fractal dimension was quite similar for groups of experiments in each of these 3 areas, but it fluctuates over a greater range (1.59 to 1.12) when we consider individual experiments. This and consideration of variation of the generalized fractal dimension for higher moments, raise the possibility that trajectories are multifractal. The estimates we obtain for fractal dimension of relative lagrangian motion should be regarded as first estimates, pending analysis of more substantial data sets. DOI: 10.1034/j.1600-0870.1990.t01-4-00005.x

Dynamical regimes of a fully nonlinear stratified model of the Atlantic equatorial undercurrent

The dynamical regimes governing various regions of a nonlinear stratified mean equatorial undercurrent are investigated by computing zonal momentum balances in Philander and Pacanowski's (1984) numerical simulation of the equatorial Atlantic. Only in a narrow region of the simulation next to the western boundary does a subsurface regime exist where the positive zonal pressure gradient force overcomes the negative zonal frictional forces, resulting in a net inertial acceleration to the east and in the formation of the eastward undercurrent below westward surface flow. Elsewhere, the net force is westward and the undercurrent, experiencing a net loss of eastward momentum, weakens from west to east. For the parameterizations of frictional processes adopted in the model, zonal momentum is dissipated mostly by vertical friction in the upper thermocline and above, and by lateral friction below. Because the net retarding effect of vertical friction on the upper layers of the undercurrent is larger than that of lateral friction on the lower layers, there is on average an apparent migration of the location of the undercurrent core velocity from above the thermocline in the west of the Atlantic basin to below in the east. The bulk of the mean midbasin model undercurrent can be described as terminating in the overlying westward flow; only a small fraction (the deeper more inertial layers) terminates at the eastern coast. It is argued that the mechanisms at work in the model are plausible ones to explain the zonal evolution of the observed undercurrent, which should motivate a reexamination of existing data. Better agreement between simulated and observed eastern undercurrent speeds might be obtained by reducing the eddy coefficient of lateral mixing used in the simulation.

Observations of the horizontal separation of the salinity core and the current core in the Atlantic Equatorial Undercurrent

Observations of the horizontal separation of the salinity core and the current core in the Atlantic Equatorial Undercurrent

Horizontal mixing in the Atlantic Equatorial Undercurrent estimated from drifting buoy clusters

During 1978 and 1979, four experiments with clusters of 6 to 10 drifting buoys were carried out in the Atlantic Equatorial Undercurrent. The measurements were intended to estimate horizontal mixing and its possible contribution to the salt loss of the high‐salinity core related to the Equatorial Undercurrent. The buoys were drogued at the estimated depth of the salinity core of 70 m to 90 m. The diameters of the spreading clusters ranged from 3 to 10 km. Each experiment was maintained for about 2 days. During this period the buoys were tracked by radar from a nearby operating research vessel. From the observed tracks, horizontal turbulent mixing coefficients were deduced. The results show a dependence on the horizontal scale of l1.43. This is not significantly different from an expected l4/3 law. Using the obtained scale dependence, the observations are extrapolated to the scale of the salinity core of 150 km obtained from hydrographic observations, resulting in a turbulent mixing coefficient of 6.5 × 106 cm2 s−1. Combining these numbers with the horizontal gradients of the salinity core, one obtains a salt loss of 1.42 g s−1 cm−1, which amounts to 20% of the complete salt loss of 7.2 g s−1 cm−1 compiled by Katz et al. (1980).

Zonal pressure gradient, velocity and transport in the Atlantic Equatorial Undercurrent from focal cruises (July 1982‐February 1984)

The zonal pressure gradient (ZPG) along the Atlantic equator and the Equatorial Undercurrent (EUC) transport are discussed for four cruises representative of each season. A very clear sea surface slope reversal occurred in the eastern area during autumn as far west as 14°W. An early onset of the equatorial thermocline rising was evident during spring 1983. An eastward equatorial surface jet clearly distinct from the EUC was observed at 35°W and 29°W. The greatest ZPG but the lowest EUC transport were observed during summer 1983. A nearly total absence of the ZPG and a large surfacing of the EUC as far as 10°W characterized the 1984 winter.

Observation of near-surface layer changes related to the Atlantic equatorial undercurrent

Data on stratification and currents obtained during the GATE-Equatorial Experiment appear to show a coupling between high current shear related to the Equatorial Undercurrent and intensified mixing in the near-surface layers.

Origin of the Atlantic Equatorial Undercurrent

The distribution of temperature, salinity, and oxygen in the western equatorial Atlantic, combined with parachute drogue observations and an analysis of the pressure fields in the upper 300 meters, shows the current pattern associated with the origin of the Equatorial Undercurrent. The temperature/oxygen relationship indicates that most of the Undercurrent water comes from the South Atlantic by way of the North Brazilian Coastal Current and that the contribution of North Atlantic water is very minor.

Drogue measurements of shallow currents on the equator in the Western Atlantic Ocean

Parachute drogue measurements in the Atlantic Equatorial Undercurrent during February, March and April 1963 between 25° and 35°W at depths between 75 and 200 meters show velocities towards the east of up to 75 cm/sec. Surface currents during this period varied between 60 cm/sec towards the south-west to 60 cm/sec towards the east. This shift in direction of the surface flow may be related to the decrease in wind stress as the inter-tropical convergence moves through the area.

Observations of the equatorial undercurrent in the Atlantic Ocean at 15°W during Equalant I

During the Equalant I program in 1963, the ship Explorer of the U. S. Coast and Geodetic Survey obtained measurements in the region of the Atlantic equatorial undercurrent between 15°W and 14°W. The average speed of the current as derived from drogue tracking at a depth of 50 m during phase I of the operation was 80 cm/sec and during phase II 75 cm/sec. Near the equator, observations of the undercurrent indicate meanders with a meridional amplitude of as much as 3 miles. Temperature and salinity sections across the undercurrent show clearly the spreading of the thermocline and the high-salinity core, respectively, associated with this current.