Northward Undercurrent Research Articles

Capturing the Transport Variability of a Western Boundary Jet: Results from the Agulhas Current Time-Series Experiment (ACT)

AbstractThe volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current- and pressure-recording inverted echo sounders (CPIES) deployed at 34°S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived from CPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000 m deep, with peak surface speeds of 1.8 m s−1 and a weak northward undercurrent on the continental slope below 1200 m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is −84 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore Tbox, gives a mean transport of −77 ± 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32°S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and Tbox show important differences during solitary meander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.

Ocean variability east of Mindanao: Mooring observations at 7°N, revisited

AbstractTwo subsurface moorings were deployed east of Mindanao Island, the Philippines, at 7°01′N, 126°55′E and 7°01′N, 127°46′E, at the location of the inshore and offshore cores of the Mindanao Undercurrent (MUC) suggested by past studies, from September 2011 to October 2012 and March 2013. A steady northward undercurrent, the MUC, was not confirmed by these observations, not only at the location of its inshore core but also of the offshore core. The observed mean flow at the mooring sites seems to be part of an anticyclonic eddy rather than the MUC. A particle‐tracking experiment using a high‐resolution general circulation model output showed that the northward mean flow, called the MUC by past studies, was too weak to advect water to the north. The Mindanao Current during 2011–2012 was weaker than during 1999–2002 because the sea surface height in the Philippine Sea during 2011–2012 was lower than that during 1999–2002. Intraseasonal variability with periods of 50–100 days was observed at the mooring sites, comparable to the previous observations during 1999–2002. Westward signal propagations were observed with periods and speeds of 50 days and 0.20 m s−1 at 300 m depth and of 60–72 days and 0.11–0.14 m s−1 at 960 m depth.

Observed features of the Halmahera and Mindanao Eddies

The structure and variability of the Halmahera and Mindanao Eddies (HE and ME), which are thought to be quasi‐stationary eddies, are described based on onboard and time‐series observations in the gateway region of the Indonesian Throughflow (ITF). The HE was found to have a clear anticyclonic eddy structure during all cruises. It tilted northward and/or westward with increasing depth. The HE mixes waters from the northern and southern hemispheres and transfers this mixed water to the eastern route of the ITF. The HE shifted northwestward in the boreal summer and the cold phase of ENSO (El Niño and Southern Oscillation) and southeastward in the boreal winter and the warm phase of ENSO. In contrast, the ME, with its cyclonic eddy structure, was observed only once and seems not to be a quasi‐stationary eddy. It appears in the mean field and is confined above the thermocline. The stationary northward undercurrent, the Mindanao Undercurrent, was also not found at 7°N east of Mindanao, but the northern tip of the HE was found at this latitude.

Monsoon-induced upwelling off the Vietnamese coast

During the southwest monsoon from July 8 to 28, 2003, an interdisciplinary cruise took place in the central area of Vietnamese upwelling with “MV Nghien Cuu Bien” in the South China Sea. Physical observations in the upwelling area are analyzed with respect to local/regional wind forcing and far field forcing. Nutrients and phytoplankton measurements are discussed with respect to exchange processes between different water masses. The wind-induced coastal upwelling by local wind forcing is much weaker than in the previous years due to weaker-than-normal winds. This can be attributed to the far field forcing of the 2002/2003 El Nino event which modulates the upwelling intensity. The atmospheric conditions reflect the typical situation after an El Nino event which weakens the wind-induced coastal upwelling, reduces the latent heat flux, and results in higher-than-normal sea-surface temperatures. The general circulation pattern during SW monsoon is driven by the spatial asymmetry in the monsoon forcing. The flow pattern is characterized by an upwelling-induced northward undercurrent and a recently detected southward countercurrent. The resulting stretching deformation of this flow pattern forms an offshore jet between ∼12°N and 12.5°N and causes a local enhancement of the upwelling intensity. The upwelling due to stretching deformation is a peculiarity, which makes the Vietnamese upwelling area different to other upwelling areas. A budget of the upwelling components is presented: the strongest contribution in 2003 to the Vietnamese upwelling is the dynamical upwelling due to the clockwise rotation of the northward undercurrent. The internal radius of deformation separates the upwelling area from the offshore area as well as different water masses. Mekong River and the Gulf of Thailand waters which are offshore show nutrient depletion. Therefore, high chlorophyll maxima cannot be explained by nutrient supply from river runoff. The dynamical upwelling brings in nutrient-rich Maximum Salinity Water into the euphotic zone. This causes a subsurface chlorophyll maximum between 20 and 40 m water depth along the northward undercurrent. Deflection from the Redfield ratio in the C:N ratio and negative excess nitrogen identifies the region as nitrogen-limited which may favor cyanobacteria blooms. The consequence is a unique feature in new production: in the upwelling area, new production is based on upwelled nitrate, whereas offshore in the nutrient-depleted Mekong and Gulf of Thailand water, new production is based in addition on nitrogen fixation.

A Large-Scale Seasonal Modeling Study of the California Current System

A high-resolution, multi-level, primitive equation ocean model has been used to investigate the combined role of seasonal wind forcing, seasonal thermohaline gradients, and coastline irregularities on the formation of currents, meanders, eddies, and filaments in the entire California Current System (CCS) region, from Baja to the Washington-Canada border. Additional objectives are to further characterize the meandering jet south of Cape Blanco and the seasonal variability off Baja. Model results show the following: All of the major currents of the CCS (i.e., the California Current, the California Undercurrent, the Davidson Current, the Southern California Countercurrent, and the Southern California Eddy) as well as filaments, meanders and eddies are generated. The results are consistent with the generation of eddies from instabilities of the southward current and northward undercurrent via barotropic and baroclinic instability processes. The meandering southward jet, which divides coastally-influenced water from water of offshore origin, is a continuous feature in the CCS, and covers an alongshore distance of over 2000 km from south of Cape Blanco to Baja. Off Baja, the southward jet strengthens (weakens) during spring and summer (fall and winter). The area off southern Baja is a highly dynamic environment for meanders, filaments, and eddies, while the region off Point Eugenia, which represents the largest coastline perturbation along the Baja peninsula, is shown to be a persistent cyclonic eddy generation region.

The Eastern Boundary Current system between the Canary Islands and the African Coast

To study the EasternBoundaryCurrentsystem off Northwest Africa in detail several CTD/ADCP-sections and long-term mooring work were carried out in the channel between Lanzarote and Africa. The observations are compared with a fine-resolution model, which was developed in the framework of the CANIGO project. The water masses, which are observed in this area, are characterised and classified in density ranges. The current field shows a high spatial and temporal variability with maximum velocities of about 35 cm/s. Seasonal means as well as currents averaged across the channel are only a few cm/s. In the surface water a steady southward flow in the middle of the channel indicates the CanaryCurrent in this area. During fall a strong northward current is observed close to the African shelf. Though the CanaryCurrent strengthens during summer and fall due to an increase of the trade winds, the transport in the channel decreases or turns northward during that time due to the enhanced poleward current at the eastern side. A northward undercurrent with a mean velocity of +2.3 cm/s is observed at the African slope in 950 m depth. The poleward transport of AAIW increases during fall and a strong influence of relatively fresh AAIW is observed during that time. Most of the observations fit well to the results of the CANIGO model, but the occurrence of MW at the bottom of the channel and the corresponding southward flow cannot be resolved by the model.

A view of the 1993–1994 California Current based on surface drifters, floats, and remotely sensed data

Near‐surface drifters were deployed in the California Current System during the period 1993–1994 in order to study mean and eddy currents. The deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5° grid. Eddy variability, which tended to be isotropic, was higher near the coast (standard deviations around 0.2 m s−1) and decreased offshore (down to ∼0.1 m s−1). Seasonal mean surface flow west of California was southward during all seasons: if a Davidson Current existed during 1993–1995, it was probably either short‐lived or only found closer to the coast than our study area, i.e., within 100–150 km of shore. In some cases, drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior. We note a tendency for individual eddies (which can last for up to 3–5 months) to drift westward at rates of O(5 km day−1). Not all eddies in the region are surface‐intensified. There is clear evidence for at least the occasional presence of a thin, meandering southward jet in the region that can be 1000 km or more long. This jet can be identified with that found in the Coastal Transition Zone program in association with cool filaments. The jet appears to become broader and slower south of Point Conception. We hypothesize (on the basis of others' results as well as ours) that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the northward undercurrent.

Winter upwelling off Luzon in the northeastern South China Sea

A major upwelling region centered about 100 km offshore between 16° and 19°N off northwest Luzon in the South China Sea is revealed in the distributions of temperature, salinity, and dissolved oxygen concentration during a cruise in December 1990. Upward movement of water low in dissolved oxygen can be identified to a depth of 300 m. Climatological distributions of temperature and upper ocean heat content and tracer distribution obtained from a recent numerical experiment lend support to the existence of this upwelling region. Results from a general circulation model of the South China Sea further show that upwelling off Luzon occurs between October and January, in agreement with the climatological data. Current structures obtained from a simple two‐layer model and the general circulation model both suggest that this upwelling is not driven by local winds but contains a large, remotely forced component arising from the basin circulation. The upwelling is sustained by offshore Ekman drift from above and a converging northward undercurrent from below. The would‐be downwind coastal jet in the surface layer is largely offset by currents associated with the winter circulation gyre. Upwelled water with high nutrient content spreads southwestward in January and could be an important nutrient source for the surface water in the interior South China Sea in spring.

Intensification of Subtidal Surface Currents Over a Deep Channel in the Upper Chesapeake Bay

A three-dimensional hydrodynamic model of the Upper Chesapeake Bay was used to examine the nature and cause of an intensification of subtidal, southward surface current in the middle reaches of the basin. The deep navigation channel along the eastern boundary was found to be ultimately responsible. The deep channel allows the density and tidally-induced subtidal currents to intensify over it, producing the eastern intensification. Both mechanisms operate in the non-rotating limit and consequently do not diminish with vanishing effect of the earth's rotation. Density-induced forcing is predominantly baroclinic, generating a northward undercurrent in the deep channel and a southward current aloft which attenuates westward. Tidal forcing is mostly barotropic, producing southward mean current in the deep channel and return flow to the west. Historic data lend support to the model results.

On the flow and thermohaline structure off Cochin during pre-monsoon season

The pre-monsoon characteristics of thermohaline and current structure off Cochin (9.75°N and 75.75°E) in the Arabian Sea suggest the occurrence of wind induced upwelling. A zone of high temperature (>30°C) noticed is believed to be the part of the Indian Ocean warm pool. The surface flow is predominantly southward and is baroclinic. The presence of a northward undercurrent, a characteristic feature associated with coastal upwelling systems, is clearly established with direct current measurements. This zone is associated with intense vertical mixing. Oxygen analysis and T—S confirmed that this current is from the equatorial region. A water mass of local origin ( T ∼- 30°C, S ⋍ 35.5 PSU, oxygen ⋍ 3.5–4 ml l −1, sigma- t22 kg m −3) is formed in the surface layers due to pre-monsoon heating and southward advection of Arabian Sea high saline water (ASHSW) over the remnants of low saline Bay of Bengal/Equatorial Indian Ocean water. Mixing characteristics indicated that the instability is mainly controlled by shear rather than buoyancy.

The WOCE model in the western equatorial Atlantic: Upper layer circulation

A high‐resolution general circulation model of the North Atlantic, first developed at the National Center for Atmospheric Research and then run at the Institut für Meereskunde in Kiel for two different wind climatologies and reduced vertical friction, is evaluated in the upper 500 m for the western tropical Atlantic, 5°S to 15°N. Although the general features of the vigorous seasonal circulation changes documented in previous studies and in the earlier high‐resolution model of Philander and Pacanowski (1986a) are reproduced, there are some interesting differences. Lack of eastward penetration of the Equatorial Undercurrent (EUC) and a thermocline that is too diffuse are model deficiencies due to the constant vertical eddy diffusion coefficient. In the lower friction case the undercurrent partially surfaces in the west, causing an eastward surface current on the equator, which is not apparent in the earlier model studies. Further, the zonal currents, in the low‐friction version, have high‐velocity bands, resulting, e.g., in two separate current cores in the North Equatorial Countercurrent (NECC) region; and an eastward surface core just south of the equator, connected to the EUC. Particularly interesting are equatorward undercurrents along the western boundary, one of which has already been confirmed in recent measurements off French Guyana. In winter it connects with the EUC in the model, in summer with the NECC. A northward undercurrent in the model exists off Brazil, between 5° and 10°S, but that is already close to the southern boundary of the model domain. The annual mean throughflow from the southern hemisphere into the Caribbean along the western boundary is small in the model, and in particular, there is no enhanced throughflow in winter, when the cross‐equatorial North Brazil Current transport is not taken up by the NECC.

The Somali current at the equator: annual cycle of currents and transports in the upper 1000 m and connection to neighbouring latitudes

Current measurements were obtained with moored stations during October 1984 to October 1986 in two consecutive deployments across the Somali Current on the equator. For the transport calculations the deficiency of conventional subsurface moorings, i.e. no data from close to the surface, had to be overcome using the historical ship drift climatology. While the current structure during the summer monsoon is that typical of a western boundary current, the profile in winter is far from being a weaker southward reverse of the summer situation. Below a thin surface layer of southward flow, there is a northward undercurrent between about 120 and 400 m depth. Below that, the flow reverses again to southward. This results in drastic differences in cross-equatorial monsoon season transports. While the summer mean transport is 21 Sv for the upper 500 m, the winter monsoon mean for that depth range is close to zero. The annual mean transport in the upper 500 m is 10 Sv northward. Very little transport is measured in the 500–1000 m depth range in either season or the annual mean. The almost closed mass budget of the boundary current system during the winter circulation allows a calculation of cross-equatorial heat transport, which comes out to −3 × 10 14 W (southward) for the northeast monsoon season mean. The heat flux associated with the annually varying part of the boundary current is small, only about −0.3 × 10 14 W or about 5% of the total cross-equatorial heat flux as estimated by other methods. By combining the equatorial measurements with earlier off-equatorial current observations, particularly at 2°−4°S and 5°N, and with property distributions (salinity and oxygen) on isopycnal surfaces, analysed from the historical data file, a synopsis of the seasonal circulation changes of the entire Somali Current system between about 4°S and 12°N is then derived.

Seasonal changes in the denitrification regime of the Arabian Sea

Extensive observations made in the northeastern Arabian Sea during successive southwest (SW) and northeast (NE) monsoon seasons reveal significant changes in chemical composition of the oxygen-deficient waters. Distributions of nitrite and nitrate deficits, the latter computed from linear relationships established between the nitrate tracer ‘NO’ and potential temperature, and evaluated using a two end-member mixing model, are studied at four isopycnal surfaces to deduce some major features of the mid-depth circulation. The results suggest a complete reversal of the subsurface coastal circulation associated with the monsoons. During the SW monsoon, a northward undercurrent appears to carry low nitrite, low deficit and relatively well-oxygenated waters off the Indian continental margin, suppressing denitrification; this feature is absent during the NE monsoon. Vigorous horizontal mixing resulting from a seasonal reversal of currents at a depth of a few hundred metres, together with the lack of large east-west differences in the downward particulate flux, as revealed by some recent sediment trap experiments, could explain why the oxygen-deficient zone in the Arabian Sea is not confined to the upwelling centres along the boundaries. The results reinforce the assertion that mixed layer-induced nutrient pumping could account for a large fraction of new production in this region. Substantially higher deficits in inorganic combined nitrogen seem to occur during the NE monsoon suggesting that the renewal processes are most vigorous during the SW monsoon. It is suggested that the dominance of the NE monsoon circulation could lead to an intensification in denitrification during the glacial times.

The Bass Strait water cascade during summer 1981–1982

Water in Bass Strait, a shallow extension of the Australian shelf between the Great Australian Bight and the Tasman Sea, is denser than Tasman Sea surface water in winter but of comparable density in summer. Winter downflow of denser water creates high-salinity intrusions along the continental slope, which are carried northward in a narrow undercurrent at 300–400 m depth. This paper gives a detailed description of the summer situation based on cruise data from November 1981. It is found that no downflow from Bass Strait occurred during the cruise. Parcels, intrusions, and filaments of Bass Strait Water were found along the continental slope in the Tasman Sea over a large area. They are interpreted as remnants from the winter downflow. The two-fold structure of the Cascade known from winter observations, with downflow at two closely adjacent but distinctly different density levels, was clearly reflected in the data. Contrary to the winter situation, parcels of Bass Strait Water were found both north and south of the Cascade and both close to, and at distance from, the continental shelf. This suggests that the northward undercurrent may not exist in summer.

Satellite Observations and Modeling of Meanders in the California Current System off Oregon and Northern California

Abstract Infrared satellite images from the west coasts of Oregon and northern California are used to identify meander patterns in sea surface temperature which appear as large cold tongues extending offshore. Two relatively long series of images from 1982 and a few examples from 1980 and 1983 demonstrate the evolution of the cold tongues from an initial variety of scales (60–200 km), to the fastest growing waves (110–130 km) and then finally to tongues with longer wavelengths (400 km). This is observed to occur over periods of about three months in summer and fall when the coastal circulation is composed of a southward surface current over a northward undercurrent. The initial shorter scale features are believed to be excited by the interaction between the mean current and the coastal topography. Baroclinic instability associated with the vertical shear between the surface current and the undercurrent is found to be responsible for the growth of the features observed in the satellite imagery. A nonlinear...

A Permanent Undercurrent Adjacent to the Great Barrier Reef

Abstract A hydrological section adjacent to the Great Barrier Reef (GBR) was completed several times during 1980 and 1981. The data indicate a maximum southward current near the surface and a northward undercurrent at depths of 300–900 m. A detailed section indicates a double cell structure in the northward undercurrent. The cell structure in the geostrophic velocity is also clearly evident in the dissolved oxygen distribution where distinct core layers can he seen. Such a current offers considerable scope for the recycling of biological material along this section or the GBR.

The Role of a Longshore Pressure Gradient in Pacific Northwest Coastal Dynamics

In this paper we demonstrate the importance of the seasonal barotropic longshore pressure gradient force to Pacific Northwest coastal dynamics. Values of the seasonal longshore pressure gradient corrected for gauge height relative to a level surface (Hickey and Pola, 1982) and for year-to-year variations (Enfield and Alien, 1980) were included in the two-dimensional, time-dependent, baroclinic finite-difference model of Hamilton and Rattray (1978) as an external force. Observed wind stress, stratification and bottom topography were included in the model, and comparisons were made with current meter data in each of the three seasonal situations observed in the Northwest: pressure gradient force southward opposing local wind stress (winter), pressure gradient force northward opposing local wind stress (summer), and pressure gradient force and local wind stress both southward (spring). Three important features of the seasonal circulation are shown to depend on the existence of the pressure gradient force: the northward undercurrent (the California Undercurrent) that exists along most of the West Coast during summer, a southward undercurrent (which we denote the Washington Undercurrent) that the model predicts and observations shown herein substantiate for the Pacific Northwest slope during winter, and the anomalously strong southward flow that occurs subsequent to the spring transition. Finally, the seasonal variation of the balance between wind stress, bottom stress and vertically integrated longshore pressure gradient force as a function of bottom depth is addressed. In particular, it is shown that bottom stress is significant in the mid-shelf region during both winter and spring. The commonly made assumption that offshore transport in the surface layer is balanced by onshore transport in the inviscid interior is shown to be invalid except during summer.