Ekman Veering Research Articles

Observations of northeastward flow on a narrow shelf dominated by the Agulhas Current

Water column currents and bottom temperature from September 2005 to September 2006 across the narrow shelf region off Port Edward on the east coast of South Africa, combined with satellite observations, highlighted the dominant influence of the Agulhas Current (AC) on shelf circulation. Stronger southwestward flow occurred across the shelf during austral summer and autumn suggesting an intensification and shoreward movement of the AC during these seasons. In situ moored observations also showed that the AC extended as far inshore as 3.2 km from the coast. Weak and non-significant correlations between wind and current, and between wind and bottom temperature, indicated minimal influence of local wind forcing on current variations and temperature responses. A total of 11 northeasterly flow events, ranging from 1 to 15 days, were observed, with only 4 events associated with the passage of mesoscale cyclonic eddies along the inshore edge of the AC. The remaining events all suggested small-scale movements of the AC resulting in enhanced Ekman veering in the bottom layers further offshore, and full water column current reversals close inshore. Contrary to expectation, no clear seasonal variation in bottom temperatures was evident, and temperatures and currents were poorly correlated, since both warming and cooling occurred during southwesterly and northeasterly flows. Substantial short-term fluctuations of up to 4 °C were associated with shoreward/offshore movements of the AC, and southward propagation of eddies. Advection of warmer AC waters around eddies and southward advection of waters upwelled further north served to complicate the observed temperature responses.

Wind and Current Forcing Combine to Drive Strong Upwelling in the Agulhas Current

AbstractStrong upwelling events inshore of the Agulhas Current close to 33.5°S are investigated. These events are important to the exchange of shelf and slope waters, potentially enhancing primary productivity and advecting larvae offshore. Using hydrographic observations, this study shows that a wind-driven upwelling event and a current-driven upwelling event can each advect central waters more than 130 m upward, resulting in a maximum 9°C cooling at 50-m depth over the continental shelf and surface cooling greater than 4°C. The authors use satellite data to assess the frequency and forcing mechanisms of similar cold events from January 2003 through December 2011, defining cold events as days when the sea surface temperature (SST) anomaly is significantly correlated with a local current or wind forcing. The authors identify 47 events with an average length of 2.2 days and SST anomaly of −1.6°C, corresponding to an average 13 days of surface cold events along the Agulhas Current front per year. This study uses combined EOF analysis to characterize these cold events based on four highly correlated forcing mechanisms: alongshore wind speed, wind stress curl, current meandering, and current speed over the slope. The authors find that meanders act in combination with upwelling-favorable winds to force the strongest cold events, while upwelling-favorable winds alone, possibly primed by Ekman veering, force weaker cold events. Most significantly, it is found that the frontal curvature of warm Agulhas Current meanders couples with the atmosphere to drive local wind stress curl anomalies that reinforce upwelling.

Observations and mechanisms of upwelling in the northern KwaZulu-Natal Bight, South Africa

New in situ time-series data were acquired by two ADCP moorings placed on the shelf off Richards Bay on the east coast of South Africa at depths of 25 m and 582 m between October 2009 and August 2010. The 11-month inshore bottom-temperature record revealed five substantial upwelling events lasting 5–10 days each where temperatures decreased by about 7 °C to 17–18 °C. Satellite sea surface temperature data showed these events to coincide with cold-water plumes occupying the northern wedge of the KwaZulu-Natal (KZN) Bight. Numerous shorter duration (1–2 days) upwelling events with less vivid surface expressions were also observed throughout the entire record where bottom temperature dropped by 2–3 °C. The last four months of the record were characterised by a protracted cool period lacking a seasonal trend but punctuated with oscillations of warm and cooler bottom water. In contrast to earlier studies that suggested upwelling was topographically and dynamically driven by the juxtaposition of the Cape St Lucia offset and the Agulhas Current (a solitary mechanism), our analysis showed almost all major and minor cold-water intrusions to coincide with upwelling-favourable north-easterly winds that simultaneously force a south-westerly coastal current. Ekman veering in the bottom boundary layer of the Agulhas Current, and the concomitant movement of cold water up the slope, was found to coexist at times with coastal upwelling, but its absence did not impede inshore cold-water intrusions, calling into question its role as a primary driver of upwelling. Both major and minor upwelling events were observed to promote phytoplankton blooms in the northern KZN Bight which commonly extended to the Thukela River. Wind-driven upwelling was also observed in the inner bight between Richards Bay and Port Durnford, explaining the ribbon of coastal chlorophyll continuously observed on ocean colour images between Cape St Lucia and the Thukela River. Similarities in upwelling character and mechanisms are observed between the northern KZN Bight and the Florida Current shelf systems.

Agulhas Current Influence on the Shelf Dynamics of the KwaZulu-Natal Bight

AbstractThe Agulhas Current is a fast western boundary current flowing along the South African east coast, strongly influencing oceanographic variability on the adjacent KwaZulu-Natal shelf. Hydrographic observations in January 2010, July 2010, and July 1989 have been used to describe variations in geostrophic currents and water masses and to elucidate mechanisms of change in circulation and thermohaline structure. Differences in wind forcing, precipitation, and solar insolation caused modifications in stratification and mixing that resulted in seasonal changes in surface temperature, salinity, and mixed layer depth ZMLD between the austral summer and winter surveys. During January 2010, surface temperatures were elevated and there was a large range in salinity as the upper 10 m was influenced by river outflow and rainfall. In July 2010 and 1989, surface temperatures were lower and the salinity range was significantly less. The ZMLD in January 2010 was much shallower, varying between 5 and 40 m, while deeper ZMLD, from 9 to 107 m, was found in July 2010 and 1989. Cross-shelf, alongshore, and vertical variations in water masses during July 2010 and July 1989 were driven by deviations in the position of the inshore edge of the Agulhas Current and the influence of offshore cyclonic eddies. In January 2010, the close proximity of the current to the coast and shelf break caused increased Ekman veering in the bottom layers along the shelf edge and slope in the north and recirculation in the south.

Ekman Veering, Internal Waves, and Turbulence Observed under Arctic Sea Ice

Abstract The ice–ocean system is investigated on inertial to monthly time scales using winter 2009–10 observations from the first ice-tethered profiler (ITP) equipped with a velocity sensor (ITP-V). Fluctuations in surface winds, ice velocity, and ocean velocity at 7-m depth were correlated. Observed ocean velocity was primarily directed to the right of the ice velocity and spiraled clockwise while decaying with depth through the mixed layer. Inertial and tidal motions of the ice and in the underlying ocean were observed throughout the record. Just below the ice–ocean interface, direct estimates of the turbulent vertical heat, salt, and momentum fluxes and the turbulent dissipation rate were obtained. Periods of elevated internal wave activity were associated with changes to the turbulent heat and salt fluxes as well as stratification primarily within the mixed layer. Turbulent heat and salt fluxes were correlated particularly when the mixed layer was closest to the freezing temperature. Momentum flux is adequately related to velocity shear using a constant ice–ocean drag coefficient, mixing length based on the planetary and geometric scales, or Rossby similarity theory. Ekman viscosity described velocity shear over the mixed layer. The ice–ocean drag coefficient was elevated for certain directions of the ice–ocean shear, implying an ice topography that was characterized by linear ridges. Mixing length was best estimated using the wavenumber of the beginning of the inertial subrange or a variable drag coefficient. Analyses of this and future ITP-V datasets will advance understanding of ice–ocean interactions and their parameterizations in numerical models.

On the numerical resolution of the bottom layer in simulations of oceanic gravity currents

Abstract. The role of an increased numerical vertical resolution, leading to an explicit resolution of the bottom Ekman layer dynamics, is investigated. Using the hydrostatic ocean model NEMO-OPA9, we demonstrate that the dynamics of an idealised gravity current (on an inclined plane), is well captured when a few (around five) sigma-coordinate levels are added near the ocean floor. Such resolution allows to considerably improve the representation of the descent and transport of the gravity current and the Ekman dynamics near the ocean floor, including the important effect of Ekman veering, which is usually neglected in today's simulations of the ocean dynamics. Results from high resolution simulations (with σ and z-coordinates) are compared to simulations with a vertical resolution commonly employed in today's ocean models. The latter show a downslope transport that is reduced by almost an order of magnitude and the decrease in the along slope transport is reduced six-fold. We strongly advocate for an increase of the numerical resolution at the ocean floor, similar to the way it is done at the ocean surface and at the lower boundary in atmospheric models.

Observation of the bottom boundary layer off the Soya Warm Current

A mooring observation of current velocity, temperature and bottom pressure was carried out approximately 30 km off the coast of Monbetsu, between August 7 and September 2, 2005, to investigate the characteristics of bottom boundary layer (BBL) off the Soya Warm Current (SWC). We succeeded in measuring the Ekman veering and bottom Ekman transport in the BBL. On comparing the observed current velocity with that represented by the classical theoretical equation, the observed alongshore current velocity in BBL disagreed with that represented by the classical theoretical equation, but the cross-shore one agreed well. However after applying a linear extrapolation for the alongshore current velocity to estimate the alongshore geostrophic current velocity above the bottom, we could explain the alongshore current velocity by that represented in the classical theoretical equation. Consequently, our observations strongly support one of the proposed formation mechanisms of the cold-water belt observed off the SWC, that is, the convergence of bottom Ekman transport. The volume transport of vertical pumping velocity was estimated to be (0.12–0.25) Sv. In addition, the vertical profile of average temperature in all observation periods shows that slightly warmer water lies beneath the homogenous temperature layer, in the BBL. The result is considered to imply that the down-slope advection due to bottom Ekman transport supplies the SWC water in BBL and the eddy diffusivity of order of 10−3 m2s−1 maintains the oceanic structure in the bottom mixed layer.

Variability of currents in front of the Venice Lagoon, Northern Adriatic Sea

Abstract. Time scales and modes of variability of the flow in the water column in the Northern Adriatic Sea for late summer 2002 are described based on current record from a single bottom-mounted ADCP in the shallow-water area in front of the Venice Lagoon. The time averaged flow was directed 277° E (CCW), roughly aligned with the coastline, with typical magnitudes in the range 4–6 cm/s and a limited, not significant clockwise veering with depth. Tidal forcing was weak and mainly concentrated in the semidiurnal frequency band, with a barotropic (depth-independent) structure. On a diurnal time scale, tidal signal was biased by the sea-breeze regime and was characterized by a clockwise veering with depth according to the Ekman spiral. A complex EOF analysis on the velocity profile time series extracted two dominant spatial modes of variability, which explained more than 90% of the total variance in the current field. More than 78% of the total variance was accounted for by the first EOF mode, with a barotropic structure that contained the low-frequency components and the barotropic tidal signal at semidiurnal and diurnal frequencies. The second mode had a baroclinic structure with a zero-crossing at mid-depth, which was related with the response of the water column to the high-frequency wind-driven diurnal sea breeze variability. The response of low-passed non-tidal currents to local wind stress was fast and immediate, with negligible temporal lag up to mid-depth. Currents vectors were pointing to the right of wind stress, as expected from the surface Ekman veering, but with angles smaller than the expected ones. A time lag in the range 10 to 11 h was found below 8 m depth, with current vectors pointing to the left of wind stress and a counterclockwise veering towards the bottom. The delay was consistent with the frictional adjustment time scale describing the dynamics of a frictionally dominated flow in shallow water, thus suggesting the importance of bottom friction on the motion over the entire water column.

Organization of stratification, turbulence, and veering in bottom Ekman layers

Detailed observations of the Ekman spiral in the stratified bottom boundary layer during a 3‐month period in an upwelling season over the Oregon shelf suggest a systematic organization. Counter‐clockwise veering in the bottom boundary layer is constrained to the weakly stratified layer below the pycnocline, and its height is nearly identical to the turbulent boundary layer height. Veering reaches 13+/−4 degrees near the bottom and exhibits a very weak dependence on the speed and direction of the interior flow and the thickness of the veering layer. A simple Ekman balance model with turbulent viscosity consistent with the law‐of‐the‐wall parameterization modified to account for stratification at the top of the mixed layer is used to demonstrate the importance of stratification on the Ekman veering. The model agrees reasonably well with observations in the lower 60–70% of the bottom mixed layer, above which it diverges from the data due to the unaccounted physics in the interior. Neglect of stratification in an otherwise identical model results in far worse agreement with the data yielding veering in the bottom Ekman layer which is much smaller than measured, but distributed over a much thicker layer.

A 2.5D model for sand transport in a shallow sea: effect of Ekman veering

A process-based sediment transport model is presented that combines the knowledge of sediment transport in rivers and estuaries with the understanding of physical processes specific for the shelf seas. Due to formation of bottom Ekman spiral, the near-bottom current and hence the sediment transport deviate from the direction of surface current. The model takes into account velocity veering induced by Ekman spiral and estimates both direction and rate of transport of suspended particulate matter (SPM) generated by a steady or slowly varying current, through suspension, relocation and deposition of sediment in a shallow sea. The model uses a combination of analytical and numerical methods: analytical integration, in the vertical, and numerical integration, in the horizontal. The model predicts also vertical erosion/deposition fluxes at the seabed. The deviation angles and other parameters of SPM transport are computed for a range of water depths 5–50 m, particle settling velocities 0.1–6 cm/s, and current speeds 0.4–1.2 m/s. The model converges automatically to traditional engineering-style formulations, in extreme case of strong current in very shallow water, where velocity veering is of minor importance.

Numerical studies of the current response at Ormen Lange to a travelling storm

Mean temporal circulation at Ormen Lange (OL) off mid-Norway is strongly dominated by the Norwegian Atlantic Current. Tidal effects are weak. Observed events of short peak value in current speed along with peak in temperature close to the seabed at the shelf slope may be driven by strong pressure gradients. That is, strong atmospheric low pressures and/or internal pressure fronts between warmer Atlantic Water (AW) and colder Norwegian Sea Water (NSW). Along the shelf slope at OL there may be steepening of the isopycnals, separating AW and NSW, due to wind generated downwelling and strong Ekman veering during storms or approaching internal density fronts. During such events the density surfaces tend to undershoot their equilibrium level, and as the forcing weakens, the suppressed water may run up along the shelf slope. In this run up phase, peak values in the velocities are often found.Since the atmospheric forcing may be a major forcing mechanism behind the observed events, we have performed a numerical study of the sensitivity of near bottom velocities at OL to the pathway of low pressure disturbances and the strength and the radii of them.The results indicate how depth dependent currents at different locations respond to varying atmospheric forcing, with highest currents close to the surface, time delay of response at greater depths and highest impact of storms passing further out in the Nordic Seas than immediately over OL.

Observations of the Storfjorden overflow

The mixing and spreading of the Storfjorden overflow were investigated with density and horizontal velocity profiles collected at closely spaced stations. The dense bottom water generated by strong winter cooling, enhanced ice formation and the consequent brine rejection drains into and fills the depression of the fjord and upon reaching a 120-m deep sill, descends like a gravity current following the bathymetry towards the shelf edge. The observations covered an approximate 37-km path of the plume starting from about 68 km downstream of the sill. The plume is identified as two layers: a dense layer 1 with relatively uniform vertical structure underlying a thicker layer 2 with larger vertical density gradients. Layer 1, probably remnants from earlier overflows, almost maintains its temperature–salinity characteristics and spreads to a width of about 6 km over its path, comparable to spread resulting from Ekman veering. Layer 2, on the other hand, is a mixing layer and widens to about 16 km. The overflow, in its core, is observed to have salinities greater than 34.9, temperatures close to the freezing point, and light transmissivity typically 5% less than that of the ambient waters. The overall properties of the observed part of the plume suggest dynamical stability with weak entrainment. However local mixing is observed through profiles of the gradient Richardson number, the non-dimensional ratio of density gradient over velocity gradient, which show portions with supercritical values in the vicinity of the plume–ambient water interface. The net volume transport associated with the overflow is estimated to be 0.06 Sv (Sv≡10 6 m 3 s −1) out of a section closest to the sill and almost double that as it leaves the section furthest downstream. The weak entrainment is estimated to account for the doubling of the volume transport between the two sections. A simple model proposed by Killworth (J. Geophys. Res. 106 (2001) 22267), giving the path of the overflow from a constant rate of vertical descent along the slope, compares well with our observations.

Wintertime shoreward near-surface currents south of Cape Hatteras

[1] Cross-isobath flow on continental shelves is of interest for a variety of reasons. Near Cape Hatteras, North Carolina, the transport of larval organisms, pollutants, and oceanic carbon budget constituents to and from the adjacent Albemarle and Pamlico Sounds may depend critically on cross-isobath currents. Shoreward currents in the near-surface waters south of Cape Hatteras are documented herein, on the basis of continuous 2-year time series, encompassing all or part of three consecutive winters. Energetic shoreward currents exist ∼30% of the time from midfall through late spring. These currents are evident over the 20 and 35 m isobaths along a mooring line situated ∼40 km southwestward from Cape Hatteras. Shoreward velocities average ∼12 cm/s, and events persist from 0.5 to 4 days, occurring every 2.5–5 days, except in summer. These events often coincide with southwestward winds but occur under both upwelling and downwelling favorable conditions, such that Ekman veering in the surface layer does not account for the shoreward velocities. In winter the mooring line south of Cape Hatteras is frequently traversed by a strong temperature and salinity front, with light, relatively fresh, cold, stratified water on one side, and denser, more saline, warmer, unstratified water on the other. Hydrography and satellite sea surface temperature imagery help identify this front as the boundary between South Atlantic Bight and Mid-Atlantic Bight coastal shelf waters, the “Hatteras Front.” Flow along the Hatteras Front where it crosses the shelf appears to account for the observed shoreward currents. The along-shelf advection of the Hatteras Front may depend on both winds and Gulf Stream distance offshore.

Observations of near-bottom currents in the Gulf of Riga, Baltic Sea

Near-bottom currents and temperature were measured at 2 and 4 m above bottom (m.a.b.) in the Gulf of Riga, a semi-enclosed sub basin of the Baltic Sea. Measurements were taken during three periods of different vertical stratification and weather conditions. Water depth was 54 meters. The studied area comprised the outer region of the bottom boundary layer, as judged by the observed Ekman veering of the current vector.¶Local wind was the primary driving force of near-bottom currents in the deepest area of the Gulf of Riga. Imposed wind stress induced surface-layer flow to the right, while compensating return flow developed in the bottom boundary layer. The response of current field was dominated by low-frequency variability with a clear indication of topographic steering and minor variability due to near-inertial frequency processes. The near-inertial frequency processes were generated also by interaction of low-frequency waves with the topography. The low-frequency and near-inertial frequency processes were of equal value in producing vertical shear of horizontal velocity in the near-bottom layer. The quasi-steady flow in autumn was forced by strong net inflow to the Gulf of Riga through the Irbe Strait. Lateral advection of water is important for the deep-water renewal that is due to dominating low-frequency currents.

A Coupled Hydrodynamic–Bottom Boundary Layer Model of Ekman Flow on Stratified Continental Shelves

Abstract This paper describes a hydrodynamic model with turbulent energy closure that uses a simplified wave-current interaction model of the bottom boundary layer to compute bed drag coefficients. The coupled model is used to investigate the interaction of the upper and lower boundary layers with the geostrophic core flow for simple shelf geometry and forcing, and to evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification. The thickness of the bottom boundary layer predicted by the model ranges from 10 to 35 m and is consistent with observations from the California shelf. The increased bottom friction calculated by the coupled model in intermediate water depths increases bottom Ekman veering (leftward in the Northern Hemisphere) by as much as 10° if stratification is strong, thus enhancing downwelling and upwelling. Currents along isobaths in shallow water are uniformly decreased by as much as 25% in the coupled model for both summer and winter init...

Tidal and subtidal variability in the sloping benthic boundary layer

Observations are presented of the benthic boundary layer (BBL) structure for two sites on the continental slope, west of the British Isles. Variability at the tidal (M2) and subtidal frequencies is discussed. A mean poleward, along‐isobath current is present at both sites, with periodic (5–8 days) reversals observed in the flow. These reversals cause a change in the water column stratification close (<100 m) to the seabed. The relationship between the direction of the along‐ and cross‐slope flow, stratification, and the change of temperature in the frictional layer, relative to that above it, is consistent with Ekman veering induced up/downwelling close to the slope. The BBL temperature (density) structure is highly variable at the tidal frequency. There is a periodic variability in the current shear and associated Richardson number (Ri), particularly for the region where the bottom slope is close to critical for the M2 internal tide. Mixed or gravitationally unstable density layers are generated about one buoyancy period after the minimum Ri are observed. The mixed layers are transient, however, and do not persist throughout the tidal cycle. Indirect estimates of the vertical turbulent eddy diffusivity (Kυ) were made from conductivity‐temperature‐depth (CTD) yoyo casts, and its variability through a tidal cycle measured. A background level of 10−4 m2 s−1 is estimated with high values O(10−2 m2 s−1) measured for the times associated with the generation of mixed or inverted density layers. A mean value ofKυ = 15–50×10−4 m2 s−1 is found. The observations are compared to recent models of the sloping BBL and laboratory experiments, with particular emphasis on the transient nature of the BBL and the influence of internal wave reflection from the slope.

Observation of Ekman Veering at the Bottom of Lake Michigan

An experiment in 100 m of water off the east coast of Lake Michigan during 1984 provided continuous current velocity recordings at four levels located from 1 m to 9 m above the lake bottom and also at 50 m. Near-inertial-period current oscillations were prominent and superimposed on longer-period velocity variations. Current speed at 1 m elevation varied from less than the threshold speed of the Savonious rotor sensor to 13 cm s −1. Profiles of current speed were close to logarithmic during many episodes of measurable currents over a 4-month-long recording interval. Over 50% of hourly-averaged profiles were in the nearly-logarithmic category (R > 0.987) during the month of September 1984. Veering averaging 11° was observed between low-pass-filtered current velocities measured at elevations of 1 m and at 9 m. Boundary layer theories developed from ocean studies are surveyed and the Lake Michigan data are examined from that perspective.

A comparison between geostrophic and observed winds at a Japan Meteorological Agency Buoy in the East China Sea

Geostrophic winds (vg) calculated over the East China Sea from surface pressure maps are compared to observed winds (vo) from a Japan Meteorological Agency Ocean Data Buoy for the period of January-April 1986. For mean winds, the average counterclockwise veering angle fromvg tov0 is 32° and there is no difference in speed. For fluctuating winds, the Ekman veering and speed reduction fromvg tov0 are respectively 16° and 2%. Co-spectral analysis of the two time series indicates very high coherency and a slight lead in phase by the buoy observations. The phase difference is consistent with the general movement of fronts from west to east and the fact that the geostrophic winds are calculated for a point 155 km to the southeast of the buoy.

Water stress and ocean current measurements under first‐year sea ice in the Canadian Arctic

Ocean current profiles were measured to calculate the ice‐water drag coefficent for tidal currents under landfast ice at the intersection of Lancaster Sound and Navy Board Inlet in the Canadian Arctic. Under‐ice current profiles showed 1–3 m of a logarithmic layer with a friction velocity ranging from 0.3 to 1.0 cm/s. No significant Ekman veering was found below the logarithmic layer, which suggests a strong modulation of the surface current by tidal currents. The ice‐water drag coefficients at 1‐m depth, calculated from the mean current profiles, ranged from 0.0053 to 0.0093, reflecting a nonhomogeneous rough surface and ridges. It was shown that the present estimate of the drag coefficient was consistent with previously measured values of the ice stress.

Observations and simulation of a bottom Ekman layer on a continental shelf

A numerical model was used to simulate the bottom Ekman layer of a continental shelf region. The basis for the model was the Mellor and Yamada level turbulence closure scheme. Conservation equations for momentum, turbulent kinetic energy, and turbulent length scale were utilized in the model. The model was used to simulate data taken from a Cyclesonde mooring on the Peruvian continental shelf in May 1976 as part of the Joint II Coastal Upwelling Ecosystems Analysis program. The Cyclesonde provided mean horizontal velocity, temperature, salinity, and pressure data. An intense poleward undercurrent drove the bottom flow regime. The most striking feature of the data was the clockwise Ekman veering of velocity vectors as the bottom was approached. A 48‐hour period was chosen for the model simulations. The vertical profile of speed (48 hours mean) simulated by the model fell within the error bars of the data. The corresponding Ekman spiral display of the model results also showed good agreement with the observations.