Vertical Eddy Fluxes in the Southern Ocean
Jan D. ZikaJulien Le SommerCarolina O. DufourJean‐Marc MolinesBernard BarnierPierre BrasseurRaphaël DussinThierry PenduffDaniele IudiconeAndrew LentonGurvan MadecPierre MathiotJames C. OrrEmily ShuckburghFrédéric Vivier
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Abstract The overturning circulation of the Southern Ocean has been investigated using eddying coupled ocean–sea ice models. The circulation is diagnosed in both density–latitude coordinates and in depth–density coordinates. Depth–density coordinates follow streamlines where the Antarctic Circumpolar Current is equivalent barotropic, capture the descent of Antarctic Bottom Water, follow density outcrops at the surface, and can be interpreted energetically. In density–latitude coordinates, wind-driven northward transport of light water and southward transport of dense water are compensated by standing meanders and to a lesser degree by transient eddies, consistent with previous results. In depth–density coordinates, however, wind-driven upwelling of dense water and downwelling of light water are compensated more strongly by transient eddy fluxes than fluxes because of standing meanders. Model realizations are discussed where the wind pattern of the southern annular mode is amplified. In density–latitude coordinates, meridional fluxes because of transient eddies can increase to counter changes in Ekman transport and decrease in response to changes in the standing meanders. In depth–density coordinates, vertical fluxes because of transient eddies directly counter changes in Ekman pumping.Keywords:
Downwelling
Eddy
Ekman transport
Barotropic fluid
Antarctic Intermediate Water
Downwelling
Deep ocean water
Physical oceanography
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Abstract We present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003–14 average of 2.3 m yr −1 downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30 m yr −1 downward to 10 m yr −1 upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18 m yr −1 is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed.
Downwelling
Ekman transport
Canada Basin
Beaufort scale
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Downwelling
Ekman transport
Antarctic Intermediate Water
Ekman layer
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The structure of the annual-mean shallow meridional overturning circulation(SMOC) in the South China Sea(SCS) and the related water movement are investigated,using simple ocean data assimilation(SODA) outputs.The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale,which consists of downwelling in the northern SCS,a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow,with a strength of about 1×10~6 m~3/s.The formation mechanisms of its branches are studied separately.The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m.The annual-mean Ekman transport across 18°N is about 1.2×10~6 m~3/s.An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework.An annual subduction rate of about 0.66×10~6m~3/s is obtained between 17° and 20°N,of which 87% is due to vertical pumping and 13% is due to lateral induction.The subduction rate implies that the subdution contributes significantly to the downwelling branch.The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward.The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents.Significant upwelling mainly occurs off the Vietnam coast in the southern SCS.An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×10~6m~3/s,of which a large portion is contributed by summer upwelling,with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.
Downwelling
Ekman transport
Antarctic Intermediate Water
Ekman layer
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Winds observed at an island in the Gulf of Alaska display a large annual signal with maximum easterlies in winter and weak easterlies in summer. For the northern Gulf of Alaska, the monthly mean Ekman transport is onshore in all months leading to sea surface setup and downwelling, creating westward barotropic and baroclinic longshore currents. While downwelling dominates over any month, shorter period reversals always occur, leading to rapid alternation of upwelling and downwelling situations. The cross-shelf Ekman transport is skewed onshore so that extreme transport events tend to be onshore and hence be downwelling events. Upwelling indices calculated from synoptic pressure maps over the Gulf of Alaska exhibit a larger annual cycle than does the same parameter determined from observed winds. The observed winds do not contain the summer reversal predicted from the upwelling indices. This discrepancy between the onshore Ekman transport determined by the synoptic pressure data and that determined from observed winds is attributed to orographic effects where the pressure gradient is interrupted by a coastal mountain range. Similar effects could exist with other estimates of upwelling indices based on a large-scale grid.
Downwelling
Ekman transport
Barotropic fluid
Ekman layer
Pressure gradient
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Geostrophic transport can affect the structure of the wind-driven coastal upwelling/downwelling. Focusing on quantifying this impact is vital to understanding circulation dynamics in the Persian Gulf. To this end, in this study, after the investigation of wind patterns, the temporal and spatial structure of coastal upwelling/downwelling using the Ekman transport upwelling index, and the intra-annual vertical variability of temperature are investigated based on the daily wind, and monthly temperature data time series of 28 years (1993–2020). Then, the geostrophic transport using an improved methodology and the total cross-shore transport as a sum of Ekman and geostrophic transport are estimated based on the monthly SLA data time series. The results indicated that the region, located around 51.5 and 28 (48 and 29 and 50.5 and 25.5) experienced the most intense coastal upwelling (downwelling) at a peak in June with larger mixed and thermocline layers than other regions. The intensity of Ekman transport is higher than the geostrophic transport in the Persian Gulf due to the presence of the prevailing wind and the shallowness of the mixed layers’ depth. We found that the intensity of the coastal upwelling (downwelling) decreases (increases) under favorable spatial and temporal conditions by considering the geostrophic transport in the upwelling index.
Downwelling
Ekman transport
Geostrophic current
Wind Stress
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Circumpolar deep water
Ekman transport
Outflow
Antarctic Intermediate Water
Deep ocean water
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Abstract The circulation induced by the interaction of surface Ekman transport with an island is considered using both numerical models and linear theory. The basic response is similar to that found for the interaction of Ekman layers and an infinite boundary, namely downwelling (upwelling) in narrow boundary layers and deformation-scale baroclinic boundary layers with associated strong geostrophic flows. The presence of the island boundary, however, allows the pressure signal to propagate around the island so that the regions of upwelling and downwelling are dynamically connected. In the absence of stratification the island acts as an effective barrier to the Ekman transport. The presence of stratification supports baroclinic boundary currents that provide an advective pathway from one side of the island to the other. The resulting steady circulation is quite complex. Near the island, both geostrophic and ageostrophic velocity components are typically large. The density anomaly is maximum below the surface and, for positive wind stress, exhibits an anticyclonic phase rotation with depth (direction of Kelvin wave propagation) such that anomalously warm water can lie below regions of Ekman upwelling. The horizontal and vertical velocities exhibit similar phase changes with depth. The addition of a sloping bottom can act to shield the deep return flow from interacting with the island and providing mass transport into/out of the surface Ekman layer. In these cases, the required transport is provided by a pair of recirculation gyres that connect the narrow upwelling/downwelling boundary layers on the eastern and western sides of the island, thus directly connecting the Ekman transport across the island.
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Ekman transport
Ekman layer
Boundary current
Ekman number
Stratification (seeds)
Kelvin wave
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Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results.
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Drifter
Ekman transport
Eddy diffusion
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Downwelling
Halocline
Ekman transport
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