Water Mass Transformations Driven by Ekman Upwelling and Surface Warming in Subpolar Gyres

AbstractThe Sverdrup relationship when applied to the Southern Ocean suggests that some isopycnals that are deep in the eastern Pacific will shoal in the Atlantic. Cold waters surfacing in the South Atlantic at midlatitudes would be warmed by the atmosphere. The potential for water mass transformations in this region is studied by applying a three-layer planetary geostrophic model to a wide ocean basin driven by the Ekman upwelling typical of the Southern Ocean surface winds. The model uses a simple physically based parameterization of the entrainment of mass into the surface layer with zonally symmetric atmospheric surface fields to find steady-state subpolar gyre solutions. The solutions are found numerically by specifying suitable boundary conditions and integrating along characteristics. With reasonable parameter settings, transformations of more than 10 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water between layers are obtained. The water mass transformations are sensitive to the strength of the wind stress curl and the width of the basin and relatively insensitive to the parameterization of the surface heat fluxes. On the western side of the basin where the cold waters are near the surface, there is a large region where there is a local balance between the Ekman pumping and the exchange of mass between layers. Simple formulas are derived for the water mass transformation rates in terms of the difference between the maximum and minimum northward Ekman transports integrated across the basin and the depths of the isopycnal layers on the eastern boundary. The relevance of the model to the Southern Ocean and the Atlantic meridional overturning circulation is briefly discussed.

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Net surface heat fluxes and Meridional Overturning Circulations

10.5194/egusphere-egu21-2111 ◽

2021 ◽

Author(s):

Mike Bell ◽

Pat Hyder ◽

Twm Jonathan ◽

Helen Johnson ◽

David Marshall ◽

...

Keyword(s):

Cold Water ◽

Surface Heat ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

Cold Pool ◽

Ekman Pumping ◽

Surface Heat Fluxes ◽

Creative Commons ◽

Geographical Patterns ◽

Meridional Overturning

The geographical patterns of the annual mean net surface heat fluxes (NSHF) simulated by the HadGEM3 GC3.1 coupled atmosphere-ocean models are shown to agree well with the CDEEP analyses. The patterns for the coarse resolution (N96O1) and high resolution (N512O12) simulations are shown to be similar (except near the &#8220;cold pool of death&#8221;). We argue that they can be interpreted relatively simply in terms of (a) regions of net surface heating where Ekman pumping provides a supply of cold water at the sea surface and (b) regions of net cooling where boundary currents have taken warm water poleward. We extend the simple models of Gnanadesikan (1999), Nikurashin & Vallis (2011) and Bell (2015) for the mid-depth Meridional Overturning Circulation (MOC) to a simple model describing the upper and mid-depth MOC cells. As a first step in investigating whether these ideas simulate the model circulations &#8220;realistically&#8221;, we show that in the HadGEM3 Pacific Ocean, time-variations in the annual and zonal mean NSHF within 5o of the equator are well correlated (r2=0.6) with those in the annual and zonal mean wind stress along the equator. Finally we explore a warm, salty wedge of water next to the eastern boundary in the north Atlantic N96O1 pre-industrial simulations and interpret its northward heat transport in terms suggested by Bell (2015).&#160; &#160;&#160;&#160;This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.

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Meridional Overturning Circulations Driven by Surface Wind and Buoyancy Forcing

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0255.1 ◽

2015 ◽

Vol 45 (11) ◽

pp. 2701-2714 ◽

Cited By ~ 2

Author(s):

Michael J. Bell

Keyword(s):

Heat Loss ◽

Water Mass ◽

Surface Wind ◽

Surface Heat ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Eastern Boundary ◽

Transformation Rates ◽

Meridional Overturning ◽

Water Mass Transformations

AbstractThe meridional overturning circulation (MOC) can be considered to consist of a downwelling limb in the Northern Hemisphere (NH) and an upwelling limb in the Southern Hemisphere (SH) that are connected via western boundary currents. Steady-state analytical gyre-scale solutions of the planetary geostrophic equations are derived for a downwelling limb driven in the NH solely by surface heat loss. In these solutions the rates of the water mass transformations between layers driven by the surface heat loss determine the strength of the downwelling limb. Simple expressions are obtained for these transformation rates that depend on the most southerly latitudes where heat loss occurs and the depths of the isopycnals on the eastern boundary. Previously derived expressions for the water mass transformation rates in subpolar gyres driven by the Ekman upwelling characteristic of the SH are also summarized. Explicit expressions for the MOC transport and the depths of isopycnals on the eastern boundary are then derived by equating the water mass transformations in the upwelling and downwelling limbs. The MOC obtained for a “single-basin” two-layer model is shown to be generally consistent with that obtained by Gnanadesikan. The model’s energetics are derived and discussed. In a world without a circumpolar channel in the SH, it is suggested that the upwelling limb would feed downwelling limbs in both hemispheres. In a world with two basins in the NH, if one of them has a strong halocline the model suggests that the MOC would be very weak in that basin.

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Southern Ocean Deep Circulation and Heat Uptake in a High-Resolution Climate Model

Journal of Climate ◽

10.1175/jcli-d-15-0513.1 ◽

2016 ◽

Vol 29 (7) ◽

pp. 2597-2619 ◽

Cited By ~ 26

Author(s):

Emily R. Newsom ◽

Cecilia M. Bitz ◽

Frank O. Bryan ◽

Ryan Abernathey ◽

Peter R. Gent

Keyword(s):

High Resolution ◽

Southern Ocean ◽

Sea Ice ◽

Climate Model ◽

Water Mass ◽

Global Climate Model ◽

Substantial Reduction ◽

Meridional Overturning Circulation ◽

Heat Uptake ◽

Water Mass Transformation

Abstract The dynamics of the lower cell of the meridional overturning circulation (MOC) in the Southern Ocean are compared in two versions of a global climate model: one with high-resolution (0.1°) ocean and sea ice and the other a lower-resolution (1.0°) counterpart. In the high-resolution version, the lower cell circulation is stronger and extends farther northward into the abyssal ocean. Using the water-mass-transformation framework, it is shown that the differences in the lower cell circulation between resolutions are explained by greater rates of surface water-mass transformation within the higher-resolution Antarctic sea ice pack and by differences in diapycnal-mixing-induced transformation in the abyssal ocean. While both surface and interior transformation processes work in tandem to sustain the lower cell in the control climate, the circulation is far more sensitive to changes in surface transformation in response to atmospheric warming from raising carbon dioxide levels. The substantial reduction in overturning is primarily attributed to reduced surface heat loss. At high resolution, the circulation slows more dramatically, with an anomaly that reaches deeper into the abyssal ocean and alters the distribution of Southern Ocean warming. The resolution dependence of associated heat uptake is particularly pronounced in the abyssal ocean (below 4000 m), where the higher-resolution version of the model warms 4.5 times more than its lower-resolution counterpart.

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Absorption of Ocean Heat Along and Across Isopycnals in HadCM3

10.5194/egusphere-egu21-12769 ◽

2021 ◽

Author(s):

Louis Clement ◽

Elaine McDonagh ◽

Jonathan Gregory ◽

Quran Wu ◽

Alice Marzocchi ◽

...

Keyword(s):

North Atlantic ◽

Heat Fluxes ◽

High Latitudes ◽

Ekman Pumping ◽

Temperature Changes ◽

Surface Heat Fluxes ◽

The North ◽

Winter Mixed Layer ◽

The North Atlantic ◽

Heat Budgets

Anthropogenic warming added to the climate system accumulates mostly in the ocean interior and discrepancies in how this is modelled contribute to uncertainties in predicting sea level rise. Temperature changes are partitioned between excess, due to perturbed surface heat fluxes, and redistribution, that arises from the changing circulation and perturbations to mixing. In a model (HadCM3) with realistic historical forcing (anthropogenic and natural) from 1960 to 2011, we firstly compare this excess-redistribution partitioning with the spice and heave decomposition, in which ocean interior temperature anomalies occur along or across isopycnals, respectively. This comparison reveals that in subtropical gyres (except in the North Atlantic) heave mostly captures excess warming in the top 2000 m, as expected from Ekman pumping, whereas spice captures redistributive cooling. At high-latitudes and in the subtropical Atlantic, however, spice predicts excess warming at the winter mixed layer whereas below this layer, spice represents redistributive warming in southern high latitudes.&#160;Secondly, we use Eulerian heat budgets of the ocean interior to identify the process responsible for excess and redistributive warming. In southern high latitudes, spice warming results from reduced convective cooling and increased warming by isopycnal diffusion, which account for the deep redistributive and shallow excess warming, respectively. In the North Atlantic, excess warming due to advection contains both cross-isopycnal warming (heave found in subtropical gyres) and along-isopycnal warming (spice). Finally, projections of heat budgets &#8212;coupled with salinity budgets&#8212; into thermohaline and spiciness-density coordinates inform us about how water mass formation occurs with varying T-S slopes. Such formation happens preferentially along isopycnal surfaces at high-latitudes and along isospiciness surfaces at mid-latitudes, and along both coordinates in the subtropical Atlantic. Because spice and heave depend only on temperature and salinity, our study suggests a method to detect excess warming in observations.

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Observations on Seychelles-Chagos Thermocline Ridge (SCTR) upwelling during April-May 2019 in the western tropical Indian Ocean

10.5194/egusphere-egu21-14203 ◽

2021 ◽

Author(s):

Suyun Noh ◽

SungHyun Nam

Keyword(s):

Indian Ocean ◽

Tropical Indian Ocean ◽

Meridional Overturning Circulation ◽

Time Varying ◽

Ekman Pumping ◽

Wind Stress Curl ◽

The Pacific ◽

Equatorial Upwelling ◽

Meridional Overturning ◽

Upwelling Intensity

The Seychelles-Chagos Thermocline Ridge (SCTR) in the western tropical Indian Ocean is known as a region of off-equatorial upwelling contrasting to equatorial upwelling in the Pacific and Atlantic where the most wide open-ocean upwelling occurs corresponding to ascending branch of one of the meridional overturning cells in the Indian Ocean, yet detailed stratification, upwelling intensity, and dynamics of SCTR upwelling variability are still poorly understood. Here, we present observational results on the SCTR upwelling based on ship-based data collected during April-May 2019 as a part of the Korea-US inDian Ocean Scientific Research Program (KUDOS). The upwelling structure is confirmed from 20 &#8451; and 10 &#8451; isotherms (D20 and D10) shoaling up in the center of SCTR, from 200 m to 100 m (D20) and from 600 m to 400 m (D10), respectively. Horizonal divergence at the upper 250 m within an 1&#176; by 1&#176; area in the SCTR center (8 &#176;S, 61 &#176;E) estimated from currents measurements along the boundaries (1.0 x 10-3 Sv) supports a mean upwelling intensity of 7.0 x 10-3 m day-1 (1.0 x 10-3 Sv divided by the area). The upwelling intensity generally decreases with depth but shows multiple peaks within the upper water column, yielding the maximum peak (5.0 x 10-2 m day-1) at 60 m and the minimum peak (1.4 x 10-2 m day-1) at 230 m, with negative peaks (downwelling) at depths around 100 and 210 m. Our results on the observed structure and intensity of SCTR upwelling are discussed in comparison to time-varying local wind stress curl-driven Ekman pumping, D20-based Seychelles Upwelling Index (SUI), and Indian Dipole Mode Index (DMI). Detailed observations on the structure and intensity of SCTR upwelling presented here have important implications on time-varying SCTR upwelling (e.g., weakened upwelling peaked in fall 2019) and climate via meridional overturning circulation in the upper Indian Ocean.

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Sea-ice retreat suggests re-organization of water mass transformation in the Nordic and Barents Seas

Nature Communications ◽

10.1038/s41467-021-27641-6 ◽

2022 ◽

Vol 13 (1) ◽

Author(s):

G. W. K. Moore ◽

K. Våge ◽

I. A. Renfrew ◽

R. S. Pickart

Keyword(s):

Sea Ice ◽

Water Mass ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

Spatiotemporal Variability ◽

Warming Climate ◽

Ice Retreat ◽

Meridional Overturning ◽

Sea Ice Retreat ◽

Water Mass Transformation

AbstractWater mass transformation in the Nordic and Barents Seas, triggered by air-sea heat fluxes, is an integral component of the Atlantic Meridional Overturning Circulation (AMOC). These regions are undergoing rapid warming, associated with a retreat in ice cover. Here we present an analysis covering 1950−2020 of the spatiotemporal variability of the air-sea heat fluxes along the region’s boundary currents, where water mass transformation impacts are large. We find there is an increase in the air-sea heat fluxes along these currents that is a function of the currents’ orientation relative to the axis of sea-ice change suggesting enhanced water mass transformation is occurring. Previous work has shown a reduction in heat fluxes in the interior of the Nordic Seas. As a result, a reorganization seems to be underway in where water mass transformation occurs, that needs to be considered when ascertaining how the AMOC will respond to a warming climate.

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Eddy Cancellation of the Ekman Cell in Subtropical Gyres

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0097.1 ◽

2016 ◽

Vol 46 (10) ◽

pp. 2995-3010 ◽

Cited By ~ 9

Author(s):

Edward W. Doddridge ◽

David P. Marshall ◽

Andrew McC. Hogg

Keyword(s):

Biological Activity ◽

Southern Ocean ◽

Wind Stress ◽

Particle Tracking ◽

Potential Vorticity ◽

Large Scale ◽

Ekman Pumping ◽

Lagrangian Particle Tracking ◽

Lagrangian Particle ◽

Wind Stress Curl

AbstractThe presence of large-scale Ekman pumping associated with the climatological wind stress curl is the textbook explanation for low biological activity in the subtropical gyres. Using an idealized, eddy-resolving model, it is shown that Eulerian-mean Ekman pumping may be opposed by an eddy-driven circulation, analogous to the way in which the atmospheric Ferrel cell and the Southern Ocean Deacon cell are opposed by eddy-driven circulations. Lagrangian particle tracking, potential vorticity fluxes, and depth–density streamfunctions are used to show that, in the model, the rectified effect of eddies acts to largely cancel the Eulerian-mean Ekman downwelling. To distinguish this effect from eddy compensation, it is proposed that the suppression of Eulerian-mean downwelling by eddies be called “eddy cancellation.”

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Wind-induced upwelling in the Kerguelen Plateau region

Biogeosciences ◽

10.5194/bg-11-6389-2014 ◽

2014 ◽

Vol 11 (22) ◽

pp. 6389-6400 ◽

Cited By ~ 14

Author(s):

S. T. Gille ◽

M. M. Carranza ◽

R. Cambra ◽

R. Morrow

Keyword(s):

Wind Stress ◽

Vertical Mixing ◽

Euphotic Zone ◽

Ekman Pumping ◽

Kerguelen Plateau ◽

Wind Stress Curl ◽

Chl A ◽

The North ◽

The Impact ◽

Ekman Upwelling

Abstract. In contrast to most of the Southern Ocean, the Kerguelen Plateau supports an unusually strong spring chlorophyll (Chl a) bloom, likely because the euphotic zone in the region is supplied with higher iron concentrations. This study uses satellite wind, sea surface temperature (SST), and ocean color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the euphotic zone. Results show that, in the Kerguelen region, cold SSTs correlate with high wind speeds, implying that wind-mixing leads to enhanced vertical mixing. Cold SSTs also correlate with negative wind-stress curl, implying that Ekman pumping can further enhance upwelling. In the moderate to high eddy kinetic energy (EKE) regions surrounding Kerguelen, we find evidence of coupling between winds and SST gradients associated with mesoscale eddies, which can locally modulate the wind-stress curl. This coupling introduces persistent wind-stress curl patterns and Ekman pumping around these long-lived eddies, which may modulate the evolution of Chl a in the downstream plume far offshore. Close to the plateau, this eddy coupling breaks down. Kerguelen has a significant wind shadow on its downwind side, which changes position depending on the prevailing wind and which generates a wind-stress curl dipole that shifts location depending on wind direction. This leads to locally enhanced Ekman pumping for a few hundred kilometers downstream from the Kerguelen Plateau; Chl a values tend to be more elevated in places where wind-stress curl induces Ekman upwelling than in locations of downwelling, although the estimated upwelling rates are too small for this relationship to derive from direct effects on upward iron supply, and thus other processes, which remain to be determined, must also be involved in the establishment of these correlations. During the October and November (2011) KErguelen Ocean and Plateau compared Study (KEOPS-2) field program, wind conditions were fairly typical for the region, with enhanced Ekman upwelling expected to the north of the Kerguelen Islands.

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Evolving air-sea interaction due to sea-ice retreat points to a re-organisation of water mass transformation in the Nordic and Barents Seas

10.5194/egusphere-egu21-10382 ◽

2021 ◽

Author(s):

Kent Moore ◽

Kjetil Våge ◽

Ian Renfrew ◽

Bob Pickart

Keyword(s):

Sea Ice ◽

Water Mass ◽

Critical Role ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

Spatiotemporal Variability ◽

Ice Retreat ◽

Meridional Overturning ◽

Sea Ice Retreat ◽

Water Mass Transformation

The Nordic and Barents Seas play a critical role in the climate system resulting from water mass transformation, triggered by intense air-sea heat fluxes, that is an integral component of the Atlantic Meridional Overturning Circulation (AMOC). These seas are undergoing rapid warming, associated with a retreat in ice cover. Here we present a novel analysis, covering the period 1950-2020, of the spatiotemporal variability of the air-sea heat fluxes along the region&#8217;s boundary currents, where the impacts on the water mass transformation are large. &#160;We find that the variability is a function of the relative orientation of the current and the axis of sea-ice change that can result in up to a doubling of the heat fluxes over the period of interest. This implies enhanced water mass transformation is occurring along these currents. In contrast, previous work has shown a reduction in fluxes in the interior sites of the Nordic Seas, where ocean convection is also observed, suggesting that a reorganization may be underway in the nature of the water mass transformation, that needs to be considered when ascertaining how the AMOC will respond to a warming climate.

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Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic

10.5194/egusphere-egu21-15972 ◽

2021 ◽

Author(s):

D. Gwyn Evans ◽

N. Penny Holliday ◽

Marilena Oltmanns

Keyword(s):

North Atlantic ◽

Water Mass ◽

Meridional Overturning Circulation ◽

Mass Analysis ◽

Overturning Circulation ◽

Water Mass Analysis ◽

Meridional Overturning ◽

The Mean ◽

Water Mass Transformation ◽

Water Mass Transformations

The OSNAP (Overturning in the Subpolar North Atlantic Program) array at ~60&#176;N has provided new and unprecedented insight into the strength and variability of the meridional overturning circulation in the subpolar North Atlantic. OSNAP has identified the region of the subpolar North Atlantic east of Greenland as a key region for the water mass transformation and densification that sets the strength and variability of the overturning circulation. Here, we will investigate the drivers of this water mass transformation and their roles in driving the overturning circulation at OSNAP. Using a water mass analysis on both model-based and observational-based datasets, we isolate diathermal (across surfaces of constant temperature) and diahaline (across surfaces of constant salinity) transformations due to air-sea buoyancy fluxes, and mixing. We show that the time-mean overturning strength is set by both the air-sea buoyancy fluxes and the strength of subsurface mixing. This balance is apparent on a seasonal timescale, where we resolve large seasonal fluctuations in the both the air-sea buoyancy fluxes and mixing. The residual of this seasonal cycle then corresponds to the mean overturning strength. On interannual timescales, mixing becomes the dominant driver of variability in the overturning circulation. To determine the location of these water mass transformations and the dynamical processes responsible for the mixing-driven variability, our water mass analysis is projected onto geographical coordinates.

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