Tidal and near-inertial energy density and energy fluxes over the Reykjanes Ridge

The Reykjanes Ridge is a key topographic structure stretching south of Iceland and located at the crossroad of the Atlantic Meridional Overturning Circulation upper and lower limbs. It has been inferred to host significant mixing and water mass transformation, yet the mechanisms at play remain obscure. Using data from an array of 7 moorings deployed for two years over the Reykjanes Ridge in a cross-ridge direction, we computed internal waves&#8217; energy density and energy fluxes in the dominating wavebands, i.e., near-inertial and semi-diurnal (tidal) bands to assess the contribution of several mechanisms at fuelling a route to energy dissipation and mixing. Internal tide fluxes are dominating the energy fluxes by an order of magnitude right on top of the ridge and follow a clear spring-neap cycle; but rapidly fade away and become decoherent O(100) km away from the ridge, suggesting a strong scattering by mesoscale turbulence and seafloor topography. Near-inertial energy fluxes and density are surface-intensified and follow a seasonal cycle, with a winter intensification due to storms and intense low-pressure weather systems. The level of near-inertial energy density is roughly explained by the local wind power input, and rapid decay of energy with depth suggests that most of the dissipation occurs in the surface layers, thus is not important for deep water mass transformation.&#160;

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Tidal and near-inertial internal waves over the Reykjanes Ridge

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0097.1 ◽

2020 ◽

Author(s):

Clément Vic ◽

Bruno Ferron ◽

Virginie Thierry ◽

Herlé Mercier ◽

Pascale Lherminier

Keyword(s):

Energy Density ◽

Kinetic Energy ◽

Time Scales ◽

Internal Waves ◽

Phase Locking ◽

Tidal Energy ◽

Kinetic Energy Density ◽

Energy Fluxes ◽

Reykjanes Ridge ◽

Inertial Energy

AbstractInternal waves in the semi-diurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6-59.1°N, 28.5-33.3°W). Continuous measurements of horizontal velocity and temperature for more than two years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semi-diurnal energy density and the tidal energy conversion at the spring-neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4-5 days on the ridge top vs 7-9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally-driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semi-diurnal kinetic energy by a factor 2-3, but is much more variable in time. It features a strong seasonal cycle with a winter intensification and sub-seasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13-15 days, which is consistent with the short time scales of observed variability of near-inertial energy. Finally, in the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase compared to cyclonic structures, illustrating the funneling effect of anticyclones.

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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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The temporal variability of near-inertial internal waves in an internal tide beam

10.5194/egusphere-egu21-792 ◽

2021 ◽

Author(s):

Jonas Löb ◽

Monika Rhein

Keyword(s):

Internal Wave ◽

Internal Waves ◽

Temporal Variability ◽

Internal Tide ◽

Internal Tides ◽

Inertial Waves ◽

Energy Fluxes ◽

Wind Event ◽

Mode 2 ◽

Inertial Energy

Low mode internal waves in the stratified ocean are generated by the interaction between barotropic tides and seafloor topography and by the wind field in the near-inertial range. They are crucial for interior mixing and for the oceanic energy pathways, since they carry a large portion of the energy of the entire internal wave field. Long-term observations of energy fluxes of internal waves are sparse. The aim of this work is to study the temporal variability of wind generated low mode near-inertial internal waves inside an internal tide beam emanating from seamounts south of the Azores. For this, 20 months of consecutive mooring observations are used to calculate the mode 1 and mode 2 near-inertial energy fluxes as well as kinetic and potential energies. The gathered time series of near-inertial internal wave energy flux is not steady due to its intermittent forcing and is neither dominated by either mode 1 or mode 2. It shows a peak induced by a distinct strong wind event which is directly linked to wind-power input into the mixed layer north-east of the mooring location, and allows a comparison between the wind event and a background state. Furthermore, indications of non-linear interactions of the near-inertial waves with the internal tides in the form of resonant triad interaction and non-linear self-interaction have been found. This study provides new insights on the relative importance of single wind events and reinforces the assumption of a global non-uniform distribution of near-inertial energy with emphasis in regions where these events occur often and regularly. It furthermore displays its importance to be adequately incorporated into ocean general circulation models and in generating ocean mixing estimates by near-inertial waves as a similarly important component next to the internal tides.

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Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0238.1 ◽

2015 ◽

Vol 45 (6) ◽

pp. 1735-1756 ◽

Cited By ~ 24

Author(s):

Clark G. Richards ◽

Fiammetta Straneo

Keyword(s):

Water Mass ◽

Heat Budget ◽

Mesoscale Eddy ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Boundary Current ◽

Mode Water ◽

Nordic Seas ◽

Eddy Field ◽

Water Mass Transformation

AbstractThe Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.

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Influences of Precipitation on Water Mass Transformation and Deep Convection

Journal of Physical Oceanography ◽

10.1175/jpo-d-11-0230.1 ◽

2012 ◽

Vol 42 (10) ◽

pp. 1684-1700 ◽

Cited By ~ 19

Author(s):

Michael A. Spall

Keyword(s):

Numerical Model ◽

Water Mass ◽

Numerical Models ◽

Deep Convection ◽

Meridional Overturning Circulation ◽

Critical Threshold ◽

Overturning Circulation ◽

Model Calculations ◽

Meridional Overturning ◽

Water Mass Transformation

Abstract The influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models. Nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses, making explicit their dependence on both geometric parameters such as basin area, sill depth, and latitude, as well as on the strength of atmospheric forcing. In addition to the properties of the convective water, the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode. High-resolution numerical model calculations compare well with the theory for the properties of the convective water mass, the strength of the meridional overturning circulation, and also the shutdown of deep convection. However, the numerical model also shows that, for precipitation levels that exceed this critical threshold, the circulation retains downwelling and northward heat transport, even in the absence of deep convection.

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Climatic Impacts of Parameterized Local and Remote Tidal Mixing

Journal of Climate ◽

10.1175/jcli-d-15-0153.1 ◽

2016 ◽

Vol 29 (10) ◽

pp. 3473-3500 ◽

Cited By ~ 43

Author(s):

Angélique Melet ◽

Sonya Legg ◽

Robert Hallberg

Keyword(s):

Turbulent Mixing ◽

Water Mass ◽

Internal Tide ◽

Meridional Overturning Circulation ◽

Internal Tides ◽

Tidal Mixing ◽

Primary Role ◽

Ocean Heat Uptake ◽

Meridional Overturning ◽

The Impact

Abstract Turbulent mixing driven by breaking internal tides plays a primary role in the meridional overturning and oceanic heat budget. Most current climate models explicitly parameterize only the local dissipation of internal tides at the generation sites, representing the remote dissipation of low-mode internal tides that propagate away through a uniform background diffusivity. In this study, a simple energetically consistent parameterization of the low-mode internal-tide dissipation is derived and implemented in the Geophysical Fluid Dynamics Laboratory Earth System Model with GOLD component (GFDL-ESM2G). The impact of remote and local internal-tide dissipation on the ocean state is examined using a series of simulations with the same total amount of energy input for mixing, but with different scalings of the vertical profile of dissipation with the stratification and with different idealized scenarios for the distribution of the low-mode internal-tide energy dissipation: uniformly over ocean basins, continental slopes, or continental shelves. In these idealized scenarios, the ocean state, including the meridional overturning circulation, ocean ventilation, main thermocline thickness, and ocean heat uptake, is particularly sensitive to the vertical distribution of mixing by breaking low-mode internal tides. Less sensitivity is found to the horizontal distribution of mixing, provided that distribution is in the open ocean. Mixing on coastal shelves only impacts the large-scale circulation and water mass properties where it modifies water masses originating on shelves. More complete descriptions of the distribution of the remote part of internal-tide-driven mixing, particularly in the vertical and relative to water mass formation regions, are therefore required to fully parameterize ocean turbulent mixing.

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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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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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The Irminger Sea – air-ice-ocean interactions in a 5 km coupled simulation with ICON-ESM

10.5194/egusphere-egu21-11010 ◽

2021 ◽

Author(s):

Oliver Gutjahr ◽

Johann H. Jungclaus ◽

Nils Brüggemann ◽

Helmuth Haak ◽

Jochem Marotzke

Keyword(s):

Heat Loss ◽

Water Mass ◽

Deep Convection ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Katabatic Winds ◽

Cold Air ◽

Irminger Sea ◽

Water Mass Transformation ◽

Cold Air Outbreaks

Recent observations suggest that deep convection and water mass transformation in the Irminger Sea southeast of Greenland, together with overflows from the Nordic Seas, may be more important for the variability of the Atlantic meridional overturning circulation (AMOC) than the Labrador Sea. The preconditioning for and triggering of deep convection in the Irminger Sea is strongly associated with topography-induced mesoscale wind phenomena, such as Greenland tip jets, katabatic winds and marine cold air outbreaks. However, the resolution of current coupled climate models is too coarse to capture all the properties of these wind systems or to capture them at all. Here we explore the air-ice-ocean interactions induced by mesoscale wind phenomena in the Irminger Sea in a 1-year global coupled 5km simulation with ICON-ESM. The model is able to capture the complex interactions of the wind field and the ocean. We find that strong downward katabatic winds cause substantial heat loss from the Irminger Sea in addition to Greenland tip jets. The outflowing katabatic winds form narrow streaks of cold air that extend across the entire Irminger basin from southeast Greenland to Iceland. In addition, cold air outbreaks from the sea ice lead to the genesis of mesoscale cyclones, which in turn can cause Greenland tip jets before moving off to the east. All these wind phenomena cause substantial heat loss that preconditions the ocean for deep convection. If these wind systems are not resolved, the water mass transformation in the Irminger Sea could be too weak, contributing to why the Labrador Sea dominates AMOC variability in models. We conclude that resolving these mesoscale wind systems in an Earth system model could have significant implications for deep convection and water mass transformation in the Irminger Sea, and thus for AMOC variability.

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