scholarly journals How does salinity shape ocean circulation and ice geometry on Enceladus and other icy satellites?

2021 ◽  
Author(s):  
Wanying Kang ◽  
Tushar Mittal ◽  
Suyash Bire ◽  
Jean Michel ◽  
John Marshall
Keyword(s):  
Ocean Circulation ◽  
Freezing Point ◽  
Ocean Dynamics ◽  
Icy Satellites ◽  
Heating Rates ◽  
Ocean Salinity ◽  
Thickness Variations ◽  
Tidal Heating ◽  
Ice Shell ◽  
Shell Geometry

Abstract Of profound astrobiological interest is that not only does Enceladus have a water ocean, but it also appears to be salty, important for its likely habitability. Here, we investigate how salinity affects ocean dynamics and equilibrium ice shell geometry and use knowledge of ice shell geometry and tidal heating rates to help constrain ocean salinity. We show that the vertical overturning circulation of the ocean, driven from above by melting and freezing and the temperature dependence of the freezing point of water on pressure, has opposing signs at very low and very high salinities. In both cases, heat and freshwater converges toward the equator, where the ice is thick, acting to homogenise thickness variations. In order to maintain observed ice thickness variations, ocean heat transport should not overwhelm tidal heating rates within the ice, which are small in equatorial regions. This can only happen when the ocean’s salinity has intermediate values, order 20 psu. In this case polar-sinking driven by meridional temperature variations is largely canceled by equatorial-sinking circulation driven by salinity variations and a consistent ocean circulation, ice shell geometry and tidal heating rate can be achieved.

scholarly journals Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model

1994 ◽  
Vol 20 ◽  
pp. 413-419 ◽  
Author(s):  
J. Determann ◽  
R. Gerdes
Keyword(s):  
Ocean Circulation ◽  
Thermohaline Circulation ◽  
Sea Water ◽  
Freezing Point ◽  
Bottom Topography ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Ice Shelf

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.

scholarly journals Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model

1994 ◽  
Vol 20 ◽  
pp. 413-419 ◽  
Author(s):  
J. Determann ◽  
R. Gerdes
Keyword(s):  
Ocean Circulation ◽  
Thermohaline Circulation ◽  
Sea Water ◽  
Freezing Point ◽  
Bottom Topography ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Ice Shelf

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.

scholarly journals Ocean Convection

Fluids ◽  
2021 ◽  
Vol 6 (10) ◽  
pp. 360
Author(s):  
Catherine Vreugdenhil ◽  
Bishakhdatta Gayen
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Mixed Layer ◽  
Water Mass ◽  
Current Knowledge ◽  
Open Ocean ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Global Circulation Models ◽  
Ocean Convection

Ocean convection is a key mechanism that regulates heat uptake, water-mass transformation, CO2 exchange, and nutrient transport with crucial implications for ocean dynamics and climate change. Both cooling to the atmosphere and salinification, from evaporation or sea-ice formation, cause surface waters to become dense and down-well as turbulent convective plumes. The upper mixed layer in the ocean is significantly deepened and sustained by convection. In the tropics and subtropics, night-time cooling is a main driver of mixed layer convection, while in the mid- and high-latitude regions, winter cooling is key to mixed layer convection. Additionally, at higher latitudes, and particularly in the sub-polar North Atlantic Ocean, the extensive surface heat loss during winter drives open-ocean convection that can reach thousands of meters in depth. On the Antarctic continental shelf, polynya convection regulates the formation of dense bottom slope currents. These strong convection events help to drive the immense water-mass transport of the globally-spanning meridional overturning circulation (MOC). However, convection is often highly localised in time and space, making it extremely difficult to accurately measure in field observations. Ocean models such as global circulation models (GCMs) are unable to resolve convection and turbulence and, instead, rely on simple convective parameterizations that result in a poor representation of convective processes and their impact on ocean circulation, air–sea exchange, and ocean biology. In the past few decades there has been markedly more observations, advancements in high-resolution numerical simulations, continued innovation in laboratory experiments and improvement of theory for ocean convection. The impacts of anthropogenic climate change on ocean convection are beginning to be observed, but key questions remain regarding future climate scenarios. Here, we review the current knowledge and future direction of ocean convection arising from sea–surface interactions, with a focus on mixed layer, open-ocean, and polynya convection.

scholarly journals Multi-grid algorithm for passive tracer transport in the NEMO ocean circulation model: a case study with the NEMO OGCM (version 3.6)

2020 ◽  
Vol 13 (11) ◽  
pp. 5465-5483
Author(s):  
Clément Bricaud ◽  
Julien Le Sommer ◽  
Gurvan Madec ◽  
Christophe Calone ◽  
Julie Deshayes ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Ocean Circulation ◽  
Circulation Model ◽  
Ocean Model ◽  
Ocean Dynamics ◽  
Global Ocean ◽  
Passive Tracer ◽  
Tracer Transport ◽  
Ocean Circulation Model ◽  
Grid Algorithm

Abstract. Ocean biogeochemical models are key tools for both scientific and operational applications. Nevertheless the cost of these models is often expensive because of the large number of biogeochemical tracers. This has motivated the development of multi-grid approaches where ocean dynamics and tracer transport are computed on grids of different spatial resolution. However, existing multi-grid approaches to tracer transport in ocean modelling do not allow the computation of ocean dynamics and tracer transport simultaneously. This paper describes a new multi-grid approach developed for accelerating the computation of passive tracer transport in the Nucleus for European Modelling of the Ocean (NEMO) ocean circulation model. In practice, passive tracer transport is computed at runtime on a grid with coarser spatial resolution than the hydrodynamics, which reduces the CPU cost of computing the evolution of tracers. We describe the multi-grid algorithm, its practical implementation in the NEMO ocean model, and discuss its performance on the basis of a series of sensitivity experiments with global ocean model configurations. Our experiments confirm that the spatial resolution of hydrodynamical fields can be coarsened by a factor of 3 in both horizontal directions without significantly affecting the resolved passive tracer fields. Overall, the proposed algorithm yields a reduction by a factor of 7 of the overhead associated with running a full biogeochemical model like PISCES (with 24 passive tracers). Propositions for further reducing this cost without affecting the resolved solution are discussed.

scholarly journals Last Glacial Maximum (LGM) climate forcing and ocean dynamical feedback and their implications for estimating climate sensitivity

2021 ◽  
Vol 17 (1) ◽  
pp. 253-267
Author(s):  
Jiang Zhu ◽  
Christopher J. Poulsen
Keyword(s):  
Last Glacial Maximum ◽  
Ocean Circulation ◽  
Climate Sensitivity ◽  
Radiative Forcing ◽  
Ice Sheets ◽  
Glacial Maximum ◽  
Ocean Dynamics ◽  
Last Glacial ◽  
True Value ◽  
Climate Forcings

Abstract. Equilibrium climate sensitivity (ECS) has been directly estimated using reconstructions of past climates that are different than today's. A challenge to this approach is that temperature proxies integrate over the timescales of the fast feedback processes (e.g., changes in water vapor, snow, and clouds) that are captured in ECS as well as the slower feedback processes (e.g., changes in ice sheets and ocean circulation) that are not. A way around this issue is to treat the slow feedbacks as climate forcings and independently account for their impact on global temperature. Here we conduct a suite of Last Glacial Maximum (LGM) simulations using the Community Earth System Model version 1.2 (CESM1.2) to quantify the forcing and efficacy of land ice sheets (LISs) and greenhouse gases (GHGs) in order to estimate ECS. Our forcing and efficacy quantification adopts the effective radiative forcing (ERF) and adjustment framework and provides a complete accounting for the radiative, topographic, and dynamical impacts of LIS on surface temperatures. ERF and efficacy of LGM LIS are −3.2 W m−2 and 1.1, respectively. The larger-than-unity efficacy is caused by the temperature changes over land and the Northern Hemisphere subtropical oceans which are relatively larger than those in response to a doubling of atmospheric CO2. The subtropical sea-surface temperature (SST) response is linked to LIS-induced wind changes and feedbacks in ocean–atmosphere coupling and clouds. ERF and efficacy of LGM GHG are −2.8 W m−2 and 0.9, respectively. The lower efficacy is primarily attributed to a smaller cloud feedback at colder temperatures. Our simulations further demonstrate that the direct ECS calculation using the forcing, efficacy, and temperature response in CESM1.2 overestimates the true value in the model by approximately 25 % due to the neglect of slow ocean dynamical feedback. This is supported by the greater cooling (6.8 ∘C) in a fully coupled LGM simulation than that (5.3 ∘C) in a slab ocean model simulation with ocean dynamics disabled. The majority (67 %) of the ocean dynamical feedback is attributed to dynamical changes in the Southern Ocean, where interactions between upper-ocean stratification, heat transport, and sea-ice cover are found to amplify the LGM cooling. Our study demonstrates the value of climate models in the quantification of climate forcings and the ocean dynamical feedback, which is necessary for an accurate direct ECS estimation.

The Role of Oceanic Feedback in the Climate Response to Doubling CO2

2012 ◽  
Vol 25 (21) ◽  
pp. 7544-7563 ◽  
Author(s):  
Jian Lu ◽  
Bin Zhao
Keyword(s):  
Ocean Circulation ◽  
Global Climate ◽  
Coupled Model ◽  
Southern Annular Mode ◽  
Ocean Dynamics ◽  
Climate Response ◽  
Tropical Precipitation ◽  
Fluctuation Dissipation ◽  
Wes Feedback ◽  
Partial Coupling

Two suites of partial coupling experiments are devised with the upper-ocean dynamics version (UOM) of the CCSM3 to isolate the effects of the feedbacks from the change of the wind-driven ocean circulation and air–sea heat flux in the global climate response to the forcing of doubling CO2. The partial coupling is achieved by implementing a so-called overriding technique, which helps quantitatively partition the total response in the fully coupled model to the feedback component in question and the response to external forcing in the absence of the former. By overriding the wind stress seen by the ocean and the wind speed through the bulk formula for evaporation, the experiments help to reveal that (i) the wind–evaporation–SST (WES) feedback is the main formation mechanism for the tropical SST pattern under the CO2 forcing, verifying the hypothesis proposed by Xie et al.; (ii) the weakened tropical Pacific wind is shown in this UOM model not to be the cause for the enhanced equatorial Pacific warming, as one might expect from the thermocline and Bjerknes feedbacks; (iii) WES is also the leading mechanism for shaping the tropical precipitation response in the ocean; and (iv) both the wind-driven ocean dynamical feedback and the WES feedback act to increase the persistence of the southern annular mode (SAM) and the increased time scale of the SAM due to these feedbacks manifests itself in the response of the jet shift to an identical CO2 forcing, in a manner conforming to the fluctuation–dissipation theorem.

Internal salt content: a useful framework for understanding the oceanic branch of the water cycle

2021 ◽  
Author(s):  
Christopher Bladwell ◽  
Ryan M. Holmes ◽  
Jan D. Zika
Keyword(s):  
Fresh Water ◽  
Ocean Circulation ◽  
Water Cycle ◽  
Salt Content ◽  
Ocean Model ◽  
Global Ocean ◽  
Freshwater Forcing ◽  
Freshwater Fluxes ◽  
Ocean Salinity ◽  
Isopycnal Mixing

AbstractThe global water cycle is dominated by an atmospheric branch which transfers fresh water away from subtropical regions and an oceanic branch which returns that fresh water from subpolar and tropical regions. Salt content is commonly used to understand the oceanic branch because surface freshwater fluxes leave an imprint on ocean salinity. However, freshwater fluxes do not actually change the amount of salt in the ocean and – in the mean – no salt is transported meridionally by ocean circulation. To study the processes which determine ocean salinity we introduce a new variable: “internal salt” and its counterpart “internal fresh water”. Precise budgets for internal salt in salinity coordinates relate meridional and diahaline transport to surface freshwater forcing, ocean circulation and mixing, and reveal the pathway of fresh water in the ocean. We apply this framework to a 1° global ocean model. We find that in order for fresh water to be exported from the ocean’s tropical and subpolar regions to the subtropics, salt must be mixed across the salinity surfaces that bound those regions. In the tropics, this mixing is achieved by parameterized vertical mixing, along-isopycnal mixing, and numerical mixing associated with truncation errors in the model’s advection scheme, while along-isopycnal mixing dominates at high latitudes. We analyze the internal freshwater budgets of the Indo-Pacific and Atlantic Ocean basins and identify the transport pathways between them which redistribute fresh water added through precipitation, balancing asymmetries in freshwater forcing between the basins.

scholarly journals Contributions of Atmospheric Stochastic Forcing and Intrinsic Ocean Modes to North Atlantic Ocean Interdecadal Variability

2020 ◽  
Vol 33 (6) ◽  
pp. 2351-2370 ◽  
Author(s):  
Olivier Arzel ◽  
Thierry Huck
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Thermal Structure ◽  
Ocean Dynamics ◽  
Dimensionless Number ◽  
Stochastic Forcing ◽  
The North ◽  
Wide Range ◽  
The North Atlantic

AbstractAtmospheric stochastic forcing associated with the North Atlantic Oscillation (NAO) and intrinsic ocean modes associated with the large-scale baroclinic instability of the North Atlantic Current (NAC) are recognized as two strong paradigms for the existence of the Atlantic multidecadal oscillation (AMO). The degree to which each of these factors contribute to the low-frequency variability of the North Atlantic is the central question in this paper. This issue is addressed here using an ocean general circulation model run under a wide range of background conditions extending from a supercritical regime where the oceanic variability spontaneously develops in the absence of any atmospheric noise forcing to a damped regime where the variability requires some noise to appear. The answer to the question is captured by a single dimensionless number Γ measuring the ratio between the oceanic and atmospheric contributions, as inferred from the buoyancy variance budget of the western subpolar region. Using this diagnostic, about two-thirds of the sea surface temperature (SST) variance in the damped regime is shown to originate from atmospheric stochastic forcing whereas heat content is dominated by internal ocean dynamics. Stochastic wind stress forcing is shown to substantially increase the role played by damped ocean modes in the variability. The thermal structure of the variability is shown to differ fundamentally between the supercritical and damped regimes, with abrupt modifications around the transition between the two regimes. Ocean circulation changes are further shown to be unimportant for setting the pattern of SST variability in the damped regime but are fundamental for a preferred time scale to emerge.

scholarly journals Middle Holocene expansion of Pacific Deep Water into the Southern Ocean

2019 ◽  
Vol 117 (2) ◽  
pp. 889-894
Author(s):  
Torben Struve ◽  
David J. Wilson ◽  
Tina van de Flierdt ◽  
Naomi Pratt ◽  
Kirsty C. Crocket
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Deep Water ◽  
Water Masses ◽  
Ocean Dynamics ◽  
Global Ocean ◽  
Circulation System ◽  
The Holocene ◽  
Middle Holocene ◽  
Deep Mixing

The Southern Ocean is a key region for the overturning and mixing of water masses within the global ocean circulation system. Because Southern Ocean dynamics are influenced by the Southern Hemisphere westerly winds (SWW), changes in the westerly wind forcing could significantly affect the circulation and mixing of water masses in this important location. While changes in SWW forcing during the Holocene (i.e., the last ∼11,700 y) have been documented, evidence of the oceanic response to these changes is equivocal. Here we use the neodymium (Nd) isotopic composition of absolute-dated cold-water coral skeletons to show that there have been distinct changes in the chemistry of the Southern Ocean water column during the Holocene. Our results reveal a pronounced Middle Holocene excursion (peaking ∼7,000–6,000 y before present), at the depth level presently occupied by Upper Circumpolar Deep Water (UCDW), toward Nd isotope values more typical of Pacific waters. We suggest that poleward-reduced SWW forcing during the Middle Holocene led to both reduced Southern Ocean deep mixing and enhanced influx of Pacific Deep Water into UCDW, inducing a water mass structure that was significantly different from today. Poleward SWW intensification during the Late Holocene could then have reinforced deep mixing along and across density surfaces, thus enhancing the release of accumulated CO2 to the atmosphere.

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