The effect of the horizontal size of sea ice floes on sea ice melting is commonly formulated using the ratio between side and basal floe area. This leads to the conclusion that floe size is not ...important for sea ice evolution when floes exceed about 30 m. This paper considers a mutual interaction between floe size, ocean circulation, and melting. We find that lateral density gradients form at the boundaries of floes and drive ocean‐mixed‐layer instability and energetic eddies that spread from the ice edge. The resulting circulation mixes heat horizontally, melting floes near their edges. Idealized ocean model experiments show that the sea ice response is sensitive to floe size in the range of 1–50 km, considerably larger than previously assumed important, as smaller floes melt more rapidly per unit ice area. It is proposed that the role of eddies and floe size distribution should be incorporated into current climate models.
Key Points
Sea ice melting rates are sensitive to floe sizes in the range of 1–50 km, larger floes than previously assumed
Ocean eddies that develop due to density gradients at floe edges lead to enhanced melting of floes at their boundaries
Eddies therefore effectively melt floes laterally, with smaller floes melting more rapidly per unit of sea ice area
The Beaufort Gyre is a key circulation system of the Arctic Ocean and its main reservoir of freshwater. Freshwater storage and release affects Arctic sea ice cover, as well as North Atlantic and ...global climate. We describe a mechanism that is fundamental to the dynamics of the gyre, namely, the ice‐ocean stress governor. Wind blows over the ice, and the ice drags the ocean. But as the gyre spins up, currents catch the ice up and turn off the surface stress. This governor sets the basic properties of the gyre, such as its depth, freshwater content, and strength. Analytical and numerical modeling is employed to contrast the equilibration processes in an ice‐covered versus ice‐free gyre. We argue that as the Arctic warms, reduced sea ice extent and more mobile ice will result in a deeper and faster Beaufort Gyre, accumulating more freshwater that will be released by Ekman upwelling or baroclinic instability.
Plain Language Summary
The Beaufort Gyre, located north of Alaska and Canada, is a key circulation system of the Arctic Ocean. Changes in its depth and circulation influence the evolution of the Arctic sea ice cover, the North Atlantic circulation, and the global climate. The gyre is driven by persistent, ice‐mediated winds, accumulating surface freshwater toward the center, deepening the gyre, and spinning up its currents. We describe a mechanism, dubbed here the ice‐ocean governor, in which the interaction of surface currents with the ice regulates the depth of the Beaufort Gyre: The spinning up of the gyre reduces the relative speed between the ocean and the ice, and hence the freshwater accumulation. This competes with, and we argue is more important than, the release of freshwater by flow instability, which moves water from the center toward the periphery. In the current climate the depth and speed of the Beaufort Gyre are mainly set by the governor, but this may change in a warming world where reduced ice cover will render the ice‐ocean governor less effective. The resulting deeper, swifter gyre will likely exhibit more variability in its freshwater storage and flow speeds.
Key Points
Spin‐up of the Beaufort Gyre is regulated by a negative feedback between ice‐ocean stress and surface currents: an ice‐ocean stress governor
In the present Arctic system, the governor is likely a key process controlling the equilibration of the gyre
Continued sea ice loss will likely lead to reduced effectiveness of the governor and change the fundamental internal dynamics of the gyre
Across a range of simulations with a coupled atmosphere–ocean climate model, shifts in the intertropical convergence zone (ITCZ) are induced by an interhemispheric heating contrast. The response to ...heating anomalies which are polar amplified are contrasted with those which are largest in the tropics. First, we find that ITCZ shifts are always damped relative to simulations in which the ocean circulation is held fixed, irrespective of the heating distribution, keeping the ITCZ “stuck” to latitudes near the equator. The damping is primarily due to the ocean’s anomalous cross-equatorial energy transport associated with the coupling of the trade winds to an oceanic cross-equatorial cell (CEC). Second, we find that the damping effect is strongest when the forcing distribution is polar-amplified, which enhances the gross stability of the CEC and maximizes the efficiency of its cross-equatorial energy transport. Third, we argue that the ocean’s energy transport can have secondary impacts on ITCZ shifts through its interaction with climate feedbacks. Finally, we discuss the implications of our study for our understanding of the role of CECs in damping ITCZ shifts and the atmosphere’s energy balance.
Anthropogenic influences have led to a strengthening and poleward shift of westerly winds over the Southern Ocean (SO), especially during austral summer. We use observations, an idealized ...eddy‐resolving ocean‐sea ice channel model, and a global coupled model to explore the SO response to a step‐change in westerly winds. Previous work hypothesized a two timescale response for sea surface temperature. Initially, Ekman transport cools the surface before sustained upwelling causes warming on decadal timescales. The fast response is robust across our models and the observations: we find Ekman‐driven cooling in the mixed layer, mixing‐driven warming below the mixed layer, and a small upwelling‐driven warming at the temperature inversion. The long‐term response is inaccessible from observations. Neither of our models shows a persistent upwelling anomaly, or long‐term, upwelling‐driven subsurface warming. Mesoscale eddies act to oppose the anomalous wind‐driven upwelling, through a process known as eddy compensation, thereby preventing long‐term warming.
Analysis of coupled ocean‐atmosphere numerical simulations revealed a widespread oscillation of sea surface temperature (SST) and surface wind anomalies with a cyclic nature characterized by a period ...of 3 to 6 days. Similar oscillations were also found in buoy data at different locations around the globe and in an atmospheric reanalysis, albeit the oscillations were noisier and stronger in the observations, and they were weaker and longer period in the atmospheric reanalysis. These oscillations did not, however, develop in an atmosphere‐only simulation, suggesting the importance of ocean–atmosphere coupling for simulating this phenomenon. We propose a mechanism for this interaction involving a feedback between SST and surface wind.
Key Points
Goddard Earth Observing System‐Massachusetts Institute of Technology general circulation mode is able to reproduce past observed sea surface temperature (SST) and wind stress correlations
Analysis of higher temporal resolution SST and wind speed reveals several days SST wind speed cycle
We suggest a mechanism for the cycle
Abstract
Observations of ocean currents in the Arctic interior show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found ...close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below. Informed by baroclinic instability analysis, we explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. A high-resolution pan-Arctic ocean model confirms that the interior Arctic basin is baroclinically unstable all year long at depth. We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.
We numerically explore convection and general circulation of an ocean, encased in a spherical shell of uniform thickness, heated from below by a spatially uniform heat flux, and whose temperature at ...the upper surface is relaxed to the freezing point of water. The role of salt is not considered. We describe the phenomenology and equilibrium solutions across a broad range of two key non‐dimensional numbers: the natural Rossby number, a measure of the influence of rotation, and the ratio of inner to outer radius of the moon's ocean, a measure of the geometry of the moon's tangent cylinder. Two distinct regimes of circulation are identified, both dominated by Taylor columns aligned with the rotation axis—“plumes” and “rolls” which predominate inside and outside the tangent cylinder, respectively. Inside the tangent cylinder, convective plumes align with Taylor columns that extend from the bottom to the ice shell. The plumes energize geostrophic turbulence which in turn generates a general circulation consisting counter‐rotating zonal jets. Moreover, the plumes are efficient at transferring heat from the bottom to the surface, resulting in loss of heat from the ocean to the polar ice. If the plumes are suppressed rolls outside the tangent cylinder become the dominant mode of heat transfer resulting in equatorial cooling. We conclude that if moons such as Enceladus and Europa were to be predominantly heated from below, they will likely have a “Jovian‐like” circulation: an unstratified, turbulent, geostrophically controlled ocean with strong “Taylor column” behavior and a circulation dominated by counter‐rotating zonal jets.
Plain Language Summary
Subsurface oceans on the icy moons of our Solar System are receiving increasing attention in the search for extraterrestrial life. It is thus important to understand the dynamics of these oceans and identify circulation pathways and timescales. Here, we use numerical simulations to explore likely ocean circulations if the primary energy source is geothermal heating from below. We describe the phenomenology of the resulting circulation patterns as the assumed rotation rate and ocean depth is varied. Key non‐dimensional numbers are identified, and used to organize our study, informing speculations about the nature of the circulation of real icy moons. Based on best estimates of controlling non‐dimensional parameters, we argue that the ocean circulations on Europa and Enceladus are likely to be very strongly influenced by rotation and manifest multiple alternating bands of zonal currents flowing in opposite direction, with distinctly different tropical and extratropical regimes. The likely nature of the small‐scale turbulence and convection excited by bottom heating, which energizes the zonal jets, is also described.
Key Points
Ocean circulation on icy moons induced by heating from below is explored by numerical simulation
Non‐dimensional numbers characterize the influence of the moon's rotation and geometry of the tangent cylinder
Scaling laws backed by our experiments are extended to parameters typical of real icy moons allowing us to speculate on their circulation
The observational record shows a substantial 40-yr upward trend in summertime westerly winds over the Southern Ocean, as characterized by the southern annular mode (SAM) index. Enhanced summertime ...westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downward, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.
This paper describes the sea ice component of the Massachusetts Institute of Technology general circulation model (MITgcm); it presents example Arctic and Antarctic results from a realistic, ...eddy-admitting, global ocean and sea ice configuration; and it compares B-grid and C-grid dynamic solvers and other numerical details of the parameterized dynamics and thermodynamics in a regional Arctic configuration. Ice mechanics follow a viscous-plastic rheology and the ice momentum equations are solved numerically using either line-successive-over-relaxation (LSOR) or elastic–viscous-plastic (EVP) dynamic models. Ice thermodynamics are represented using either a zero-heat-capacity formulation or a two-layer formulation that conserves enthalpy. The model includes prognostic variables for snow thickness and for sea ice salinity. The above sea ice model components were borrowed from current generation climate models but they were reformulated on an Arakawa C grid in order to match the MITgcm oceanic grid and they were modified in many ways to permit efficient and accurate automatic differentiation. Both stress tensor divergence and advective terms are discretized with the finite-volume method. The choice of the dynamic solver has a considerable effect on the solution; this effect can be larger than, for example, the choice of lateral boundary conditions, of ice rheology, and of ice-ocean stress coupling. The solutions obtained with different dynamic solvers typically differ by a few cm
s
−1 in ice drift speeds, 50
cm in ice thickness, and order 200
km
3
yr
−1 in freshwater (ice and snow) export out of the Arctic.
The seasonal ice zone around both the Arctic and the Antarctic coasts is typically characterized by warm and salty waters underlying a cold and fresh layer that insulates sea‐ice floating at the ...surface from vertical heat fluxes. Here, we explore how a mesoscale eddy field rubbing against ice at the surface can, through Ekman‐induced vertical motion, bring warm waters up to the surface and partially melt the ice. We dub this the “Eddy‐Ice‐Pumping” (EIP) mechanism. When sea‐ice is relatively motionless, underlying mesoscale eddies experience a surface drag that generates Ekman upwelling in anticyclones and downwelling in cyclones. An eddy composite analysis of a Southern Ocean eddying channel model, capturing the interaction of the mesoscale with sea‐ice, shows that within the compact ice zone, the mixed layer depth (MLD) is shallow in anticyclones (∼20 m) due to sea‐ice melt and deep in cyclones (∼50–200m) due to brine rejection. “EIP” warms the core of anticyclones without significantly affecting the temperature of cyclones, producing a net upward vertical heat flux that reduces the mean sea‐ice thickness by 10% and shoals the MLD by 60% over the course of winter and spring. In the following months, the sea‐ice thickness recovers with an overshoot, due to strong negative feedbacks associated with atmospheric cooling and salt stratification. Consequently, the effect of “EIP” does not accumulate over the years, but modulates the seasonal cycle of ice within the compact ice zone.
Plain Language Summary
Polar oceans typically have cold, fresh waters at the surface and warmer, saltier waters below that. When the atmospheric temperature is cold enough, the top layer of the ocean cools to the freezing point and sea‐ice forms at the surface. This growth can be impeded by warmer waters at the subsurface, whose heat can melt sea‐ice at its base. The effectiveness of this melting depends on the amount of heat transported from the deeper layers of the ocean up to the surface underneath the ice. This study explores a novel mechanism by which frictional interactions between ocean and sea‐ice can increase the amount of heat delivered to the surface. At small scales (30–100 km), when the ocean rubs against relatively stationary sea‐ice, it experiences a torque that drives vertical motions in the water column. This process brings warmer waters in contact with sea‐ice and can reduces its mean thickness by 10% over the course of winter and spring. In the following months, the ice thickness recovers due to restoring processes, such that this mechanism does not lead to accumulated melt over the years, but changes the seasonality of sea‐ice.
Key Points
Sea‐ice drag on mesoscale ocean eddies can generate Ekman pumping and bring warm waters up to the surface
In a process model, this reduces the ice thickness by 10% and the mixed layer depth by up to 60% in winter and spring
Sea‐ice formation (melt) in cyclones (anticyclones) produce deeper (shallower) mixed layer depths
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