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Adele K. Morrison and Andrew McC. Hogg

Abstract

The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.

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Ruth Moorman, Adele K. Morrison, and Andrew McC. Hogg

Abstract

The response of near-Antarctic waters to freshening by increased glacial melt is investigated using a high-resolution (0.1°) global ocean–sea ice model with realistic Antarctic water-mass properties. Two meltwater perturbation experiments are conducted where the ocean model is forced with constant elevated glacial melt rates of 1.5 and 2.8 times the control rate. Within 10 years of the onset of enhanced meltwater forcing, the generation of Antarctic Bottom Water from Dense Shelf Water ceases, as shelf waters become increasingly buoyant. Increased ocean stratification triggers subsurface warming in Dense Shelf Water source regions, suggesting a localized positive feedback to melt. In a parallel response, meltwater forcing enhances the subsurface lateral density gradients of the Antarctic Slope Front that modulate the transport of warm Circumpolar Deep Water across the continental slope toward ice shelf grounding lines. Consequently, coastal freshening acts to isolate the Antarctic Ice Sheet from open ocean heat, suggesting a cooling response to melt that counteracts warming associated with stratification. Further, these strengthening density gradients accelerate westward geostrophic currents along the coast and shelf break, homogenizing shelf waters and amplifying remote feedbacks. The net effect on the continental shelf is transient warming, followed by cooling in both experiments; however, this signal is the aggregate of a complex pattern of regional warming and cooling responses. These results suggest coastal freshening by meltwater may alter the thermal forcing of the Antarctic ice sheet in ways that both accelerate and inhibit ice shelf melt at different locations along the Antarctic coastline.

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Adele K. Morrison, Matthew H. England, and Andrew McC. Hogg

Abstract

This study explores how buoyancy-driven modulations in the abyssal overturning circulation affect Southern Ocean temperature and salinity in an eddy-permitting ocean model. Consistent with previous studies, the modeled surface ocean south of 50°S cools and freshens in response to enhanced surface freshwater fluxes. Paradoxically, upper-ocean cooling also occurs for small increases in the surface relaxation temperature. In both cases, the surface cooling and freshening trends are linked to reduced convection and a slowing of the abyssal overturning circulation, with associated changes in oceanic transport of heat and salt. For small perturbations, convective shutdown does not begin immediately, but instead develops via a slow feedback between the weakened overturning circulation and buoyancy anomalies. Two distinct phases of surface cooling are found: an initial smaller trend associated with the advective (overturning) adjustment of up to ~60 yr, followed by more rapid surface cooling during the convective shutdown period. The duration of the first advective phase decreases for larger forcing perturbations. As may be expected during the convective shutdown phase, the deep ocean warms and salinifies for both types of buoyancy perturbation. However, during the advective phase, the deep ocean freshens in response to freshwater perturbations but salinifies in the surface warming perturbations. The magnitudes of the modeled surface and abyssal trends during the advective phase are comparable to the recent observed multidecadal Southern Ocean temperature and salinity changes.

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Adele K. Morrison, Stephen M. Griffies, Michael Winton, Whit G. Anderson, and Jorge L. Sarmiento

Abstract

The Southern Ocean plays a dominant role in anthropogenic oceanic heat uptake. Strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region and allows the heat uptake to be sustained. Using an eddy-rich global climate model, the processes controlling the northward transport and convergence of the heat anomaly in the midlatitude Southern Ocean are investigated in an idealized 1% yr−1 increasing CO2 simulation. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the uptake region in the south is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller 20% contribution from enhanced upwelling. The heat anomaly converges in the midlatitude deep mixed layers because there is not a corresponding increase in the mean heat transport out of the deep mixed layers northward into the mode waters. To the north of the deep mixed layers, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. The eddy transport mechanism results from a reduction in both the diffusive and advective southward eddy heat transports, driven by decreasing isopycnal slopes and decreasing along-isopycnal temperature gradients on the northern edge of the peak warming.

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Carolina O. Dufour, Adele K. Morrison, Stephen M. Griffies, Ivy Frenger, Hannah Zanowski, and Michael Winton

Abstract

The Weddell Sea polynya is a large opening in the open-ocean sea ice cover associated with intense deep convection in the ocean. A necessary condition to form and maintain a polynya is the presence of a strong subsurface heat reservoir. This study investigates the processes that control the stratification and hence the buildup of the subsurface heat reservoir in the Weddell Sea. To do so, a climate model run for 200 years under preindustrial forcing with two eddying resolutions in the ocean (0.25° CM2.5 and 0.10° CM2.6) is investigated. Over the course of the simulation, CM2.6 develops two polynyas in the Weddell Sea, while CM2.5 exhibits quasi-continuous deep convection but no polynyas, exemplifying that deep convection is not a sufficient condition for a polynya to occur. CM2.5 features a weaker subsurface heat reservoir than CM2.6 owing to weak stratification associated with episodes of gravitational instability and enhanced vertical mixing of heat, resulting in an erosion of the reservoir. In contrast, in CM2.6, the water column is more stably stratified, allowing the subsurface heat reservoir to build up. The enhanced stratification in CM2.6 arises from its refined horizontal grid spacing and resolution of topography, which allows, in particular, a better representation of the restratifying effect by transient mesoscale eddies and of the overflows of dense waters along the continental slope.

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Stephen M. Griffies, Michael Winton, Whit G. Anderson, Rusty Benson, Thomas L. Delworth, Carolina O. Dufour, John P. Dunne, Paul Goddard, Adele K. Morrison, Anthony Rosati, Andrew T. Wittenberg, Jianjun Yin, and Rong Zhang

Abstract

The authors characterize impacts on heat in the ocean climate system from transient ocean mesoscale eddies. Their tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three GFDL coupled models comprising the Climate Model, version 2.0–Ocean (CM2-O), model suite. CM2-O models differ in their ocean resolution: CM2.6 uses a 0.1° ocean grid, CM2.5 uses an intermediate grid with 0.25° spacing, and CM2-1deg uses a nominal 1.0° grid.

Analysis of the ocean heat budget reveals that mesoscale eddies act to transport heat upward in a manner that partially compensates (or offsets) for the downward heat transport from the time-mean currents. Stronger vertical eddy heat transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also imparts an upward heat transport, yet it differs systematically from that found in CM2.6. This analysis points to the fundamental role that ocean mesoscale features play in transient ocean heat uptake. In general, the more accurate simulation found in CM2.6 provides an argument for either including a rich representation of the ocean mesoscale in model simulations of the mean and transient climate or for employing parameterizations that faithfully reflect the role of eddies in both lateral and vertical heat transport.

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Carolina O. Dufour, Stephen M. Griffies, Gregory F. de Souza, Ivy Frenger, Adele K. Morrison, Jaime B. Palter, Jorge L. Sarmiento, Eric D. Galbraith, John P. Dunne, Whit G. Anderson, and Richard D. Slater

Abstract

This study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which this study refers to as the “PF core.” A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model [GFDL Climate Model, version 2.6 (CM2.6)– simplified version of the Biogeochemistry with Light Iron Nutrients and Gas (miniBLING) 0.1° ocean model] is used to investigate the transport of heat, carbon, oxygen, and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. The authors find that the total transport across the PF core results from a ubiquitous Ekman transport that drives the upwelled tracers to the north and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the southward eddy transport dominates the total transport but remains much smaller in magnitude than the near-surface northward transport. Most of the southward branch of the total transport is achieved below the PF core, mainly through geostrophic currents. This study finds that the eddy-diffusive transport reinforces the southward eddy-advective transport for carbon and heat, and opposes it for oxygen and phosphate. Eddy-advective transport is likely to be the leading-order component of eddy-induced transport for all four tracers. However, eddy-diffusive transport may provide a significant contribution to the southward eddy heat transport due to strong along-isopycnal temperature gradients.

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