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Abstract The Antarctic continental shelf (ACS) hosts processes that impact the climate system globally, which has motivated ongoing efforts to characterize its state, circulation, and variability. However, the nature and consequences of eddies over the ACS, and their contributions to the budgets of heat and freshwater, remain systematically understudied. This study uses hydrographic measurements collected from instrumented seals, supported by a high‐resolution model of the southern Weddell Sea, to characterize eddies and their role in vertical heat transport around the entire ACS. A key finding is that eddies are ubiquitous, and exhibit frequent (2%–10% of hydrographic casts) occurrences of bulk Richardson numbers, indicative of submesoscale variability. However, along‐track density power spectra exhibit wavenumber dependences of , consistent with quasigeostrophic turbulence. Approximately of the points in the surface mixed layer satisfy conditions favorable for symmetric instability, although its prevalence is likely higher than this due to the relatively coarse resolution of the seal tracks. Vertical heat transports, estimated from a regional model‐calibrated parameterization of submesoscale restratification, are largest in shelf regions hosting dense water, which have previously been identified as key sites of warm water intrusions onto the ACS. These regions also exhibit the largest seasonal cycles, with elevated winter eddy activity and heat fluxes accompanying the formation of high salinity shelf waters. These findings indicate that eddies may contribute substantially to ACS heat and tracer budgets, and motivate further study of their role in determining the pathways and fate of heat that intrudes onto the ACS. 

Abstract Antarctic bottom water (AABW) forms through the descent of dense shelf waters (DSW) into the abyssal ocean, with tides playing a key role in DSW transport and entrainment. Previous studies suggest that tides can suppress the net overflow entrainment, favoring the formation of denser AABW. However, how tidal effects on AABW formation and associated material sequestration vary with a changing climate remains unclear. In this study, an idealized numerical model is used to investigate potential climatic influences on tidally influenced AABW properties. Experiments are conducted with varying ambient stratifications and rates of DSW supply, inspired by projected future changes over the Antarctic continental shelf. The results show that tidal advection and associated V‐shaped front can modify the vertical diffusivity and the exchanges between DSW and its ambient waters, thereby altering the properties of AABW. For a future warm and salty shelf, AABW will become warmer and saltier accordingly, with the effects of the V‐shaped front weakening significantly. Conversely, for a future cold and fresh shelf, AABW formation is nearly nonexistent due to the stronger dilution of DSW, and the tidal effects become much weaker. Additionally, tidal suppression of overflow mixing is only significant for large DSW fluxes (thickness) and becomes negligible for small DSW fluxes. These findings suggest that the contribution of tides to DSW descent will weaken under global warming, thereby accelerating the slowdown of AABW formation. 

In a recent paper [Chu (2023; Chu23)], the author formulated the equations governing atmospheric motion in a spheroidal coordinate system. Since the mass distribution of the Earth is not exactly spheroidal, the true gravity is not vertical in that coordinate system. Chu23 compared the magnitude of the static horizontal component of gravity in that system to those of the dynamically active forces and concluded that the horizontal components of gravity should not be neglected. In recent papers by the authors [Chang and Wolfe (2022; CW22) and Stewart and McWilliams (2022; CW22)], we explained that the actual interpretation of the approximation made in atmospheric and oceanic modeling is not neglecting the horizontal component of the true gravity, but is a geometrical approximation, approximating nearly spheroidal geopotential surfaces with bumps on which the true gravity is vertical by exactly spheroidal surfaces. We showed that under such an interpretation, the errors due to the geometrical approximation are small. Chu23 claimed that CW22 and SM22 erroneously neglected the gravity perturbations in their analyses. Here, we explain further the differences between these approaches, in the process showing that the criticisms of Chu23 on CW22 and SM22 are invalid, further supporting our conclusion that the horizontal component of the true gravity is not relevant in ocean and atmospheric dynamics. Physically, the reason why horizontal gravity is irrelevant in the coordinate system used by Chu23 is that it is balanced by a static horizontal pressure gradient force. 

High-resolution process modeling reveals a positive feedback of poleward ocean heat transport due to Antarctic ice shelf melt. 

Abstract Antarctic Bottom Water is primarily formed via overflows of dense shelf water (DSW) around the Antarctic continental margins. The dynamics of these overflows therefore influence the global abyssal stratification and circulation. Previous studies indicate that dense overflows can be unstable, energizing topographic Rossby waves (TRW) over the continental slope. However, it remains unclear how the wavelength and frequency of the TRWs are related to the properties of the overflowing DSW and other environmental conditions, and how the TRW properties influence the downslope transport of DSW. This study uses idealized high-resolution numerical simulations to investigate the dynamics of overflow-forced TRWs and the associated downslope transport of DSW. It is shown that the propagation of TRWs is constrained by the geostrophic along-slope flow speed of the DSW and by the dynamics of linear plane waves, allowing the wavelength and frequency of the waves to be predicted a priori. The rate of downslope DSW transport depends nonmonotonically on the slope steepness: steep slopes approximately suppress TRW formation, resulting in steady, frictionally dominated DSW descent. For slopes of intermediate steepness, the overflow becomes unstable and generates TRWs, accompanied by interfacial form stresses that drive DSW downslope relatively rapidly. For gentle slopes, the TRWs lead to the formation of coherent eddies that inhibit downslope DSW transport. These findings may explain the variable properties of TRWs observed in oceanic overflows, and they imply that the rate at which DSW descends to the abyssal ocean depends sensitively on the manifestation of TRWs and/or nonlinear eddies over the continental slope. 

Abstract It is now well established that changes in the zonal wind stress over the Antarctic Circumpolar Current (ACC) do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation.” Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes in the efficiency with which transient eddies transfer momentum to the sea floor; on the other extreme, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates for changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that eddy saturation primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasigeostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance. 

Abstract Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.