Raffaele Ferrari | Massachusetts Institute of Technology (MIT) (original) (raw)

Papers by Raffaele Ferrari

Global Biogeochemical Cycles, 2018

The deep-ocean carbonate ion concentration impacts the fraction of the marine calcium carbonate p... more The deep-ocean carbonate ion concentration impacts the fraction of the marine calcium carbonate production that is buried in sediments. This gives rise to the carbonate compensation feedback, which is thought to restore the deep-ocean carbonate ion concentration on multimillennial timescales. We formulate an analytical framework to investigate the impact of carbonate compensation under various changes in the carbon cycle relevant for anthropogenic change and glacial cycles. Using this framework, we show that carbonate compensation amplifies by 15-20% changes in atmospheric CO 2 resulting from a redistribution of carbon between the atmosphere and ocean (e.g., due to changes in temperature, salinity, or nutrient utilization). A counterintuitive result emerges when the impact of organic matter burial in the ocean is examined. The organic matter burial first leads to a slight decrease in atmospheric CO 2 and an increase in the deep-ocean carbonate ion concentration. Subsequently, enhanced calcium carbonate burial leads to outgassing of carbon from the ocean to the atmosphere, which is quantified by our framework. Results from simulations with a multibox model including the minor acids and bases important for the ocean-atmosphere exchange of carbon are consistent with our analytical predictions. We discuss the potential role of carbonate compensation in glacial-interglacial cycles as an example of how our theoretical framework may be applied.

The long term goal of the "Scalable Lateral Mixing and Coherent Turbulence" DRI, under which the ... more The long term goal of the "Scalable Lateral Mixing and Coherent Turbulence" DRI, under which the PIs are funded, is to understand the processes that stir and mix tracers in the ocean on lateral scales of 100 kilometers to 10 meters, the so-called submesoscales. The specific long term goals of the PIs are to understand the influence of mesoscale strain in driving stirring and mixing at the submesoscale, and to develop a robust theoretical framework through which to interpret the observations.

Nature Communications, 2017

It is an open question whether turbulent mixing across density surfaces is sufficiently large to ... more It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

The generation and destruction of stratification in the surface mixed layer of the ocean is under... more The generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air–sea fluxes and stresses. In this paper, it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum. A classic example is the strong restratification resulting from frontogenesis in regions of confluent flow. Frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving Ekman flows that differentially advect density. Ekman flow induced during spin-down always tends to restratify the fluid, while wind-driven Ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front (i.e., directed against or with the geostrophic shear), respectively. Scalings are constru...

11 The first direct estimate of the rate at which geostrophic turbulence mixes tracers 12 across ... more 11 The first direct estimate of the rate at which geostrophic turbulence mixes tracers 12 across the Antarctic Circumpolar Current is presented. The estimate is computed from 13 the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal 14 and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional 15 eddy diffusivity, a measure of the rate at which the area of the tracer spreads along 16 an isopycnal across the Antarctic Circumpolar Current, is 710±260 m2s−1 at 1500 m 17 depth. The estimate is based on an extrapolation of the tracer based diffusivity using 18 output from numerical tracers released in a 1/20th of a degree model simulation of 19 the circulation and turbulence in the Drake Passage region. The model is shown to 20 reproduce the observed spreading rate of the DIMES tracer and suggests that the 21 meridional eddy diffusivity is weak in the upper kilometer of the water column with 22 values below 500 m2s−1 and peaks at the stee...

Journal of Physical Oceanography, 2021

Between 5 and 25% of the total momentum transferred between the atmosphere and ocean is transmitt... more Between 5 and 25% of the total momentum transferred between the atmosphere and ocean is transmitted via the growth of long surface gravity waves called ‘swell’. In this paper, we use large eddy simulations to show that swell-transmitted momentum excites near-inertial waves and drives turbulent mixing that deepens a rotating, stratified, turbulent ocean surface boundary layer. We find that swell-transmitted currents are less effective at producing turbulence and mixing the boundary layer than currents driven by an effective surface stress. Overall, however, the differences between swell-driven and surface-stress-driven boundary layers are relatively minor. In consequence, our results corroborate assumptions made in Earth system models that neglect the vertical structure of swell-transmitted momentum fluxes and instead parameterize all air-sea momentum transfer processes with an effective surface stress.

Geophysical Research Letters, 2020

A simple analytical model shows that convection shuts off for a weaker freshwa-8 ter forcing per ... more A simple analytical model shows that convection shuts off for a weaker freshwa-8 ter forcing per unit area in a wide basin compared to a narrow basin. 9 • Single-basin simulations with an ocean circulation model corroborate the predic-10 tion of the analytical model.

• A Bayesian methodology can be used to probe turbulence parameterizations and better understand ... more • A Bayesian methodology can be used to probe turbulence parameterizations and better understand their biases and uncertainties. • Parameterization parameter distributions, learned using high-resolution simulations, can be used as prior distributions for climate studies.

Journal of Physical Oceanography, 2018

Ledwell, in a comment on McDougall and Ferrari, discusses the dianeutral upwelling and downwellin... more Ledwell, in a comment on McDougall and Ferrari, discusses the dianeutral upwelling and downwelling that occurs near isolated topographic features, by performing a buoyancy budget analysis that integrates the diffusive buoyancy fluxes only out to a set horizontal distance from the topography. The consequence of this choice of control volume is that the magnitude of the area-integrated diffusive buoyancy flux decreases to zero at the base of a topographic feature resulting in a net dianeutral upwelling of water. Based on this result, Ledwell argues that isolated topographic features are preferential locations for the upwelling of waters from the abyss. However the assumptions behind Ledwell’s analysis may or may not be typical of abyssal mixing in the ocean. McDougall and Ferrari developed general expressions for the balance between area-integrated dianeutral advection and diffusion, and then illustrated these general expressions using the very simple assumption that the magnitude of ...

Journal of Physical Oceanography, 2017

Zonally averaged models of the ocean overturning circulation miss important zonal exchanges of wa... more Zonally averaged models of the ocean overturning circulation miss important zonal exchanges of waters between the Atlantic and Indo-Pacific Oceans. A two-layer, two-basin model that accounts for these exchanges is introduced and suggests that in the present-day climate the overturning circulation is best described as the combination of three circulations: an adiabatic overturning circulation in the Atlantic Ocean associated with transformation of intermediate to deep waters in the north, a diabatic overturning circulation in the Indo-Pacific Ocean associated with transformation of abyssal to deep waters by mixing, and an interbasin circulation that exchanges waters geostrophically between the two oceans through the Southern Ocean. These results are supported both by theoretical analysis of the two-layer, two-basin model and by numerical simulations of a three-dimensional ocean model.

Journal of Physical Oceanography, 2015

Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. M... more Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20–60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinity-gradient spectra are approximately flat with slopes of −0.2 ± 0.2 over horizontal wavelengths of 0.03–10 km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1 km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or...

Journal of Physical Oceanography, 2016

It is generally understood that small-scale mixing, such as is caused by breaking internal waves,... more It is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However, the observational evidence that the strong turbulent fluxes generated by small-scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal waters. Using a combination of theoretical ideas and numerical models, it is argued that abyssal waters upwell along weakly stratified boundary layers, where small-scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom. The abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters, driven by small-scale mixing in the stratified interior above the bottom boundary layers, and a slightly larger net upwe...

Paleoceanography, 2015

Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate th... more Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is a mid-depth maximum in radiocarbon age during the last glacial maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. Ferrari et al. [2014] suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation (MOC). This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern-sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay, and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2D, residual-mean dynamical model of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (~5° of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45°S by 600 m, and shoals the depth of North Atlantic Deep Water (NADW) convection at 50°N by 2500 m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45°S by 40‰, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM. Recent research on the Southern Ocean shows that the combination of eddy and Ekman transport is supported by surface buoyancy forcing [Karsten and Marshall, 2002], and so past variations in surface buoyancy forcing provides an alternative mechanism to alter Southern Ocean circulation (and potentially atmospheric CO 2) [Watson and Garabato, 2006]. Furthermore, because the extent of sea ice (which varied by ~5 degrees latitude between the LGM and today [Gersonde et al., 2005]) would strongly influence the surface buoyancy forcing in the Southern Ocean, it provides a mechanistic link that ties Antarctic temperature to Southern Ocean circulation and potentially atmospheric CO 2 rise [Fischer et al., 2010]. A

Journal of Physical Oceanography, 2015

Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-sc... more Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-scale barotropic flow of a wind-driven channel with a meridional topographic ridge. This is confirmed using both two-layer quasigeostrophic and 25-level primitive equation models at high horizontal resolution. Comparing results from simulations with and without the topographic ridge, it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge. The saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large-scale barotropic streamfunction. This suggests that the total circulation can be thought of as a superposition of a gyre mode (which has zero circumpolar transport) and a free circumpolar mode (which contains all of the transport). Basinlike gyres arise in the channel because the topography steers the barotropi...

ABSTRACT Recent studies have suggested that the abyssal ocean stratification is set by winds in t... more ABSTRACT Recent studies have suggested that the abyssal ocean stratification is set by winds in the Southern Ocean, while the rate of the abyssal overturning circulation is driven by abyssal mixing. Using a simple theoretical model and idealized numerical simulations, we show that the vertical decay of mixing away from bottom topography exerts a strong constraint onto the abyssal stratification and overturning rate. Hence changes in mixing are as important as changes in Southern Ocean winds in setting the ocean stratification and circulation in different climates.

ABSTRACT We use a combination of theory and ocean data set to show that the geothermal heat flux ... more ABSTRACT We use a combination of theory and ocean data set to show that the geothermal heat flux (GHF) plays a non-negligible role in determining the abyssal stratification and circulation strength. We find that the observed buoyancy flux above the ocean floor erodes the abyssal stratification and thereby enhances the strength of the abyssal circulation, consistent with previous work. Idealized numerical simulations are used to quantify the impact of the GHF as a function of the depth dependence of the diapycnal diffusivity that is assumed to characterize mixing in the ocean interior. In particular we show that ignoring the vertical variation of abyssal mixing leads to an under-prediction of the influence of the GHF on the abyssal circulation. We conclude by showing that the GHF leads to a steepening of density surfaces in the Southern Ocean, leading to a strengthening of the eddy-induced circulation and thereby of the Antarctic bottom water circulation. The enhanced circulation acts so as to ventilate the basal heat provided to the ocean over the entire basin floor to shallow depths primarily in the Southern Ocean.

Bulletin of the American Meteorological Society, 2015

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chem... more Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, t...

Global Biogeochemical Cycles, 2018

The deep-ocean carbonate ion concentration impacts the fraction of the marine calcium carbonate p... more The deep-ocean carbonate ion concentration impacts the fraction of the marine calcium carbonate production that is buried in sediments. This gives rise to the carbonate compensation feedback, which is thought to restore the deep-ocean carbonate ion concentration on multimillennial timescales. We formulate an analytical framework to investigate the impact of carbonate compensation under various changes in the carbon cycle relevant for anthropogenic change and glacial cycles. Using this framework, we show that carbonate compensation amplifies by 15-20% changes in atmospheric CO 2 resulting from a redistribution of carbon between the atmosphere and ocean (e.g., due to changes in temperature, salinity, or nutrient utilization). A counterintuitive result emerges when the impact of organic matter burial in the ocean is examined. The organic matter burial first leads to a slight decrease in atmospheric CO 2 and an increase in the deep-ocean carbonate ion concentration. Subsequently, enhanced calcium carbonate burial leads to outgassing of carbon from the ocean to the atmosphere, which is quantified by our framework. Results from simulations with a multibox model including the minor acids and bases important for the ocean-atmosphere exchange of carbon are consistent with our analytical predictions. We discuss the potential role of carbonate compensation in glacial-interglacial cycles as an example of how our theoretical framework may be applied.

The long term goal of the "Scalable Lateral Mixing and Coherent Turbulence" DRI, under which the ... more The long term goal of the "Scalable Lateral Mixing and Coherent Turbulence" DRI, under which the PIs are funded, is to understand the processes that stir and mix tracers in the ocean on lateral scales of 100 kilometers to 10 meters, the so-called submesoscales. The specific long term goals of the PIs are to understand the influence of mesoscale strain in driving stirring and mixing at the submesoscale, and to develop a robust theoretical framework through which to interpret the observations.

Nature Communications, 2017

It is an open question whether turbulent mixing across density surfaces is sufficiently large to ... more It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

The generation and destruction of stratification in the surface mixed layer of the ocean is under... more The generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air–sea fluxes and stresses. In this paper, it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum. A classic example is the strong restratification resulting from frontogenesis in regions of confluent flow. Frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving Ekman flows that differentially advect density. Ekman flow induced during spin-down always tends to restratify the fluid, while wind-driven Ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front (i.e., directed against or with the geostrophic shear), respectively. Scalings are constru...

11 The first direct estimate of the rate at which geostrophic turbulence mixes tracers 12 across ... more 11 The first direct estimate of the rate at which geostrophic turbulence mixes tracers 12 across the Antarctic Circumpolar Current is presented. The estimate is computed from 13 the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal 14 and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional 15 eddy diffusivity, a measure of the rate at which the area of the tracer spreads along 16 an isopycnal across the Antarctic Circumpolar Current, is 710±260 m2s−1 at 1500 m 17 depth. The estimate is based on an extrapolation of the tracer based diffusivity using 18 output from numerical tracers released in a 1/20th of a degree model simulation of 19 the circulation and turbulence in the Drake Passage region. The model is shown to 20 reproduce the observed spreading rate of the DIMES tracer and suggests that the 21 meridional eddy diffusivity is weak in the upper kilometer of the water column with 22 values below 500 m2s−1 and peaks at the stee...

Journal of Physical Oceanography, 2021

Between 5 and 25% of the total momentum transferred between the atmosphere and ocean is transmitt... more Between 5 and 25% of the total momentum transferred between the atmosphere and ocean is transmitted via the growth of long surface gravity waves called ‘swell’. In this paper, we use large eddy simulations to show that swell-transmitted momentum excites near-inertial waves and drives turbulent mixing that deepens a rotating, stratified, turbulent ocean surface boundary layer. We find that swell-transmitted currents are less effective at producing turbulence and mixing the boundary layer than currents driven by an effective surface stress. Overall, however, the differences between swell-driven and surface-stress-driven boundary layers are relatively minor. In consequence, our results corroborate assumptions made in Earth system models that neglect the vertical structure of swell-transmitted momentum fluxes and instead parameterize all air-sea momentum transfer processes with an effective surface stress.

Geophysical Research Letters, 2020

A simple analytical model shows that convection shuts off for a weaker freshwa-8 ter forcing per ... more A simple analytical model shows that convection shuts off for a weaker freshwa-8 ter forcing per unit area in a wide basin compared to a narrow basin. 9 • Single-basin simulations with an ocean circulation model corroborate the predic-10 tion of the analytical model.

• A Bayesian methodology can be used to probe turbulence parameterizations and better understand ... more • A Bayesian methodology can be used to probe turbulence parameterizations and better understand their biases and uncertainties. • Parameterization parameter distributions, learned using high-resolution simulations, can be used as prior distributions for climate studies.

Journal of Physical Oceanography, 2018

Ledwell, in a comment on McDougall and Ferrari, discusses the dianeutral upwelling and downwellin... more Ledwell, in a comment on McDougall and Ferrari, discusses the dianeutral upwelling and downwelling that occurs near isolated topographic features, by performing a buoyancy budget analysis that integrates the diffusive buoyancy fluxes only out to a set horizontal distance from the topography. The consequence of this choice of control volume is that the magnitude of the area-integrated diffusive buoyancy flux decreases to zero at the base of a topographic feature resulting in a net dianeutral upwelling of water. Based on this result, Ledwell argues that isolated topographic features are preferential locations for the upwelling of waters from the abyss. However the assumptions behind Ledwell’s analysis may or may not be typical of abyssal mixing in the ocean. McDougall and Ferrari developed general expressions for the balance between area-integrated dianeutral advection and diffusion, and then illustrated these general expressions using the very simple assumption that the magnitude of ...

Journal of Physical Oceanography, 2017

Zonally averaged models of the ocean overturning circulation miss important zonal exchanges of wa... more Zonally averaged models of the ocean overturning circulation miss important zonal exchanges of waters between the Atlantic and Indo-Pacific Oceans. A two-layer, two-basin model that accounts for these exchanges is introduced and suggests that in the present-day climate the overturning circulation is best described as the combination of three circulations: an adiabatic overturning circulation in the Atlantic Ocean associated with transformation of intermediate to deep waters in the north, a diabatic overturning circulation in the Indo-Pacific Ocean associated with transformation of abyssal to deep waters by mixing, and an interbasin circulation that exchanges waters geostrophically between the two oceans through the Southern Ocean. These results are supported both by theoretical analysis of the two-layer, two-basin model and by numerical simulations of a three-dimensional ocean model.

Journal of Physical Oceanography, 2015

Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. M... more Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20–60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinity-gradient spectra are approximately flat with slopes of −0.2 ± 0.2 over horizontal wavelengths of 0.03–10 km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1 km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or...

Journal of Physical Oceanography, 2016

It is generally understood that small-scale mixing, such as is caused by breaking internal waves,... more It is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However, the observational evidence that the strong turbulent fluxes generated by small-scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal waters. Using a combination of theoretical ideas and numerical models, it is argued that abyssal waters upwell along weakly stratified boundary layers, where small-scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom. The abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters, driven by small-scale mixing in the stratified interior above the bottom boundary layers, and a slightly larger net upwe...

Paleoceanography, 2015

Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate th... more Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is a mid-depth maximum in radiocarbon age during the last glacial maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. Ferrari et al. [2014] suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation (MOC). This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern-sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay, and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2D, residual-mean dynamical model of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (~5° of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45°S by 600 m, and shoals the depth of North Atlantic Deep Water (NADW) convection at 50°N by 2500 m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45°S by 40‰, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM. Recent research on the Southern Ocean shows that the combination of eddy and Ekman transport is supported by surface buoyancy forcing [Karsten and Marshall, 2002], and so past variations in surface buoyancy forcing provides an alternative mechanism to alter Southern Ocean circulation (and potentially atmospheric CO 2) [Watson and Garabato, 2006]. Furthermore, because the extent of sea ice (which varied by ~5 degrees latitude between the LGM and today [Gersonde et al., 2005]) would strongly influence the surface buoyancy forcing in the Southern Ocean, it provides a mechanistic link that ties Antarctic temperature to Southern Ocean circulation and potentially atmospheric CO 2 rise [Fischer et al., 2010]. A

Journal of Physical Oceanography, 2015

Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-sc... more Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-scale barotropic flow of a wind-driven channel with a meridional topographic ridge. This is confirmed using both two-layer quasigeostrophic and 25-level primitive equation models at high horizontal resolution. Comparing results from simulations with and without the topographic ridge, it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge. The saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large-scale barotropic streamfunction. This suggests that the total circulation can be thought of as a superposition of a gyre mode (which has zero circumpolar transport) and a free circumpolar mode (which contains all of the transport). Basinlike gyres arise in the channel because the topography steers the barotropi...

ABSTRACT Recent studies have suggested that the abyssal ocean stratification is set by winds in t... more ABSTRACT Recent studies have suggested that the abyssal ocean stratification is set by winds in the Southern Ocean, while the rate of the abyssal overturning circulation is driven by abyssal mixing. Using a simple theoretical model and idealized numerical simulations, we show that the vertical decay of mixing away from bottom topography exerts a strong constraint onto the abyssal stratification and overturning rate. Hence changes in mixing are as important as changes in Southern Ocean winds in setting the ocean stratification and circulation in different climates.

ABSTRACT We use a combination of theory and ocean data set to show that the geothermal heat flux ... more ABSTRACT We use a combination of theory and ocean data set to show that the geothermal heat flux (GHF) plays a non-negligible role in determining the abyssal stratification and circulation strength. We find that the observed buoyancy flux above the ocean floor erodes the abyssal stratification and thereby enhances the strength of the abyssal circulation, consistent with previous work. Idealized numerical simulations are used to quantify the impact of the GHF as a function of the depth dependence of the diapycnal diffusivity that is assumed to characterize mixing in the ocean interior. In particular we show that ignoring the vertical variation of abyssal mixing leads to an under-prediction of the influence of the GHF on the abyssal circulation. We conclude by showing that the GHF leads to a steepening of density surfaces in the Southern Ocean, leading to a strengthening of the eddy-induced circulation and thereby of the Antarctic bottom water circulation. The enhanced circulation acts so as to ventilate the basal heat provided to the ocean over the entire basin floor to shallow depths primarily in the Southern Ocean.

Bulletin of the American Meteorological Society, 2015

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chem... more Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, t...