The Experts below are selected from a list of 12174 Experts worldwide ranked by ideXlab platform
Andrew Mcc. Hogg - One of the best experts on this subject based on the ideXlab platform.
-
ocean gyres driven by surface Buoyancy Forcing
Geophysical Research Letters, 2020Co-Authors: Andrew Mcc. Hogg, Bishakhdatta GayenAbstract:Midlatitude gyres in the ocean are large‐scale horizontal circulations that are intensified on the western boundary of the ocean, giving rise to currents such as the Gulf Stream. The physical mechanism underlying gyres is widely recognized to involve the curl of the wind stress, which injects potential vorticity into the upper ocean. However, model results have highlighted the role of surface Buoyancy fluxes (principally heating and cooling of the ocean surface) in driving circulation and enhancing gyre variability. Here we present two numerical simulations—one in the fully turbulent regime and the second an eddy‐permitting ocean model—which show that gyre‐like circulation can be driven by surface Buoyancy fluxes alone. We explore this phenomenon through a combination of modeling and linear theory to highlight that the transport of ocean gyres depends upon surface Buoyancy fluxes as well as wind stress.
-
The Injection of Zonal Momentum by Buoyancy Forcing in a Southern Ocean Model
Journal of Physical Oceanography, 2015Co-Authors: Emma Howard, Andrew Mcc. Hogg, Stephanie Waterman, David P. MarshallAbstract:AbstractAn overturning circulation, driven by prescribed Buoyancy Forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and Buoyancy Forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the Buoyancy Forcing and that the eddy kinetic energy is primarily set by wind stress Forcing. The zonal momentum budget integrated over each layer is considered in the Buoyancy-forced, wind-forced, and combined Forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the Buoyancy-forced case, the Forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom fric...
-
an analytical model of the response of the meridional overturning circulation to changes in wind and Buoyancy Forcing
Journal of Physical Oceanography, 2012Co-Authors: Callum J Shakespeare, Andrew Mcc. HoggAbstract:AbstractAn analytical model of the full-depth ocean stratification and meridional overturning circulation for an idealized Atlantic basin with a circumpolar channel is presented. The model explicitly describes the ocean response to both Southern Ocean winds and the global pattern and strength of prescribed surface Buoyancy fluxes. The construction of three layers, defined by the two isopycnals of overturning extrema, allows the description of circulation and stratification in both the upper and abyssal ocean. The system is fully solved in the adiabatic limit to yield scales for the surface layer thickness, buoyancies of each layer, and overturning magnitudes. The analytical model also allows scaling of the Antarctic Circumpolar Current (ACC) transport. The veracity of the three-layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry, eddy-permitting ocean general circulation model.Consistent with previous results, the abyssal overturning is found to scale ...
-
Sensitivity of the Southern Ocean overturning circulation to surface Buoyancy Forcing
Geophysical Research Letters, 2011Co-Authors: Adele K. Morrison, Andrew Mcc. Hogg, Marshall L. WardAbstract:[1] The sensitivity of the Southern Ocean overturning to altered surface Buoyancy Forcing is investigated in a series of eddy-permitting, idealised simulations. The modelled response indicates that heat and freshwater fluxes in the Southern Hemisphere mid-latitudes may play a significant role in setting the strength of the overturning circulation. Enhanced Buoyancy fluxes act to increase the meridional overturning up to a limit approaching the wind-driven Ekman transport. The sensitivity of the overturning to surface Buoyancy Forcing is strongly dependent on the relative locations of the wind stress profile, Buoyancy Forcing and upwelling region. The numerical simulations provide support for the hypothesis that changes in upwelling during deglaciations may have been driven by changes in heat and freshwater fluxes, instead of, or in addition to, changes in wind stress.
-
An Antarctic Circumpolar Current driven by surface Buoyancy Forcing
Geophysical Research Letters, 2010Co-Authors: Andrew Mcc. HoggAbstract:[1] Simulations of an idealised, but eddy-resolving, channel model of the Antarctic Circumpolar Current (ACC) are used to investigate the sensitivity of ACC transport to wind and surface Buoyancy Forcing. The results are consistent with theoretical predictions of the eddy-saturated limit, where transport is independent of wind stress. In this parameter regime, Buoyancy Forcing provides the primary control over ACC transport.
Stephen R. Rintoul - One of the best experts on this subject based on the ideXlab platform.
-
recent wind driven change in subantarctic mode water and its impact on ocean heat storage
Nature Climate Change, 2018Co-Authors: Libao Gao, Stephen R. RintoulAbstract:The subduction and export of Subantarctic Mode Water (SAMW) supplies the upper limb of the overturning circulation and makes an important contribution to global heat, freshwater, carbon and nutrient budgets. Upper ocean heat content has increased since 2006, helping to explain the so-called global warming hiatus between 1998 and 2014, with much of the ocean warming concentrated in extratropical latitudes of the Southern Hemisphere in close association with SAMW and Antarctic Intermediate Water (AAIW)6,7. Here we use Argo observations to assess changes in the thickness, depth and heat content of the SAMW layer. Between 2005 and 2015, SAMW has thickened (3.6 ± 0.3 m yr−1), deepened (2.4 ± 0.2 m yr−1) and warmed (3.9 ± 0.3 W m−2). Wind Forcing, rather than Buoyancy Forcing, is largely responsible for the observed trends in SAMW. Most (84%) of the increase in SAMW heat content is the result of changes in thickness; warming by Buoyancy Forcing (increased heat flux to the ocean) accounts for the remaining 16%. Projected increases in wind stress curl would drive further deepening of SAMW and increase in heat storage in the Southern Hemisphere oceans.
-
Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current
Ocean Science, 2007Co-Authors: Anne-marie Tréguier, Matthew H. England, Stephen R. Rintoul, Gurvan Madec, Julien Le Sommer, Jean-marc MolinesAbstract:An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. Previous model-based estimates of the meridional circulation were calculated using zonal averages: this method leads to a counter-intuitive poleward circulation of the less dense waters, and underestimates the eddy effects. We show that on the contrary, the upper ocean circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. Two model simulations, in which the Buoyancy Forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface Buoyancy flux is not as straightforward as assumed by the simplest theoretical models: the sign of the residual circulation cannot be inferred from the surface Buoyancy Forcing only. Among the other processes that likely play a part in setting the meridional circulation, our model results emphasize the complex three-dimensional structure of the ACC (probably not well accounted for in streamline-averaged, two-dimensional models) and the distinct role of temperature and salinity in the definition of the density field. Heat and salt transports by the time-mean flow are important even across time-mean streamlines. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.
-
Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current
Ocean Science Discussions, 2007Co-Authors: Anne-marie Tréguier, Matthew H. England, Stephen R. Rintoul, Gurvan Madec, Julien Le Sommer, Jean-marc MolinesAbstract:An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. In the upper layers, the meridional circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. The same calculation in a zonal average gives a completely different view and underestimates the eddy effects. Two model simulations, in which the Buoyancy Forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface Buoyancy flux is not as straightforward as assumed by some recent theoretical studies: even the sign of the residual circulation cannot be inferred from the Buoyancy Forcing. Heat and salt transports by the time-mean flow are important even in the streamline framework. Streamline-averaged, two-dimensional models cannot account quantitatively for the complex three-dimensional structure of the ACC. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.
Geoffrey K. Vallis - One of the best experts on this subject based on the ideXlab platform.
-
Meridional Overturning Circulation in a multi-basin model. Part II: Sensitivity to diffusivity and wind in warm and cool climates
Journal of Physical Oceanography, 2021Co-Authors: Jonathan A. Baker, Andrew J. Watson, Geoffrey K. VallisAbstract:AbstractThe response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind Forcing and Pacific basin vertical diffusivity is investigated under varying Buoyancy Forcings, corresponding to ‘warm’, ‘present-day’ and ‘cold’ states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all Buoyancy Forcings. The sensitivity of the AMOC to wind stress increases as the Buoyancy Forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to Buoyancy Forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively-driven upwelling sustain the AMOC in the warmer (present-day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the Buoyancy Forcing are also needed to decouple the two basins.
-
Meridional Overturning Circulation in a Multibasin Model. Part I: Dependence on Southern Ocean Buoyancy Forcing
Journal of Physical Oceanography, 2020Co-Authors: Jonathan A. Baker, Andrew J. Watson, Geoffrey K. VallisAbstract:AbstractThe variation in the strength and structure of the overturning circulation under varying Southern Ocean Buoyancy Forcing, corresponding to present day, a cooler (glacial) state, and a possi...
-
Southern Ocean Buoyancy Forcing of ocean ventilation and glacial atmospheric CO_2
Nature Geoscience, 2015Co-Authors: Andrew J. Watson, Geoffrey K. Vallis, Maxim NikurashinAbstract:Ocean circulation and dynamics can alter atmospheric CO_2 concentrations. Numerical modelling suggests that shifts in surface Buoyancy loss and the location of upwelling can sequester CO_2 in the Southern Ocean during glacial periods. Atmospheric CO_2 concentrations over glacial–interglacial cycles closely correspond to Antarctic temperature patterns^ 1 . These are distinct from temperature variations in the mid to northern latitudes^ 2 , so this suggests that the Southern Ocean is pivotal in controlling natural CO_2 concentrations^ 3 . Here we assess the sensitivity of atmospheric CO_2 concentrations to glacial–interglacial changes in the ocean’s meridional overturning circulation using a circulation model^ 4 , 5 for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface Buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO_2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO_2. We therefore suggest that the direct effect of temperatures on Southern Ocean Buoyancy Forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO_2 concentrations over glacial–interglacial cycles.
-
southern ocean Buoyancy Forcing of ocean ventilation and glacial atmospheric co2
Nature Geoscience, 2015Co-Authors: Andrew J. Watson, Geoffrey K. Vallis, Maxim NikurashinAbstract:Ocean circulation and dynamics can alter atmospheric CO2 concentrations. Numerical modelling suggests that shifts in surface Buoyancy loss and the location of upwelling can sequester CO2 in the Southern Ocean during glacial periods.
David P. Marshall - One of the best experts on this subject based on the ideXlab platform.
-
The Injection of Zonal Momentum by Buoyancy Forcing in a Southern Ocean Model
Journal of Physical Oceanography, 2015Co-Authors: Emma Howard, Andrew Mcc. Hogg, Stephanie Waterman, David P. MarshallAbstract:AbstractAn overturning circulation, driven by prescribed Buoyancy Forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and Buoyancy Forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the Buoyancy Forcing and that the eddy kinetic energy is primarily set by wind stress Forcing. The zonal momentum budget integrated over each layer is considered in the Buoyancy-forced, wind-forced, and combined Forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the Buoyancy-forced case, the Forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom fric...
-
Vertical Fluxes of Potential Vorticity and the Structure of the Thermocline
Journal of Physical Oceanography, 2000Co-Authors: David P. MarshallAbstract:Abstract A new framework for understanding the vertical structure of ocean gyres is developed based on vertical fluxes of potential vorticity. The key ingredient is an integral constraint that in a steady state prohibits a net flux of potential vorticity through any closed contour of Bernoulli potential or density. Applied to an ocean gyre, the vertical fluxes of potential vorticity associated with advection, friction, and Buoyancy Forcing must therefore balance in an integral sense. In an anticyclonic subtropical gyre, the advective and frictional potential vorticity fluxes are both directed downward, and Buoyancy Forcing is required to provide the compensating upward potential vorticity flux. Three regimes are identified: 1) a surface “ventilated thermocline” in which the upward potential vorticity flux is provided by Buoyancy Forcing within the surface mixed layer, 2) a region of weak stratification—“mode water”—in which all three components of the potential vorticity flux become vanishingly small, and...
-
Subduction of water masses in an eddying ocean
Journal of Marine Research, 1997Co-Authors: David P. MarshallAbstract:Mesoscale eddies modify the rate at which a water mass transfers from the surface mixed layer of the ocean into the interior thermocline, in particular in regions of intense baroclinic instability such as the Antarctic Circumpolar Current, open-ocean convective chimneys, and ocean fronts. Here, the time-mean subduction of a water mass, evaluated following the meandering surface density outcrops, is found to incorporate a rectified contribution from eddies, arising from correlations between the area over which the water mass is outcropped at the sea surface and the local subduction rate. Alternatively, this eddy subduction can be interpreted in terms of an eddy-driven secondary circulation associated with baroclinic instability. The net subduction rate, incorporating both Eulerian-mean and eddy contributions, can be further related to Buoyancy Forcing of the surface mixed layer using a formula by Walin (1982). Solutions from an idealized two-dimensional ocean model are presented to illustrate the eddy contribution to subduction rates in the Southern Ocean and in an open-ocean convective chimney. In the Southern Ocean, the net subduction rate is the residual of the Eulerian-mean and eddy contributions, which cancel at leading order; given plausible patterns of surface Buoyancy Forcing, one can obtain subduction of Antarctic Intermediate Water and Antarctic Bottom Water, with entrainment of North Atlantic Deep Water in between. In a convective chimney, in contrast, the Eulerian-mean subduction rate is vanishingly small and the subduction is contributed entirely by mesoscale eddies.
Luanne Thompson - One of the best experts on this subject based on the ideXlab platform.
-
The response of the North Pacific Ocean to decadal variability in atmospheric Forcing: Wind versus Buoyancy Forcing
Journal of Physical Oceanography, 2004Co-Authors: Luanne Thompson, Carol LaddAbstract:Abstract Both wind and Buoyancy Forcing result in variability in the North Pacific Ocean thermocline. A vertical modal analysis of the density deviations in a 30-yr run of an ocean general circulation model of the North Pacific forced by atmospheric variability is used to identify the spatial and temporal patterns of the different baroclinic modes. The different dynamic vertical modes show distinct propagation characteristics, with the first baroclinic mode exhibiting consistent westward propagation at all latitudes. The higher baroclinic modes show westward phase propagation at low latitudes but propagate eastward at higher latitudes. The propagation characteristics of each mode can be understood by the inclusion of the zonal mean flow in the vertical structure equation. Evaluation of the Ekman pumping and diapycnal fluxes in the quasigeostrophic potential vorticity equation for each dynamic vertical mode distinguishes their effects on the thermocline variability. Wind variability dominantly forces the f...
-
Decadal Variability of North Pacific Central Mode Water
Journal of Physical Oceanography, 2002Co-Authors: Carol Ladd, Luanne ThompsonAbstract:An isopycnal model forced with wind stress and heat fluxes from 1965 through 1993 was used to examine the effects of variable atmospheric Forcing on the ventilation of the North Pacific. During this time period, a climatic regime shift occurred that had significant impacts on heat fluxes, sea surface temperature (SST), and wind stress patterns. The climate shift, occurring in the winter of 1976/77, affected the formation rates and locations, and properties of the Central Mode Water (CMW) formed in the model. Three model runs were compared: one with variable Buoyancy Forcing and climatological wind Forcing, one with variable wind Forcing and climatological Buoyancy Forcing, and one with variability in both the Buoyancy and the wind Forcing. The comparison indicates that Buoyancy Forcing is of primary importance in the variability of mode water formation and properties surrounding the climate shift. One measure of the climate shift is the Pacific decadal oscillation (PDO), an index of SST variability in the North Pacific, which changed sign in 1976/77. A positive state for the PDO is associated with deeper model mixed layers, formation of denser varieties of CMW, and an anticyclonic circulation anomaly in the CMW density range.
