The water mass transformation in the upper limb of the overturning circulation in the Southern Hemisphere

AbstractThe oceanic connections between tidewater glaciers and continental shelf waters are modulated and controlled by geometrically complex fjords. These fjords exhibit both overturning circulations and horizontal recirculations, driven by a combination of water mass transformation at the head of the fjord, variability on the continental shelf, and atmospheric forcing. However, it remains unclear which geometric and forcing parameters are the most important in exerting control on the overturning and horizontal recirculation. To address this, idealized numerical simulations are conducted using an isopycnal model of a fjord connected to a continental shelf, which is representative of regions in Greenland and the West Antarctic Peninsula. A range of sensitivity experiments demonstrate that sill height, wind direction/strength, subglacial discharge strength, and depth of offshore warm water are of first-order importance to the overturning circulation, while fjord width is also of leading importance to the horizontal recirculation. Dynamical predictions are developed and tested for the overturning circulation of the entire shelf-to-glacierface domain, subdivided into three regions: the continental shelf extending from the open ocean to the fjord mouth, the sill-overflow at the fjord mouth, and the plume-driven water mass transformation at the fjord head. A vorticity budget is also developed to predict the strength of the horizontal recirculation, which provides a scaling in terms of the overturning and bottom friction. Based on these theories, we may predict glacial melt rates that take into account overturning and recirculation, which may be used to refine estimates of ocean-driven melting of the Greenland and Antarctic ice sheets.

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Vertical Heat Transport by Ocean Circulation and the Role of Mechanical and Haline Forcing

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0179.1 ◽

2013 ◽

Vol 43 (10) ◽

pp. 2095-2112 ◽

Cited By ~ 17

Author(s):

Jan D. Zika ◽

Willem P. Sijp ◽

Matthew H. England

Keyword(s):

Southern Hemisphere ◽

Ocean Circulation ◽

Upper Limb ◽

Thermohaline Circulation ◽

Climate Model ◽

Mechanical Energy ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Direct Cell ◽

Meridional Overturning

Abstract Vertical transport of heat by ocean circulation is investigated using a coupled climate model and novel thermodynamic methods. Using a streamfunction in temperature–depth coordinates, cells are identified by whether they are thermally direct (flux heat upward) or indirect (flux heat downward). These cells are then projected into geographical and other thermodynamic coordinates. Three cells are identified in the model: a thermally direct cell coincident with Antarctic Bottom Water, a thermally indirect deep cell coincident with the upper limb of the meridional overturning circulation, and a thermally direct shallow cell coincident with the subtropical gyres at the surface. The mechanisms maintaining the thermally indirect deep cell are investigated. Sinking water within the deep cell is more saline than that which upwells, because of the coupling between the upper limb and the subtropical gyres in a broader thermohaline circulation. Despite the higher salinity of its sinking water, the deep cell transports buoyancy downward, requiring a source of mechanical energy. Experiments run to steady state with increasing Southern Hemisphere westerlies show an increasing thermally indirect circulation. These results suggest that heat can be pumped downward by the upper limb of the meridional overturning circulation through a combination of salinity gain in the subtropics and the mechanical forcing provided by Southern Hemisphere westerly winds.

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Water mass transformation and overturning circulation in the Arabian Gulf

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0249.1 ◽

2021 ◽

Author(s):

Maryam R. Al-Shehhi ◽

Hajoon Song ◽

Jeffery Scott ◽

John Marshall

Keyword(s):

Water Mass ◽

Arabian Gulf ◽

Three Dimensional ◽

Heat Fluxes ◽

Transformation Rate ◽

Overturning Circulation ◽

Resolution Model ◽

Meridional Overturning ◽

High Resolution Model ◽

Water Mass Transformation

AbstractWe diagnose the ocean’s residual overturning circulation of the Arabian Gulf in a high-resolution model and interpret it in terms ofwater-mass transformation processes mediated by air-sea buoyancy fluxes and interior mixing. We attempt to rationalise the complex three-dimensional flow in terms of the superposition of a zonal (roughly along-axis) and meridional (transverse) overturning pattern. Rates of overturning and the seasonal cycle of air-sea fluxes sustaining them are quantified and ranked in order of importance. Air-sea fluxes dominate the budget so that, at zero order, the magnitude and sense of the overturning circulation can be inferred from air-sea fluxes, with interior mixing playing a lesser role. We find that wintertime latent heat fluxes dominate the water-mass transformation rate in the interior waters of the Gulf leading to a diapycnal volume flux directed toward higher densities. In the zonal overturning cell, fluid is drawn in from the Sea of Oman through the Strait of Hormuz, transformed and exits the Strait near the southern and bottom boundaries. Along the southern margin of the Gulf, evaporation plays an important role in the meridional overturning pattern inducing sinking there.

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Restratification of Abyssal Mixing Layers by Submesoscale Baroclinic Eddies

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0082.1 ◽

2018 ◽

Vol 48 (9) ◽

pp. 1995-2010 ◽

Cited By ~ 11

Author(s):

Jörn Callies

Keyword(s):

Large Scale ◽

Water Mass ◽

Mixing Layer ◽

Mixing Layers ◽

Small Scale ◽

Surface Mixed Layer ◽

Overturning Circulation ◽

Topographic Slope ◽

Abyssal Mixing ◽

Water Mass Transformation

AbstractFor small-scale turbulence to achieve water mass transformation and thus affect the large-scale overturning circulation, it must occur in stratified water. Observations show that abyssal turbulence is strongly enhanced in the bottom few hundred meters in regions with rough topography, and it is thought that these abyssal mixing layers are crucial for closing and shaping the overturning circulation. If it were left unopposed, however, bottom-intensified turbulence would mix away the observed mixing-layer stratification over the course of a few years. It is proposed here that the homogenizing tendency of mixing may be balanced by baroclinic restratification. It is shown that bottom-intensified mixing, if it occurs on a large-scale topographic slope such as a midocean ridge flank, not only erodes stratification but also tilts isopycnals in the bottom few hundred meters. This tilting of isopycnals generates a reservoir of potential energy that can be tapped into by submesoscale baroclinic eddies. The eddies slide dense water under light water and thus restratify the mixing layer, similar to what happens in the surface mixed layer. This restratification is shown to be effective enough to balance the homogenizing tendency of mixing and to maintain the observed mixing-layer stratification. This suggests that submesoscale baroclinic eddies may play a crucial role in providing the stratification mixing can act on, thus allowing sustained water mass transformation. Through their restratification of abyssal mixing layers, submesoscale eddies may therefore directly affect the strength and structure of the abyssal overturning circulation.

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Influences of Precipitation on Water Mass Transformation and Deep Convection

Journal of Physical Oceanography ◽

10.1175/jpo-d-11-0230.1 ◽

2012 ◽

Vol 42 (10) ◽

pp. 1684-1700 ◽

Cited By ~ 19

Author(s):

Michael A. Spall

Keyword(s):

Numerical Model ◽

Water Mass ◽

Numerical Models ◽

Deep Convection ◽

Meridional Overturning Circulation ◽

Critical Threshold ◽

Overturning Circulation ◽

Model Calculations ◽

Meridional Overturning ◽

Water Mass Transformation

Abstract The influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models. Nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses, making explicit their dependence on both geometric parameters such as basin area, sill depth, and latitude, as well as on the strength of atmospheric forcing. In addition to the properties of the convective water, the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode. High-resolution numerical model calculations compare well with the theory for the properties of the convective water mass, the strength of the meridional overturning circulation, and also the shutdown of deep convection. However, the numerical model also shows that, for precipitation levels that exceed this critical threshold, the circulation retains downwelling and northward heat transport, even in the absence of deep convection.

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Response of the North Atlantic Thermohaline Circulation and Ventilation to Increasing Carbon Dioxide in CCSM3

Journal of Climate ◽

10.1175/jcli3757.1 ◽

2006 ◽

Vol 19 (11) ◽

pp. 2382-2397 ◽

Cited By ~ 79

Author(s):

Frank O. Bryan ◽

Gokhan Danabasoglu ◽

Norikazu Nakashiki ◽

Yoshikatsu Yoshida ◽

Dong-Hoon Kim ◽

...

Keyword(s):

North Atlantic ◽

Control Experiment ◽

Thermohaline Circulation ◽

Water Mass ◽

Overturning Circulation ◽

The North ◽

Freshwater Fluxes ◽

The North Atlantic ◽

Atlantic Thermohaline Circulation ◽

Water Mass Transformation

Abstract The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high latitudes with increasing CO2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland–Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.

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Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic

10.5194/egusphere-egu21-15972 ◽

2021 ◽

Author(s):

D. Gwyn Evans ◽

N. Penny Holliday ◽

Marilena Oltmanns

Keyword(s):

North Atlantic ◽

Water Mass ◽

Meridional Overturning Circulation ◽

Mass Analysis ◽

Overturning Circulation ◽

Water Mass Analysis ◽

Meridional Overturning ◽

The Mean ◽

Water Mass Transformation ◽

Water Mass Transformations

<p>The OSNAP (Overturning in the Subpolar North Atlantic Program) array at ~60&#176;N has provided new and unprecedented insight into the strength and variability of the meridional overturning circulation in the subpolar North Atlantic. OSNAP has identified the region of the subpolar North Atlantic east of Greenland as a key region for the water mass transformation and densification that sets the strength and variability of the overturning circulation. Here, we will investigate the drivers of this water mass transformation and their roles in driving the overturning circulation at OSNAP. Using a water mass analysis on both model-based and observational-based datasets, we isolate diathermal (across surfaces of constant temperature) and diahaline (across surfaces of constant salinity) transformations due to air-sea buoyancy fluxes, and mixing. We show that the time-mean overturning strength is set by both the air-sea buoyancy fluxes and the strength of subsurface mixing. This balance is apparent on a seasonal timescale, where we resolve large seasonal fluctuations in the both the air-sea buoyancy fluxes and mixing. The residual of this seasonal cycle then corresponds to the mean overturning strength. On interannual timescales, mixing becomes the dominant driver of variability in the overturning circulation. To determine the location of these water mass transformations and the dynamical processes responsible for the mixing-driven variability, our water mass analysis is projected onto geographical coordinates.</p>

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Impacts of Ice-Shelf Melting on Water-Mass Transformation in the Southern Ocean from E3SM Simulations

Journal of Climate ◽

10.1175/jcli-d-19-0683.1 ◽

2020 ◽

Vol 33 (13) ◽

pp. 5787-5807

Author(s):

Hyein Jeong ◽

Xylar S. Asay-Davis ◽

Adrian K. Turner ◽

Darin S. Comeau ◽

Stephen F. Price ◽

...

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Water Mass ◽

Dominant Role ◽

Heat Fluxes ◽

Ice Formation ◽

Ice Shelf ◽

Overturning Circulation ◽

Antarctic Sea Ice ◽

Water Mass Transformation

AbstractThe Southern Ocean overturning circulation is driven by winds, heat fluxes, and freshwater sources. Among these sources of freshwater, Antarctic sea ice formation and melting play the dominant role. Even though ice-shelf melt is relatively small in magnitude, it is located close to regions of convection, where it may influence dense water formation. Here, we explore the impacts of ice-shelf melting on Southern Ocean water-mass transformation (WMT) using simulations from the Energy Exascale Earth System Model (E3SM) both with and without the explicit representation of melt fluxes from beneath Antarctic ice shelves. We find that ice-shelf melting enhances transformation of Upper Circumpolar Deep Water, converting it to lower density values. While the overall differences in Southern Ocean WMT between the two simulations are moderate, freshwater fluxes produced by ice-shelf melting have a further, indirect impact on the Southern Ocean overturning circulation through their interaction with sea ice formation and melting, which also cause considerable upwelling. We further find that surface freshening and cooling by ice-shelf melting cause increased Antarctic sea ice production and stronger density stratification near the Antarctic coast. In addition, ice-shelf melting causes decreasing air temperature, which may be directly related to sea ice expansion. The increased stratification reduces vertical heat transport from the deeper ocean. Although the addition of ice-shelf melting processes leads to no significant changes in Southern Ocean WMT, the simulations and analysis conducted here point to a relationship between increased Antarctic ice-shelf melting and the increased role of sea ice in Southern Ocean overturning.

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