The role of diapycnal mixing for the equilibrium response of thermohaline circulation

Abstract The role of baroclinic eddies in transferring thermal gradients laterally, and thus determining the stratification of the ocean, is examined. The hypothesis is that the density differences imposed at the surface by differential heating are a source of available potential energy that can be partially released by mesocale eddies with horizontal scales on the order of 100 km. Eddy fluxes balance the diapycnal mixing of heat and thus determine the vertical scale of penetration of horizontal thermal gradients (i.e., the depth of the thermocline). This conjecture is in contrast with the current thinking that the deep stratification is determined by a balance between diapycnal mixing and the large-scale thermohaline circulation. Eddy processes are analyzed in the context of a rapidly rotating primitive equation flow driven by specified surface temperature, with isotropic diffusion and viscosity. The barotropic component of the eddies is found to be responsible for most of the heat flux, and so the eddy transport is horizontal rather than isopycnal. This eddy transport takes place in the shallow surface layer where eddies, as well as the mean temperature, undergo diabatic, irreversible mixing. Scaling laws for the depth of the thermocline as a function of the external parameters are proposed. In the classical thermocline theory, the depth of the thermocline depends on the diffusivity, the rotation rate, and the imposed temperature gradient. In this study the authors find an additional dependence on the viscosity and on the domain width.

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Overlooked Role of Mesoscale Winds in Powering Ocean Diapycnal Mixing

Scientific Reports ◽

10.1038/srep37180 ◽

2016 ◽

Vol 6 (1) ◽

Cited By ~ 1

Author(s):

Zhao Jing ◽

Lixin Wu ◽

Xiaohui Ma ◽

Ping Chang

Keyword(s):

Diapycnal Mixing

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The use of Cs and Sr isotopes as tracers in the Arctic Mediterranean Seas

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1988.0049 ◽

1988 ◽

Vol 325 (1583) ◽

pp. 161-176 ◽

Cited By ~ 36

Keyword(s):

Deep Water ◽

Thermohaline Circulation ◽

Atlantic Water ◽

The Arctic ◽

Coastal Current ◽

Intermediate Water ◽

Sr Isotopes ◽

Arctic Ice ◽

Norwegian Coastal Current

The Arctic Mediterranean Seas constitute an oceanic region in which the thermohaline circulation has a strong advective component and deep ventilation processes are very active relative to other oceanic areas. Details of the nature of these circulation and ventilation processes have been revealed through use of Cs and Sr isotopes from bomb-fallout and nuclear-waste sources as ocean tracers. In both cases, their regional input is dominated by advective supply in the Norwegian Atlantic Current and Norwegian Coastal Current, respectively. The different temporal, spatial, and compositional input patterns of these tracers have been used to study different facets of the regional circulation. These input differences and some representative applications of the use of these tracers are reviewed. The data discussed derive from samples collected both from research vessels and from Arctic ice camps. The topics addressed include: ( a ) the role of Arctic Intermediate Water as source, supplying recent surface water to North Atlantic Deep Water via the Denmark Strait overflow; ( b ) deep convective mixing in the Greenland Sea; ( c ) circulation or recirculation of Atlantic water in the Arctic basins; and ( d ) the role of Arctic shelfwaters in the ventilation of intermediate and deep water in the Eurasian and Canadian basins.

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Resonant Forcing of the Climate System in Subharmonic Modes

Journal of Marine Science and Engineering ◽

10.3390/jmse8010060 ◽

2020 ◽

Vol 8 (1) ◽

pp. 60 ◽

Cited By ~ 1

Author(s):

Jean-Louis Pinault

Keyword(s):

Climate Variability ◽

Atmospheric Carbon Dioxide ◽

Rossby Waves ◽

Thermohaline Circulation ◽

Climate System ◽

Ice Age ◽

Polar Ice ◽

Ice Caps ◽

Resonant Forcing

During recent decades observation of climate archives has raised several questions. Concerning the mid-Pleistocene transition problem, conflicting sets of hypotheses highlight either the role of ice sheets or atmospheric carbon dioxide in causing the increase in duration and severity of ice age cycles. The role of the solar irradiance modulations in climate variability is frequently referenced but the underlying physical justifications remain most mysterious. Here, we extend the key mechanisms involving the oceanic Rossby waves in climate variability, to very long-period, multi-frequency Rossby waves winding around the subtropical gyres. Our study demonstrates that the climate system responds resonantly to solar and orbital forcing in eleven subharmonic modes. We advocate new hypotheses on the evolution of the past climate, implicating the deviation between forcing periods and natural periods according to the subharmonic modes, and the polar ice caps while challenging the role of the thermohaline circulation.

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The role of the thermohaline circulation in abrupt climate change

Nature ◽

10.1038/415863a ◽

2002 ◽

Vol 415 (6874) ◽

pp. 863-869 ◽

Cited By ~ 522

Author(s):

Peter U. Clark ◽

Nicklas G. Pisias ◽

Thomas F. Stocker ◽

Andrew J. Weaver

Keyword(s):

Climate Change ◽

Thermohaline Circulation ◽

Abrupt Climate Change

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The potential role of spectral properties in detecting thresholds in the Earth system: application to the thermohaline circulation

Ocean Dynamics ◽

10.1007/s10236-002-0023-6 ◽

2003 ◽

Vol 53 (2) ◽

pp. 53-63 ◽

Cited By ~ 126

Author(s):

Thomas Kleinen ◽

Hermann Held ◽

Gerhard Petschel-Held

Keyword(s):

System Application ◽

Spectral Properties ◽

Potential Role ◽

Thermohaline Circulation ◽

Earth System ◽

The Earth

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Equilibrium response of thermohaline circulation to large changes in atmospheric CO2 concentration

Climate Dynamics ◽

10.1007/s00382-004-0394-0 ◽

2004 ◽

Vol 22 (2-3) ◽

pp. 325-326

Author(s):

S. Manabe ◽

R. J. Stouffer

Keyword(s):

Thermohaline Circulation ◽

Co2 Concentration ◽

Atmospheric Co2 ◽

Atmospheric Co2 Concentration ◽

Equilibrium Response

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Surface and subsurface Labrador Shelf water mass conditions during the last 6000 years

Climate of the Past ◽

10.5194/cp-16-1127-2020 ◽

2020 ◽

Vol 16 (4) ◽

pp. 1127-1143

Author(s):

Annalena A. Lochte ◽

Ralph Schneider ◽

Markus Kienast ◽

Janne Repschläger ◽

Thomas Blanz ◽

...

Keyword(s):

Deep Water ◽

Late Holocene ◽

Thermohaline Circulation ◽

Sea Water ◽

Labrador Sea ◽

Circulation System ◽

Deep Water Formation ◽

Holocene Thermal Maximum ◽

Water Formation

Abstract. The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

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