The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation

2003 ◽  
Vol 21 (7-8) ◽  
pp. 707-717 ◽  
Author(s):  
J. M. Gregory ◽  
O. A. Saenko ◽  
A. J. Weaver
Keyword(s):  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Freshwater Balance

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

2021 ◽  
Author(s):  
Tomas Jonathan ◽  
Mike Bell ◽  
Helen Johnson ◽  
David Marshall
Keyword(s):  
Global Climate ◽  
Dominant Role ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Relative Role ◽  
Overturning Circulation ◽  
Near Surface ◽  
Boundary Density ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. </p><p>The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.</p>

scholarly journals Antarctic Sea Ice Control on the Depth of North Atlantic Deep Water

2019 ◽  
Vol 32 (9) ◽  
pp. 2537-2551 ◽  
Author(s):  
Louis-Philippe Nadeau ◽  
Raffaele Ferrari ◽  
Malte F. Jansen
Keyword(s):  
Sea Ice ◽  
Deep Water ◽  
Formation Rate ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Ice Formation ◽  
Overturning Circulation ◽  
Antarctic Sea Ice ◽  
Meridional Overturning

Abstract Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

The Role of Eddies in the Zonal and Meridional Overturning Circulations of Buoyancy-Forced Basins

2021 ◽  
Vol 51 (2) ◽  
pp. 575-590
Author(s):  
Suyash Bire ◽  
Christopher L.P. Wolfe
Keyword(s):  
Meridional Overturning Circulation ◽  
Swash Zone ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Southern Boundary ◽  
Eastern Boundary ◽  
Meridional Overturning ◽  
Surface Buoyancy ◽  
Upwelling And Downwelling

AbstractThe zonal and meridional overturning circulations of buoyancy-forced basins are studied in an eddy-resolving model. The zonal overturning circulation (ZOC) is driven by the meridional gradient of buoyancy at the surface and stratification at the southern boundary. The ZOC, in turn, produces zonal buoyancy gradients through upwelling and downwelling at the western and eastern boundaries, respectively. The meridional overturning circulation (MOC) is driven by these zonal gradients rather than being directly driven by meridional gradients. Eddies lead to a broadening of the upwelling and downwelling limbs of the ZOC, as well as a decoupling of the locations of vertical and diapycnal transport. This broadening is more prominent on the eastern boundary, where westward-moving eddies transport warm water away from a poleward-flowing eastern boundary current. Most of the diapycnal downwelling occurs in the “swash zone”—the region where the isopycnals intermittently come in contact with the surface and lose buoyancy to the atmosphere. A scaling for the overturning circulations, which depends on the background stratification and the surface buoyancy gradient, is derived and found to be an excellent fit to the numerical experiments.

scholarly journals Standing Eddies in the Meridional Overturning Circulation

2012 ◽  
Vol 42 (9) ◽  
pp. 1486-1508 ◽  
Author(s):  
Jan Viebahn ◽  
Carsten Eden
Keyword(s):  
Surface Layer ◽  
Southern Ocean ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Transient Eddy ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Depth Contour ◽  
Meridional Overturning

Abstract The role of standing eddies for the meridional overturning circulation (MOC) is discussed. The time-mean isopycnal meridional streamfunction is decomposed into a time- and zonal-mean part, a standing-eddy part, and a transient-eddy part. It turns out that the construction of an isopycnal MOC with an exactly vanishing standing-eddy part has to be performed by zonal integration along depth-dependent horizontal isolines of time-mean density. In contrast, zonal integration along time-mean geostrophic streamlines generally only leads to an isopycnal MOC with a reduced standing-eddy part. A generalized approach of constructing meridional transport streamfunctions by two tracer fields and the generalized way to neutralize the corresponding standing-eddy part is given to summarize the discussion. Using the results of an idealized Southern Ocean model, it is demonstrated that neglecting the depth dependence of the zonal integration paths by integrating along density contours or geostrophic streamlines of a fixed depth (“contour depth”) may represent an acceptable approximation: although the standing-eddy part then exactly vanishes only at the contour depth (except for the ageostrophic surface layer using geostrophic streamlines), the overall standing-eddy part is significantly reduced for adequate contour depths. In the idealized Southern Ocean model, density contours at middepth and surface geostrophic streamlines represent the most adequate approximations. Moreover, it is found that the effect of changing the zonal integration paths from latitude circles to curvilinear paths on the zonally averaged density is of the same order as changing from Eulerian to isopycnal averaging.

Superrotation on Venus, on Titan, and Elsewhere

2018 ◽  
Vol 46 (1) ◽  
pp. 175-202 ◽  
Author(s):  
Peter L. Read ◽  
Sebastien Lebonnois
Keyword(s):  
Solar System ◽  
Atmospheric Circulation ◽  
Numerical Models ◽  
Meridional Overturning Circulation ◽  
Dynamical Regime ◽  
Overturning Circulation ◽  
Meridional Overturning

The superrotation of the atmospheres of Venus and Titan has puzzled dynamicists for many years and seems to put these planets in a very different dynamical regime from most other planets. In this review, we consider how to define superrotation objectively and explore the constraints that determine its occurrence. Atmospheric superrotation also occurs elsewhere in the Solar System and beyond, and we compare Venus and Titan with Earth and other planets for which wind estimates are available. The extreme superrotation on Venus and Titan poses some difficult challenges for numerical models of atmospheric circulation, much more difficult than for more rapidly rotating planets such as Earth or Mars. We consider mechanisms for generating and maintaining a superrotating state, all of which involve a global meridional overturning circulation. The role of nonaxisymmetric eddies is crucial, however, but the detailed mechanisms may differ between Venus, Titan, and other planets.

scholarly journals Tides and the Climate: Some Speculations

2007 ◽  
Vol 37 (2) ◽  
pp. 135-147 ◽  
Author(s):  
Walter Munk ◽  
Bruce Bills
Keyword(s):  
Solar Radiation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Tidal Mixing ◽  
Sea Surface Temperatures ◽  
Overturning Circulation ◽  
Long Period ◽  
Meridional Overturning ◽  
Shallow Seas

Abstract The important role of tides in the mixing of the pelagic oceans has been established by recent experiments and analyses. The tide potential is modulated by long-period orbital modulations. Previously, Loder and Garrett found evidence for the 18.6-yr lunar nodal cycle in the sea surface temperatures of shallow seas. In this paper, the possible role of the 41 000-yr variation of the obliquity of the ecliptic is considered. The obliquity modulation of tidal mixing by a few percent and the associated modulation in the meridional overturning circulation (MOC) may play a role comparable to the obliquity modulation of the incoming solar radiation (insolation), a cornerstone of the Milanković theory of ice ages. This speculation involves even more than the usual number of uncertainties found in climate speculations.

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