scholarly journals Reassessing the Role of the Indo‐Pacific in the Ocean's Global Overturning Circulation

2018 ◽  
Vol 45 (22) ◽  
pp. 12,422-12,431 ◽  
Author(s):  
Emily R. Newsom ◽  
Andrew F. Thompson
Keyword(s):  
Overturning Circulation ◽  
Global Overturning Circulation

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.

scholarly journals The Energetics of Southern Ocean Upwelling

2017 ◽  
Vol 47 (1) ◽  
pp. 135-153 ◽  
Author(s):  
Andrew McC. Hogg ◽  
Paul Spence ◽  
Oleg A. Saenko ◽  
Stephanie M. Downes
Keyword(s):  
Potential Energy ◽  
Southern Ocean ◽  
Northern Hemisphere ◽  
Wind Stress ◽  
Weddell Sea ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Available Potential Energy ◽  
Ocean Winds ◽  
Global Overturning Circulation

AbstractThe ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.

scholarly journals High-latitude ocean ventilation and its role in Earth's climate transitions

2017 ◽  
Vol 375 (2102) ◽  
pp. 20160324 ◽  
Author(s):  
Alberto C. Naveira Garabato ◽  
Graeme A.  MacGilchrist ◽  
Peter J. Brown ◽  
D. Gwyn Evans ◽  
Andrew J. S. Meijers ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
High Latitude ◽  
Climate System ◽  
High Latitudes ◽  
Overturning Circulation ◽  
Ocean Ventilation ◽  
Earth’S Climate ◽  
Warming World ◽  
Climate Transitions

The processes regulating ocean ventilation at high latitudes are re-examined based on a range of observations spanning all scales of ocean circulation, from the centimetre scales of turbulence to the basin scales of gyres. It is argued that high-latitude ocean ventilation is controlled by mechanisms that differ in fundamental ways from those that set the overturning circulation. This is contrary to the assumption of broad equivalence between the two that is commonly adopted in interpreting the role of the high-latitude oceans in Earth's climate transitions. Illustrations of how recognizing this distinction may change our view of the ocean's role in the climate system are offered. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

scholarly journals Enhanced Mid-Latitude Meridional Heat Imbalance Induced by the Ocean

Atmosphere ◽  
2019 ◽  
Vol 10 (12) ◽  
pp. 746 ◽  
Author(s):  
Hu Yang ◽  
Gerrit Lohmann ◽  
Xiaoxu Shi ◽  
Chao Li
Keyword(s):  
Global Warming ◽  
Atmospheric Circulation ◽  
Water Cycle ◽  
Strong Mixing ◽  
Jet Stream ◽  
Overturning Circulation ◽  
Ocean Heat Uptake ◽  
Climate Simulations ◽  
Heat Uptake

The heat imbalance is the fundamental driver for the atmospheric circulation. Therefore, it is crucially important to understand how it responds to global warming. In this study, the role of the ocean in reshaping the atmospheric meridional heat imbalance is explored based on observations and climate simulations. We found that ocean tends to strengthen the meridional heat imbalance over the mid-latitudes. This is primarily because of the uneven ocean heat uptake between the subtropical and subpolar oceans. Under global warming, the subtropical ocean absorbs relatively less heat as the water there is well stratified. In contrast, the subpolar ocean is the primary region where the ocean heat uptake takes place, because the subpolar ocean is dominated by upwelling, strong mixing, and overturning circulation. We propose that the enhanced meridional heat imbalance may potentially contribute to strengthening the water cycle, westerlies, jet stream, and mid-latitude storms.

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.

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