scholarly journals On the glacial and interglacial thermohaline circulation and the associated transports of heat and freshwater

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 907-921 ◽  
Author(s):  
M. Ballarotta ◽  
S. Falahat ◽  
L. Brodeau ◽  
K. Döös
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Conveyor Belt ◽  
Meridional Overturning Circulation ◽  
Interglacial Period ◽  
Global Changes ◽  
The Pacific ◽  
Abyssal Circulation

Abstract. The thermohaline circulation (THC) and the oceanic heat and freshwater transports are essential for understanding the global climate system. Streamfunctions are widely used in oceanography to represent the THC and estimate the transport of heat and freshwater. In the present study, the regional and global changes of the THC, the transports of heat and freshwater and the timescale of the circulation between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present-day climate are explored using an Ocean General Circulation Model and streamfunctions projected in various coordinate systems. We found that the LGM tropical circulation is about 10% stronger than under modern conditions due to stronger wind stress. Consequently, the maximum tropical transport of heat is about 20% larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes by almost 50% and reorganising the freshwater transport. The strength of the Atlantic Meridional Overturning Circulation depends strongly on the coordinate system. It varies between 9 and 16 Sv during the LGM, and between 12 to 19 Sv for the present day. Similar to paleo-proxy reconstructions, a large intrusion of saline Antarctic Bottom Water takes place into the Northern Hemisphere basins and squeezes most of the Conveyor Belt circulation into a shallower part of the ocean. These different haline regimes between the glacial and interglacial period are illustrated by the streamfunctions in latitude–salinity coordinates and thermohaline coordinates. From these diagnostics, we found that the LGM Conveyor Belt circulation is driven by an enhanced salinity contrast between the Atlantic and the Pacific basin. The LGM abyssal circulation lifts and makes the Conveyor Belt cell deviate from the abyssal region, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimate of the timescale of the circulation reveals a sluggish abyssal circulation during the LGM, and a Conveyor Belt circulation that is more vigorous due to the combination of a stronger wind stress and a shortened circulation route.

scholarly journals On the glacial and inter-glacial thermohaline circulation and the associated transports of heat and freshwater

2014 ◽  
Vol 11 (2) ◽  
pp. 979-1022
Author(s):  
M. Ballarotta ◽  
S. Falahat ◽  
L. Brodeau ◽  
K. Döös
Keyword(s):  
Wind Stress ◽  
North Atlantic ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Conveyor Belt ◽  
Interglacial Period ◽  
The North ◽  
The Pacific ◽  
Stream Functions ◽  
The North Atlantic

Abstract. The change of the thermohaline circulation (THC) between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present day climate are explored using an Ocean General Circulation Model and stream functions projected in various coordinates. Compared to the present day period, the LGM circulation is reorganised in the Atlantic Ocean, in the Southern Ocean and particularly in the abyssal ocean, mainly due to the different haline stratification. Due to stronger wind stress, the LGM tropical circulation is more vigorous than under modern conditions. Consequently, the maximum tropical transport of heat is slightly larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes and reorganising the freshwater transport. The LGM circulation is represented as a large intrusion of saline Antarctic Bottom Water into the Northern Hemisphere basins. As a result, the North Atlantic Deep Water is shallower in the LGM simulation. The stream functions in latitude-salinity coordinates and thermohaline coordinates point out the different haline regimes between the glacial and interglacial period, as well as a LGM Conveyor Belt circulation largely driven by enhanced salinity contrast between the Atlantic and the Pacific basin. The thermohaline structure in the LGM simulation is the result of an abyssal circulation that lifts and deviates the Conveyor Belt cell from the area of maximum volumetric distribution, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimation of the turnover times reveal a deep circulation almost sluggish during the LGM, and a Conveyor Belt cell more vigorous due to the combination of stronger wind stress and shortened circulation route.

A Wind-Induced Thermohaline Circulation Hysteresis and Millennial Variability Regimes

2007 ◽  
Vol 37 (10) ◽  
pp. 2446-2457 ◽  
Author(s):  
Yosef Ashkenazy ◽  
Eli Tziperman
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Stress Amplitude ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Box Model ◽  
Freshwater Flux ◽  
Relaxation Oscillations

Abstract The multiple equilibria of the thermohaline circulation (THC: used here in the sense of the meridional overturning circulation) as function of the surface freshwater flux has been studied intensively following a Stommel paper from 1961. It is shown here that multistability and hysteresis of the THC also exist when the wind stress amplitude is varied as a control parameter. Both the Massachusetts Institute of Technology ocean general circulation model (MITgcm) and a simple three-box model are used to study and explain different dynamical regimes of the THC and THC variability as a function of the wind stress amplitude. Starting with active winds and a thermally dominant thermohaline circulation state, the wind stress amplitude is slowly reduced to zero over a time period of ∼40 000 yr (40 kyr) and then increased again to its initial value over another ∼40 kyr. It is found that during the decreasing wind stress phase, the THC remains thermally dominant until very low wind stress amplitude at which pronounced Dansgaard–Oeschger-like THC relaxation oscillations are initiated. However, while the wind stress amplitude is increased, these relaxation oscillations are present up to significantly larger wind stress amplitude. The results of this study thus suggest that under the same wind stress amplitude, the THC can be either in a stable thermally dominant state or in a pronounced relaxation oscillations state. The simple box model analysis suggests that the observed hysteresis is due to the combination of the Stommel hysteresis and the Winton and Sarachik “deep decoupling” oscillations.

scholarly journals Meridional Overturning Circulation in a multi-basin model. Part II: Sensitivity to diffusivity and wind in warm and cool climates

2021 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Primary Determinant ◽  
Meridional Overturning

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.

scholarly journals The Energetics of Global Thermohaline Circulation and Its Wind Enhancement

2009 ◽  
Vol 39 (7) ◽  
pp. 1715-1728 ◽  
Author(s):  
L. Shogo Urakawa ◽  
Hiroyasu Hasumi
Keyword(s):  
Southern Ocean ◽  
Energy Budget ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Dominant Role ◽  
Mechanical Energy ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Energy Budgets ◽  
Gravitational Potential Energy

Abstract The energy budget of global thermohaline circulation (THC) is numerically investigated using an ocean general circulation model (OGCM) under a realistic configuration. Earlier studies just discuss a globally integrated energy budget. This study intends to draw a comprehensive picture of the global THC by separately calculating the energy budgets for three basins (the Atlantic, Indo-Pacific, and Southern Ocean). The largest mechanical energy source is a kinetic energy (KE) input to the general circulation by wind. Of that, 0.3 TW is converted to gravitational potential energy (GPE), and 80% of the energy conversion occurs in the Southern Ocean. Almost the same quantity of GPE is supplied by vertical mixing. Injected GPE is almost equally dissipated by convective adjustment and the effect of cabbeling, and a large part of that is consumed in the Southern Ocean. A dominant role of the Southern Ocean in the energy balance of THC and importance of the interbasin transport of GPE are found. Then, the enhancement of the meridional overturning circulation in the Atlantic induced by wind in the Southern Ocean is examined. Calculating the energy budget anomaly enables the authors to identify its mechanism as a component of THC.

The role of the Maritime Continent SST anomalies in maintaining the Pacific–Japan pattern on decadal time scales

2021 ◽  
pp. 1-54
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Wave Pattern ◽  
Meridional Overturning Circulation ◽  
Maritime Continent ◽  
Wave Source ◽  
Rossby Wave Source ◽  
The Pacific ◽  
Decadal Climate ◽  
Vorticity Anomaly

Abstract The decadal Pacific–Japan (PJ) pattern, the dominant decadal mode of summer vorticity anomaly over East Asia, is characterized as a meridionally arranged wave pattern with one anomalous cyclone located over Taiwan, and two anomalous anticyclones around the South China Sea (SCS) and the Bohai Sea. This pattern can cause wetter and colder conditions in Southeast China and dryer and warmer conditions in North China. Local SST–rainfall relationship reveals that the Maritime Continent (MC) SST can act as an engine to regulate and maintain the decadal PJ pattern. Driven by enhanced convection over the MC, anomalous divergent flows in the upper troposphere move northward, cross the equator and then converge and subside over the SCS. The SCS low-level divergence, maintained by this meridional overturning circulation under the Sverdrup vorticity balance, further works as a Rossby wave source and excites the decadal PJ pattern pointing straight northward. The transhemispheric impacts of the MC SST are well reproduced by both the atmospheric general circulation model and the dry linear baroclinic model, with the former emphasizing the MC’s original forcing role and the latter highlighting the SCS anticyclone’s role in relaying and amplifying those climatic impacts. Thus, our results indicate that SST variations over the MC region can be viewed as a potential source of East Asian decadal climate predictability.

scholarly journals Anisotropic Gent–McWilliams Parameterization for Ocean Models

2004 ◽  
Vol 34 (11) ◽  
pp. 2541-2564 ◽  
Author(s):  
Richard D. Smith ◽  
Peter R. Gent
Keyword(s):  
High Resolution ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Tracer Transport ◽  
Meridional Heat Transport ◽  
Functional Formalism ◽  
Ocean Models ◽  
Anisotropic Viscosity

Abstract An anisotropic generalization of the Gent–McWilliams (GM) parameterization is presented for eddy-induced tracer transport and diffusion in ocean models, and it is implemented in an ocean general circulation model using a functional formalism to derive the spatial discretization. This complements the anisotropic viscosity parameterization recently developed by Smith and McWilliams. The anisotropic GM operator is potentially useful in both coarse- and high-resolution ocean models, and in this study the focus is on its application in high-resolution eddying solutions, for which it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operators. It is shown that realistically high levels of eddy energy can be simulated using harmonic anisotropic diffusion and friction operators. Isotropic forms can also be used, but these tend either to overly damp the solution when a large diffusion coefficient is used or to introduce unacceptable levels of numerical noise when a small coefficient is used. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2° resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms and to isolate changes in the solutions specifically associated with anisotropic GM. A high-resolution 0.1° simulation is then conducted using both anisotropic forms, and the results are compared with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization, which eliminates the spurious diapycnal diffusion inherent in horizontal tracer diffusion. This leads to significant improvements in the model thermohaline circulation, including the meridional heat transport, meridional overturning circulation, and deep-water formation and convection in the Labrador Sea.

scholarly journals Eddy Saturation of Equilibrated Circumpolar Currents

2013 ◽  
Vol 43 (3) ◽  
pp. 507-532 ◽  
Author(s):  
David R. Munday ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Diapycnal Diffusivity ◽  
Ocean Wind

Abstract This study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing. At eddy-permitting, and finer, resolution, the sensitivity of circumpolar transport to forcing magnitude is drastically reduced. At sufficiently high resolution, there is little or no sensitivity of circumpolar transport to wind stress, even in the limit of no wind. In contrast, the meridional overturning circulation continues to vary with Southern Ocean wind stress, but with reduced sensitivity in the limit of high wind stress. Both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions. The circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution, although sensitivity always remains. In contrast, the overturning circulation is more sensitive to change in diapycnal diffusivity when the resolution is high enough to permit mesoscale eddies.

scholarly journals Wind-Driven Oscillations in Meridional Overturning Circulations near the Equator. Part II: Idealized Simulations

2021 ◽  
Vol 51 (3) ◽  
pp. 663-683
Author(s):  
Michael J. Bell ◽  
Adam T. Blaker ◽  
Joël J.-M. Hirschi
Keyword(s):  
Time Series ◽  
General Circulation ◽  
Normal Modes ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
High Frequency Oscillations ◽  
The Pacific ◽  
Meridional Overturning ◽  
State Of Rest

AbstractLarge-amplitude [±100 Sv (1 Sv ≡ 106 m3 s−1)], high-frequency oscillations in the Pacific Ocean’s meridional overturning circulation within 10° of the equator have been found in integrations of the NEMO ocean general circulation model. Part I of this paper showed that these oscillations are dominated by two bands of frequencies with periods close to 4 and 10 days and that they are driven by the winds within about 10° of the equator. This part shows that the oscillations can be well simulated by small-amplitude, wind-driven motions on a horizontally uniform, stably stratified state of rest. Its main novelty is that, by focusing on the zonally integrated linearized equations, it presents solutions for the motions in a basin with sloping side boundaries. The solutions are found using vertical normal modes and equatorial meridional modes representing Yanai and inertia–gravity waves. Simulations of 16-day-long segments of the time series for the Pacific of each of the first three meridional and vertical modes (nine modes in all) capture between 85% and 95% of the variance of matching time series segments diagnosed from the NEMO integrations. The best agreement is obtained by driving the solutions with the full wind forcing and the full pressure forces on the bathymetry. Similar results are obtained for the corresponding modes in the Atlantic and Indian Oceans. Slower variations in the same meridional and vertical modes of the MOC are also shown to be well simulated by a quasi-stationary solution driven by zonal wind and pressure forces.

scholarly journals Meridional Overturning Circulation in a Multibasin Model. Part I: Dependence on Southern Ocean Buoyancy Forcing

2020 ◽  
Vol 50 (5) ◽  
pp. 1159-1178 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Pacific Basin ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Meridional Overturning

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 possible future warmer state is analyzed in an idealized two-basin general circulation model connected by a southern circumpolar channel. A connection between the North Atlantic Deep Water (NADW) cell in the Atlantic basin and the Pacific Deep Water (PDW) cell in the Pacific basin occurs with a direct flow of NADW into the channel’s lower cell, while PDW upwelled in the Pacific basin can flow directly into the upper wind-driven cell in the channel. The intersection of these cells along with direct zonal flows between the basins completes the interbasin circulation. The present-day Atlantic meridional overturning circulation (AMOC) in the model is upwelled both by wind-driven upwelling in the Southern Ocean and by diffusion in the Pacific and Atlantic. In a cooler climate with enhanced sea ice, the NADW cell shoals, which can then no longer flow directly into the channel’s lower cell, reducing the Pacific pathway of NADW. This leads to a substantial weakening of the AMOC, suggesting buoyancy forcing changes can play a substantial role in the transition of the AMOC to a glacial state. In contrast, in a warmer equilibrium climate state with reduced AABW formation, the NADW cell strengthens and deepens. NADW is increasingly directed along the Pacific pathway, while the direct upwelling in the channel’s wind-driven upper cell plays a smaller role.

scholarly journals A Decomposition of the Atlantic Meridional Overturning

2006 ◽  
Vol 36 (12) ◽  
pp. 2253-2270 ◽  
Author(s):  
Louise C. Sime ◽  
David P. Stevens ◽  
Karen J. Heywood ◽  
Kevin I. C. Oliver
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Boundary ◽  
Bottom Pressure ◽  
External Mode ◽  
Sverdrup Balance ◽  
Meridional Overturning

Abstract A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure–dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8° and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

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