scholarly journals Interannual Variability of the Global Meridional Overturning Circulation Dominated by Pacific Variability

2020 ◽  
Vol 50 (3) ◽  
pp. 559-574
Author(s):  
Neil F. Tandon ◽  
Oleg A. Saenko ◽  
Mark A. Cane ◽  
Paul J. Kushner
Keyword(s):  
Interannual Variability ◽  
Large Scale ◽  
Southern Oscillation ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Prominent Feature ◽  
Overturning Circulation ◽  
Near Surface ◽  
The Pacific ◽  
Meridional Overturning

AbstractThe most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.

scholarly journals The Interannual Variability of Shallow Meridional Overturning Circulation and its association with the South-West Indian Ocean Heat Content variability

2021 ◽  
Author(s):  
Rahul U Pai ◽  
Anant Parekh ◽  
Jasti S. Chowdary ◽  
C. Gnanaseelan
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Sea Level ◽  
Southern Oscillation ◽  
Heat Content ◽  
Meridional Overturning Circulation ◽  
Ocean Heat Content ◽  
The South ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The present study examines interannual variability of Shallow Meridional Overturning Circulation (SMOC) using century long reanalysis data. The strength of the transport associated with SMOC is calculated by meridional overturning streamfunction. The interannual variability in SMOC is found maximum between the 5oS and 15oS and displaying strong signals after 1940s. A year for which the meridional overturning streamfunction detrended anomaly is greater (lesser) its standard deviation is identified as strong (weak) SMOC year. For strong (weak) SMOC year composite displayed more (less) southward transport (~2.5 Sv) and shown excess (less) subduction over the South Indian Ocean. During strong (weak) years, the excess (less) southward heat transport (~0.25PW) leads to reduction (increase) in the upper 200m Ocean Heat Content (OHC) and sea level over the Southwest Indian Ocean (SWIO). The results obtained are well supported by tide gauge and satellite measured sea level data for the available period. Further analysis reveals that the SMOC variability is primarily driven by change in zonal wind stress south of the equator and displayed association with the Southern Oscillation Index. The Ocean model-based sensitivity experiments confirms that the OHC variability over SWIO is closely associated with the SMOC variability and is primarily driven by local wind forcing as a response to El Niño Southern Oscillation. However, the role of remote forcing from Pacific through Oceanic pathway over SWIO is absent. Study attempts to provide a comprehensive view on the interannual variability of SMOC and its linkage to OHC variability over SWIO during last century.

scholarly journals Difference between the North Atlantic and Pacific meridional overturning circulation in response to the uplift of the Tibetan Plateau

2018 ◽  
Vol 14 (6) ◽  
pp. 751-762 ◽  
Author(s):  
Baohuang Su ◽  
Dabang Jiang ◽  
Ran Zhang ◽  
Pierre Sepulchre ◽  
Gilles Ramstein
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
The Tibetan Plateau ◽  
Overturning Circulation ◽  
The North ◽  
The Pacific ◽  
Water Formation ◽  
Meridional Overturning

Abstract. The role of the Tibetan Plateau (TP) in maintaining the large-scale overturning circulation in the Atlantic and Pacific is investigated using a coupled atmosphere–ocean model. For the present day with a realistic topography, model simulation shows a strong Atlantic meridional overturning circulation (AMOC) but a near absence of the Pacific meridional overturning circulation (PMOC), which are in good agreement with the present observations. In contrast, the simulation without the TP depicts a collapsed AMOC and a strong PMOC that dominates deep-water formation. The switch in deep-water formation between the two basins results from changes in the large-scale atmospheric circulation and atmosphere–ocean feedback over the Atlantic and Pacific. The intensified westerly winds and increased freshwater flux over the North Atlantic cause an initial slowdown of the AMOC, while the weakened East Asian monsoon circulation and associated decreased freshwater flux over the North Pacific give rise to the initial intensification of the PMOC. The further decreased heat flux and the associated increase in sea-ice fraction promote the final AMOC collapse over the Atlantic, while the further increased heat flux leads to the final PMOC establishment over the Pacific. Although the simulations were performed in a cold world, it still importantly implicates that the uplift of the TP alone could have been a potential driver for the reorganization of PMOC–AMOC between the late Eocene and early Oligocene.

Response of Global SSTs and ENSO to the Atlantic and Pacific Meridional Overturning Circulations

2021 ◽  
pp. 1-68
Author(s):  
Maria J. Molina ◽  
Aixue Hu ◽  
Gerald A. Meehl
Keyword(s):  
Southern Oscillation ◽  
Global Climate ◽  
Coupled Model ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Enso Teleconnections ◽  
The Pacific ◽  
Meridional Overturning ◽  
Physical Reasons

AbstractConsequences from a slowdown or collapse of the Atlantic Meridional Overturning Circulation (AMOC) could include modulations to El Niño-Southern Oscillation (ENSO) and development of the Pacific Meridional Overturning Circulation (PMOC). Despite potential ramifications to the global climate, our understanding of the influence of various AMOC and PMOC states on ENSO and global sea surface temperatures (SSTs) remains limited. Five multi-centennial, fully-coupled model simulations created with the Community Earth System Model were used to explore the influence of AMOC and PMOC on global SSTs and ENSO. We found that the amplitude of annual cycle SSTs across the tropical Pacific decreases and ENSO amplitude increases as a result of an AMOC shutdown, irrespective of PMOC development. However, active deep overturning circulations in both the Atlantic and Pacific basins reduce ENSO amplitude and variance of monthly SSTs globally. The underlying physical reasons for changes to global SSTs and ENSO are also discussed, with the atmospheric and oceanic mechanisms that drive changes to ENSO amplitude differing based on PMOC state. These results suggest that if climate simulations projecting AMOC weakening are realized, compounding climate impacts could occur given the far-reaching ENSO teleconnections to extreme weather and climate events. More broadly, these results provide us with insight into past geologic era climate states, when PMOC was active.

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 A New Index for the Atlantic Meridional Overturning Circulation at 26°N

2014 ◽  
Vol 27 (17) ◽  
pp. 6439-6455 ◽  
Author(s):  
A. Duchez ◽  
J. J.-M. Hirschi ◽  
S. A. Cunningham ◽  
A. T. Blaker ◽  
H. L. Bryden ◽  
...  
Keyword(s):  
Time Scales ◽  
Decadal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Current Estimate ◽  
Overturning Circulation ◽  
Main Hypothesis ◽  
Meridional Overturning ◽  
Sverdrup Transport

Abstract The Atlantic meridional overturning circulation (AMOC) has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26°N made by the Rapid Climate Change (RAPID) array provide the best current estimate of the state of the AMOC. The period 2004–11 when RAPID AMOC is available is too short to assess decadal variability of the AMOC. This modeling study introduces a new AMOC index (called AMOCSV) at 26°N that combines the Florida Straits transport, the Ekman transport, and the southward geostrophic Sverdrup transport. The main hypothesis in this study is that the upper midocean geostrophic transport calculated using the RAPID array is also wind-driven and can be approximated by the geostrophic Sverdrup transport at interannual and longer time scales. This index is expected to reflect variations in the AMOC at interannual to decadal time scales. This estimate of the surface branch of the AMOC can be constructed as long as reliable measurements are available for the Gulf Stream and for wind stress. To test the reliability of the AMOCSV on interannual and longer time scales, two different numerical simulations are used: a forced and a coupled simulation. Using these simulations the AMOCSV captures a substantial fraction of the AMOC variability and is in good agreement with the AMOC transport at 26°N on both interannual and decadal time scales. These results indicate that it might be possible to extend the observation-based AMOC at 26°N back to the 1980s.

scholarly journals A Decomposition of the Atlantic Meridional Overturning Circulation into Physical Components Using Its Sensitivity to Vertical Diffusivity

2006 ◽  
Vol 36 (4) ◽  
pp. 636-650 ◽  
Author(s):  
Juliette Mignot ◽  
Anders Levermann ◽  
Alexa Griesel
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Scaling Law ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Small Range ◽  
Vertical Diffusion ◽  
Overturning Circulation ◽  
Vertical Diffusivity ◽  
Meridional Overturning

Abstract The sensitivity of the Atlantic Ocean meridional overturning circulation to the vertical diffusion coefficient κ in the global coupled atmosphere–ocean–sea ice model CLIMBER-3α is investigated. An important feature of the three-dimensional ocean model is its low-diffusive tracer advection scheme. The strength Mmax of the Atlantic overturning is decomposed into three components: 1) the flow MS exported southward at 30°S, 2) the large-scale upward flow that balances vertical diffusion in the Atlantic, and 3) a wind-dependent upwelling flux Wbound along the Atlantic boundaries that is not due to vertical diffusion. The export of water at 30°S varies only weakly with κ, but is strongly correlated with the strength of the overflow over the Greenland–Scotland ridge. The location of deep convection is found to be mixing dependent such that a shift from the Nordic seas to the Irminger Sea is detected for high values of κ. The ratio R = MS/Mmax gives a measure of the interhemispheric overturning efficiency and is found to decrease linearly with κ. The diffusion-induced upwelling in the Atlantic is mostly due to the uniform background value of κ while parameterization of enhanced mixing over rough topography and in stratified areas gives only a weak contribution to the overturning strength. It increases linearly with κ. This is consistent with the classic 2/3 scaling law only when taking the linear variation of the density difference to κ into account. The value of Wbound is roughly constant with κ but depends linearly on the wind stress strength in the North Atlantic. The pycnocline depth is not sensitive to changes in κ in the model used herein, and the results suggest that it is primarily set by the forcing of the Southern Ocean winds. The scaling of the total overturning strength with κ depends on the combined sensitivity of each of the terms to κ. In the range of background diffusivity values in which no switch in deep convection sites is detected, Mmax scales linearly with the vertical diffusivity. It is argued that scalings have, in general, to be interpreted with care because of the generally very small range of κ but also because of possible shifts in important physical processes such as deep convection location.

An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

2006 ◽  
Vol 19 (15) ◽  
pp. 3751-3767 ◽  
Author(s):  
Véronique Bugnion ◽  
Chris Hill ◽  
Peter H. Stone
Keyword(s):  
Boundary Conditions ◽  
Wind Stress ◽  
Ocean Circulation ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Model Parameters ◽  
Overturning Circulation ◽  
Near Surface ◽  
Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

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