scholarly journals Sensitivity of Atlantic Meridional Overturning Circulation Variability to Parameterized Nordic Sea Overflows in CCSM4

2012 ◽  
Vol 25 (6) ◽  
pp. 2077-2103 ◽  
Author(s):  
Stephen Yeager ◽  
Gokhan Danabasoglu
Keyword(s):  
Deep Ocean ◽  
Atlantic Meridional Overturning Circulation ◽  
Order Effect ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Frequency Space ◽  
Meridional Overturning

Abstract The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in a much improved representation of the North Atlantic tracer and velocity distributions compared to a control CCSM4 simulation without this parameterization. As a consequence, the variability of the Atlantic meridional overturning circulation (AMOC) on decadal and longer time scales is generally lower, but the reduction is not uniform in latitude, depth, or frequency–space. While there is dramatically less variance in the overall AMOC maximum (at about 35°N), the reduction in AMOC variance at higher latitudes is more modest. Also, it is somewhat enhanced in the deep ocean and at low latitudes (south of about 30°N). The complexity of overturning response to overflows is related to the fact that, in both simulations, the AMOC spectrum varies substantially with latitude and depth, reflecting a variety of driving mechanisms that are impacted in different ways by the overflows. The usefulness of reducing AMOC to a single index is thus called into question. This study identifies two main improvements in the ocean mean state associated with the overflow parameterization that tend to damp AMOC variability: enhanced stratification in the Labrador Sea due to the injection of dense overflow waters and a deepening of the deep western boundary current. Direct driving of deep AMOC variance by overflow transport variations is found to be a second-order effect.

scholarly journals Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

2013 ◽  
Vol 26 (18) ◽  
pp. 7167-7186 ◽  
Author(s):  
Carl Wunsch ◽  
Patrick Heimbach
Keyword(s):  
Ocean Circulation ◽  
Extreme Values ◽  
Statistical Significance ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Gaussian Stochastic Process ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

scholarly journals Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 265-284
Author(s):  
Josefine Herrford ◽  
Peter Brandt ◽  
Torsten Kanzow ◽  
Rebecca Hummels ◽  
Moacyr Araujo ◽  
...  
Keyword(s):  
Seasonal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Bottom Pressure ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Western Basin ◽  
Meridional Overturning

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11∘ S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The characteristics of the observed seasonal cycles of the AMOC and its components are compared to results from an ocean general circulation model, which is known to reproduce the variability of the Western Boundary Current on longer timescales. The observed seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, especially at the western boundary, which translates into the estimate of the upper-ocean geostrophic transport. In the model, seasonal AMOC variability at 11∘ S is governed, besides the Ekman transport, by the geostrophic transport variability in the eastern basin. The geostrophic contribution of the western basin to the seasonal cycle of the AMOC is instead comparably weak, as transport variability in the western basin interior related to local wind curl forcing is mainly compensated by the Western Boundary Current. Our analyses indicate that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes not being properly represented in the model, uncertainties in the wind forcing are particularly relevant for the resulting uncertainties of AMOC estimates at 11∘ S.

scholarly journals Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

2020 ◽  
Author(s):  
Josefine Herrford ◽  
Peter Brandt ◽  
Torsten Kanzow ◽  
Rebecca Hummels ◽  
Moacyr Araujo ◽  
...  
Keyword(s):  
Seasonal Variability ◽  
Rossby Waves ◽  
Atlantic Meridional Overturning Circulation ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Bottom Pressure ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11° S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The observed seasonal cycles of the AMOC, its components as well as the Western Boundary Current as observed with current meter moorings are in general good agreement with results of an ocean general circulation model. The seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, slightly stronger at the western boundary. In the model, seasonal AMOC variability at 11° S is governed by the variability in the eastern basin. Here, long Rossby waves originating from equatorial forcing are known to be radiated from the Angolan continental slope and propagate westward into the basin interior. The contribution of the western basin to AMOC seasonal variability is instead comparably weak as transport variability due to locally forced Rossby waves is mainly compensated by the Western Boundary Current. Our analyses indicate, that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes being not properly represented in the model, uncertainties in the wind forcing are particularly relevant for AMOC estimates at 11° S.

scholarly journals Variability of Shallow and Deep Western Boundary Currents off the Bahamas during 2004–05: Results from the 26°N RAPID–MOC Array

2008 ◽  
Vol 38 (3) ◽  
pp. 605-623 ◽  
Author(s):  
W. E. Johns ◽  
L. M. Beal ◽  
M. O. Baringer ◽  
J. R. Molina ◽  
S. A. Cunningham ◽  
...  
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
The Bahamas ◽  
Meridional Overturning ◽  
Baroclinic Transport

Abstract Data from an array of six moorings deployed east of Abaco, Bahamas, along 26.5°N during March 2004–May 2005 are analyzed. These moorings formed the western boundary array of a transbasin observing system designed to continuously monitor the meridional overturning circulation and meridional heat flux in the subtropical North Atlantic, under the framework of the joint U.K.–U.S. Rapid Climate Change (RAPID)–Meridional Overturning Circulation (MOC) Program. Important features of the western boundary circulation include the southward-flowing deep western boundary current (DWBC) below 1000 m and the northward-flowing “Antilles” Current in the upper 1000 m. Transports in the western boundary layer are estimated from direct current meter observations and from dynamic height moorings that measure the spatially integrated geostrophic flow between moorings. The results of these methods are combined to estimate the time-varying transports in the upper and deep ocean over the width of the western boundary layer to a distance of 500 km offshore of the Bahamas escarpment. The net southward transport of the DWBC across this region, inclusive of northward deep recirculation, is −26.5 Sv (Sv ≡ 106 m3 s−1), which is divided nearly equally between upper (−13.9 Sv) and lower (−12.6 Sv) North Atlantic Deep Water (NADW). In the top 1000 m, 6.0 Sv flows northward in a thermocline-intensified jet near the western boundary. These transports are found to agree well with historical current meter data in the region collected between 1986 and 1997. Variability in both shallow and deep components of the circulation is large, with transports above 1000 m varying between −15 and +25 Sv and deep transports varying between −60 and +3 Sv. Much of this transport variability, associated with barotropic fluctuations, occurs on relatively short time scales of several days to a few weeks. Upon removal of the barotropic fluctuations, slower baroclinic transport variations are revealed, including a temporary stoppage of the lower NADW transport in the DWBC during November 2004.

scholarly journals Regional imprints of changes in the Atlantic Meridional Overturning Circulation in the eddy-rich ocean model VIKING20X

Ocean Science ◽  
2021 ◽  
Vol 17 (5) ◽  
pp. 1177-1211
Author(s):  
Arne Biastoch ◽  
Franziska U. Schwarzkopf ◽  
Klaus Getzlaff ◽  
Siren Rühs ◽  
Torge Martin ◽  
...  
Keyword(s):  
Large Scale ◽  
Temporal Evolution ◽  
Dominant Role ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Basin Scale ◽  
Meridional Overturning

Abstract. A hierarchy of global 1/4∘ (ORCA025) and Atlantic Ocean 1/20∘ nested (VIKING20X) ocean–sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5∘ N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53∘ N, 26.5∘ N and 11∘ S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5∘ N, it is shown that in particular the outflow of North Atlantic Deepwater at 53∘ N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.

scholarly journals Variability of the Atlantic Meridional Overturning Circulation in CCSM4

2012 ◽  
Vol 25 (15) ◽  
pp. 5153-5172 ◽  
Author(s):  
Gokhan Danabasoglu ◽  
Steve G. Yeager ◽  
Young-Oh Kwon ◽  
Joseph J. Tribbia ◽  
Adam S. Phillips ◽  
...  
Keyword(s):  
Low Frequency ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Positive Density ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Control Simulation ◽  
Meridional Overturning

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

scholarly journals Inferring the zonal distribution of measured changes in the meridional overturning circulation

Ocean Science ◽  
2007 ◽  
Vol 3 (1) ◽  
pp. 55-57 ◽  
Author(s):  
A. M. de Boer ◽  
H. L. Johnson
Keyword(s):  
Dynamic Method ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Zonal Distribution ◽  
Deep Western Boundary Current ◽  
Meridional Overturning ◽  
Florida Straits ◽  
Southward Flow

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics) is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region immediately outside of the Florida Straits.

scholarly journals The Observed North Atlantic Meridional Overturning Circulation: Its Meridional Coherence and Ocean Bottom Pressure

2014 ◽  
Vol 44 (2) ◽  
pp. 517-537 ◽  
Author(s):  
Shane Elipot ◽  
Eleanor Frajka-Williams ◽  
Chris W. Hughes ◽  
Josh K. Willis
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Overturning Circulation ◽  
Ocean Bottom Pressure ◽  
Meridional Overturning

Abstract Analyses of meridional transport time series from the Rapid Climate Change–Meridional Overturning Circulation (RAPID MOC) array at 26°N and from Argo float and altimetry data at 41°N reveal that, at semiannual and longer time scales, the contribution from the western boundary dominates the variability of the North Atlantic meridional overturning circulation (MOC), defined as the transport in the upper 1000 m of the ocean. Because the variability of the western boundary contribution is associated with a geostrophic overturning, it is reflected in independent estimates of transports from gradient of ocean bottom pressure (OBP) relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes (26°, 39°, and 42.5°N). Time series of western meridional transports relative to and below 1000 m derived from the OBP gradient, or equivalently derived from the transport shear profile, exhibit approximately the same phase relationship between 26° and 39°–42.5°N as the western contribution to the geostrophic MOC time series do: the western geostrophic MOC at 41°N precedes the MOC at 26°N by approximately a quarter of an annual cycle, resulting in a zero correlation at this time scale. This study therefore demonstrates how OBP gradients on basin boundaries can be used to monitor the MOC and its meridional coherence.

A Diagnostic Indicator of the Stability of the Atlantic Meridional Overturning Circulation in CCSM3

2013 ◽  
Vol 26 (6) ◽  
pp. 1926-1938 ◽  
Author(s):  
Wei Liu ◽  
Zhengyu Liu
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Climate System Model ◽  
Freshwater Transport ◽  
Meridional Overturning ◽  
The Stability ◽  
Diagnostic Indicator

Abstract A diagnostic indicator ΔMov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The ΔMov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive ΔMov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative ΔMov (freshwater divergence) indicates a bistable AMOC. Based on ΔMov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative ΔMov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.

A New Pacific Influence on the Atlantic Meridional Overturning Circulation

2021 ◽  
Author(s):  
Leon Hermanson ◽  
Doug Smith ◽  
Nick Dunstone ◽  
Rosie Eade
Keyword(s):  
Decadal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Tropical Pacific ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Strong Impact ◽  
Overturning Circulation ◽  
Southern Us ◽  
Volcanic Forcing ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulation (AMOC) at 26N has been measured since 2004 by the RAPID-MOCHA array. On a multi-year timescale it shows a decline with signs of a recovery since around 2012. This variability is likely to be part of longer decadal variability. We examine here the decadal variability of the AMOC and its drivers in a coupled model run nudged to observations from 1960-2017. Temperature and winds are nudged throughout the atmosphere and potential temperature and salinity are nudged in the ocean, but the ocean velocities are allowed to vary freely. We nudge an ensemble of 10 ocean analyses into the ocean model to get an ensemble of responses, the mean of which reproduces the observed AMOC. We use these ocean-atmosphere re-analyses to study the drivers of the AMOC. The North Atlantic Oscillation (NAO) is well known to have an impact on the AMOC and is an important driver here. We find that the tropical Pacific also has a strong impact on the subtropical AMOC on multi-annual to decadal timescales. Together these two factors can explain more than half of all variability of the AMOC at 26N through wind forcing associated with Rossby waves and western boundary waves. This Pacific impact, not reported on before, is from windstress curl anomalies close to the East Coast of the southern US due to changes in the Pacific storm track and the Walker Circulation. As both the NAO and tropical Pacific variability is associated with solar and volcanic forcing, it is possible that solar and volcanic forcing are important for multi-annual to multi-decadal AMOC variability. We use observations of the NAO and tropical Pacific to reconstruct the AMOC from 1870 to present day and predict a continued recovery in the future.</p>

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