scholarly journals Ocean circulation - Does large-scale ocean overturning circulation vary with climate change? [Past]

PAGES news ◽  
2012 ◽  
Vol 20 (1) ◽  
pp. 15-15
Author(s):  
Luke Skinner
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Large Scale ◽  
Overturning Circulation

scholarly journals Large-scale ocean circulation-cloud interactions reduce the pace of transient climate change

2016 ◽  
Vol 43 (8) ◽  
pp. 3935-3943 ◽  
Author(s):  
D. S. Trossman ◽  
J. B. Palter ◽  
T. M. Merlis ◽  
Y. Huang ◽  
Y. Xia
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Large Scale ◽  
Transient Climate Change

Low-Frequency Climate Response of Quasigeostrophic Wind-Driven Ocean Circulation

2012 ◽  
Vol 42 (2) ◽  
pp. 243-260 ◽  
Author(s):  
Rafail V. Abramov ◽  
Andrew J. Majda
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Linear Response ◽  
Large Scale ◽  
Response Times ◽  
Low Frequency ◽  
External Perturbation ◽  
Climate Dynamics ◽  
The Mean ◽  
General Response

Abstract Linear response to external perturbation through the fluctuation–dissipation theorem has recently become a popular topic in the climate research community. It relates an external perturbation of climate dynamics to climate change in a simple linear fashion, which provides key insight into physics of the climate change phenomenon. Recently, the authors developed a suite of linear response algorithms for low-frequency response of large-scale climate dynamics to external perturbation, including the novel blended response algorithm, which combines the geometrically exact general response formula using integration of a linear tangent model at short response times and the classical quasi-Gaussian response algorithm at longer response times, overcoming numerical instability of the tangent linear model for longer times due to positive Lyapunov exponents. Here, the authors apply the linear response framework to several leading empirical orthogonal functions (EOFs) of a quasigeostrophic model of wind-driven ocean circulation. It is demonstrated that the actual nonlinear response of this system under external perturbation at leading EOFs can be predicted by the linear response algorithms with adequate skill with moderate errors; in particular, the blended response algorithm has a pattern correlation with the ideal response operator on the four leading EOFs of the mean state response of 94% after 5 yr. In addition, interesting properties of the mean flow response to large-scale changes in wind stress at the leading EOFs are observed.

scholarly journals Momentum Balance of the Wind-Driven and Meridional Overturning Circulation

2011 ◽  
Vol 41 (5) ◽  
pp. 960-978 ◽  
Author(s):  
David P. Marshall ◽  
Helen R. Pillar
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Boundary Layer Separation ◽  
Momentum Equation ◽  
Meridional Overturning Circulation ◽  
Momentum Balance ◽  
Overturning Circulation ◽  
Potential Applications ◽  
Rotational Momentum ◽  
Meridional Overturning

Abstract When a force is applied to the ocean, fluid parcels are accelerated both locally, by the applied force, and nonlocally, by the pressure gradient forces established to maintain continuity and satisfy the kinematic boundary condition. The net acceleration can be represented through a “rotational force” in the rotational component of the momentum equation. This approach elucidates the correspondence between momentum and vorticity descriptions of the large-scale ocean circulation: if two terms balance pointwise in the rotational momentum equation, then the equivalent two terms balance pointwise in the vorticity equation. The utility of the approach is illustrated for three classical problems: barotropic Rossby waves, wind-driven circulation in a homogeneous basin, and the meridional overturning circulation in an interhemispheric basin. In the hydrostatic limit, it is shown that the rotational forces further decompose into depth-integrated forces that drive the wind-driven gyres and overturning forces that are confined to the basin boundaries and drive the overturning circulation. Potential applications of the approach to diagnosing the output of ocean circulation models, alternative and more accurate formulations of numerical ocean models, the dynamics of boundary layer separation, and eddy forcing of the large-scale ocean circulation are discussed.

Estimating the potential for twenty-first century sudden climate change

2007 ◽  
Vol 365 (1860) ◽  
pp. 2675-2694 ◽  
Author(s):  
Drew Shindell
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Large Scale ◽  
Climate Models ◽  
Sudden Change ◽  
Climate Forcing ◽  
Climate Feedbacks ◽  
Circulation Patterns ◽  
Climate Shifts ◽  
Current Century

I investigate the potential for sudden climate change during the current century. This investigation takes into account evidence from the Earth's history, from climate models and our understanding of the physical processes governing climate shifts. Sudden alterations to climate forcing seem to be improbable, with sudden changes instead most likely to arise from climate feedbacks. Based on projections from models validated against historical events, dramatic changes in ocean circulation appear unlikely. Ecosystem–climate feedbacks clearly have the potential to induce sudden change, but are relatively poorly understood at present. More probable sudden changes are large increases in the frequency of summer heatwaves and changes resulting from feedbacks involving hydrology. These include ice sheet decay, which may be set in motion this century. The most devastating consequences are likely to occur further in the future, however. Reductions in subtropical precipitation are likely to be the most severe hydrologic effects this century, with rapid changes due to the feedbacks of relatively well-understood large-scale circulation patterns. Water stress may become particularly acute in the Southwest US and Mexico, and in the Mediterranean and Middle East, where rainfall decreases of 10–25% (regionally) and up to 40% (locally) are projected.

scholarly journals Pending recovery in the strength of the meridional overturning circulation at 26° N

2020 ◽  
Author(s):  
Ben I. Moat ◽  
David A. Smeed ◽  
Eleanor Frajka-Williams ◽  
Damien G. Desbruyères ◽  
Claudie Beaulieu ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Ocean Reanalysis ◽  
Buoyancy Forcing ◽  
Modelling Studies ◽  
Meridional Overturning ◽  
Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26° N has now been continuously measured by the RAPID array over the period Apr 2004–Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy-forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45° N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26° N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26° N is higher than its previous low. Extending the record at 26° N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45° N by 0–2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

A dynamically based method for estimating the Atlantic overturning circulation at 26°N from satellite altimetry

2021 ◽  
Author(s):  
Alejandra Sanchez-Franks ◽  
Eleanor Frajka-Williams ◽  
Ben Moat ◽  
David Smeed
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
Satellite Altimetry ◽  
Large Scale ◽  
Vertical Structure ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Large Scale Circulation ◽  
Surface Ocean ◽  
Starting Point

<p>The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude.</p><p>High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26°N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83% of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26°N lays down the starting point for monitoring large-scale circulation at all latitudes.</p>

scholarly journals Pending recovery in the strength of the meridional overturning circulation at 26° N

Ocean Science ◽  
2020 ◽  
Vol 16 (4) ◽  
pp. 863-874 ◽  
Author(s):  
Ben I. Moat ◽  
David A. Smeed ◽  
Eleanor Frajka-Williams ◽  
Damien G. Desbruyères ◽  
Claudie Beaulieu ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Ocean Reanalysis ◽  
Buoyancy Forcing ◽  
Modelling Studies ◽  
Meridional Overturning ◽  
Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26∘ N has now been continuously measured by the RAPID array over the period April 2004–September 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic multidecadal variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy forcing by the atmosphere drove intense water-mass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45∘ N, observations suggest that the AMOC may already be increasing. Examining 26∘ N, we find that the AMOC is no longer weakening, though the recent transport is not above the long-term mean. Extending the record backwards in time at 26∘ N with ocean reanalysis from GloSea5, the transport fluctuations at 26∘ N are consistent with a 0- to 2-year lag from those at 45∘ N, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering nor how the AMOC at 26∘ N responds to intense buoyancy forcing.

scholarly journals Thermodynamic Analysis of Ocean Circulation

2007 ◽  
Vol 37 (8) ◽  
pp. 2038-2052 ◽  
Author(s):  
J. Nycander ◽  
J. Nilsson ◽  
K. Döös ◽  
G. Broström
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Three Dimensional ◽  
Boussinesq Equations ◽  
Ocean Model ◽  
Global Ocean ◽  
Small Scale ◽  
Ekman Pumping ◽  
Overturning Circulation ◽  
Thermodynamic Work

Abstract Calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this streamfunction directly shows whether it is driven mechanically by large-scale wind stress or thermally by heat conduction and small-scale mixing. It is also shown that the integral of this streamfunction gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution Ocean Circulation and Climate Advanced Model (OCCAM). It is shown that the overturning circulation in OCCAM between the 200- and 1000-m depth is dominated by a thermally indirect cell of 24 Sverdrups (1 Sv ≡ 106 m3 s−1), forced by Ekman pumping. In the densest and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.

scholarly journals Influences of Local and Remote Conditions on Tropical Precipitation and Its Response to Climate Change

2020 ◽  
Vol 33 (10) ◽  
pp. 4045-4063
Author(s):  
Marion Saint-Lu ◽  
Robin Chadwick ◽  
F. Hugo Lambert ◽  
Matthew Collins ◽  
Ian Boutle ◽  
...  
Keyword(s):  
Climate Change ◽  
Surface Temperature ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Pattern ◽  
General Circulation Models ◽  
Overturning Circulation ◽  
Surface Warming ◽  
Single Column ◽  
Precipitation Changes

AbstractBy comparing a single-column model (SCM) with closely related general circulation models (GCMs), precipitation changes that can be diagnosed from local changes in surface temperature (TS) and relative humidity (RHS) are separated from more complex responses. In the SCM setup, the large-scale tropical circulation is parameterized to respond to the surface temperature departure from a prescribed environment, following the weak temperature gradient (WTG) approximation and using the damped gravity wave (DGW) parameterization. The SCM is also forced with moisture variations. First, it is found that most of the present-day mean tropical rainfall and circulation pattern is associated with TS and RHS patterns. Climate change experiments with the SCM are performed, imposing separately surface warming and CO2 increase. The rainfall responses to future changes in sea surface temperature patterns and plant physiology are successfully reproduced, suggesting that these are direct responses to local changes in convective instability. However, the SCM increases oceanic rainfall too much, and fails to reproduce the land rainfall decrease, both of which are associated with uniform ocean warming. It is argued that remote atmospheric teleconnections play a crucial role in both weakening the atmospheric overturning circulation and constraining precipitation changes. Results suggest that the overturning circulation weakens, both as a direct local response to increased CO2 and in response to energy-imbalance driven exchanges between ascent and descent regions.

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