scholarly journals On the Role of Eddies and Surface Forcing in the Heat Transport and Overturning Circulation in Marginal Seas

2011 ◽  
Vol 24 (18) ◽  
pp. 4844-4858 ◽  
Author(s):  
Michael A. Spall
Keyword(s):  
Conceptual Model ◽  
Heat Transport ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Nordic Seas ◽  
Marginal Seas ◽  
Eddy Heat Flux ◽  
The Mean

Abstract The factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high-resolution ocean circulation model and a simple conceptual model. The study is motivated by the exchange between the subpolar North Atlantic Ocean and the Nordic Seas, a region that is of central importance to the oceanic thermohaline circulation. It is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill. The balance between the oceanic eddy heat flux and atmospheric cooling, as characterized by a nondimensional number, is shown to be the primary factor in determining the properties of the exchange. Results from a series of eddy-resolving primitive equation model calculations for the meridional heat transport, overturning circulation, density of convective waters, and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space. Scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced. These results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the North Atlantic and the Nordic Seas, and thus between the Nordic Seas and the atmosphere, in climate models.

scholarly journals Optimal Surface Salinity Perturbations of the Meridional Overturning and Heat Transport in a Global Ocean General Circulation Model

2008 ◽  
Vol 38 (12) ◽  
pp. 2739-2754 ◽  
Author(s):  
Florian Sévellec ◽  
Thierry Huck ◽  
Mahdi Ben Jelloul ◽  
Nicolas Grima ◽  
Jérôme Vialard ◽  
...  
Keyword(s):  
Heat Transport ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Transient Growth ◽  
Surface Salinity ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Optimal Surface ◽  
Meridional Overturning

Abstract Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Océan Parallélisé (OPA)]; optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization using Lagrangian multipliers leads to explicit solutions (rather than eigenvalue problems), involving the integration of the model adjoint for each value to maximize. The most efficient transient growth for the intensity of the meridional overturning circulation appears for a delay of 10.5 yr after the perturbation by the optimal surface salinity anomaly. This optimal growth is induced by an initial anomaly located north of 50°N. In the same way, the most efficient transient growth for the intensity of the meridional heat transport appears for a shorter delay of 2.2 yr after the perturbation by the optimal surface salinity anomaly. This initial optimal perturbation corresponds to a zonal salinity gradient around 24°N. The optimal surface salinity perturbations studied herein yield upper bounds on the intensity of the response in meridional overturning circulation and meridional heat transport. Using typical amplitudes of the Great Salinity Anomalies, the upper bounds for the associated variability are 0.8 Sv (1 Sv ≡ 106 m3 s−1) (11% of the mean circulation) and 0.03 PW (5% of the mean circulation), respectively.

scholarly journals Predictability of the Atlantic Overturning Circulation and Associated Surface Patterns in Two CCSM3 Climate Change Ensemble Experiments

2011 ◽  
Vol 24 (23) ◽  
pp. 6054-6076 ◽  
Author(s):  
Haiyan Teng ◽  
Grant Branstator ◽  
Gerald A. Meehl
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Overturning Circulation ◽  
The North ◽  
Surface Patterns ◽  
The Mean ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract Predictability of the Atlantic meridional overturning circulation (AMOC) and associated oceanic and atmospheric fields on decadal time scales in the Community Climate System Model, version 3 (CCSM3) at T42 resolution is quantified with a 700-yr control run and two 40-member “perfect model” climate change experiments. After taking into account both the mean and spread about the mean of the forecast distributions and allowing for the possibility of time-evolving modes, the natural variability of the AMOC is found to be predictable for about a decade; beyond that range the forced predictability resulting from greenhouse gas forcing becomes dominant. The upper 500-m temperature in the North Atlantic is even more predictable than the AMOC by several years. This predictability is associated with subsurface and sea surface temperature (SST) anomalies that propagate in an anticlockwise direction along the subpolar gyre and tend to be prominent during the 10 yr following peaks in the amplitude of AMOC anomalies. Predictability in the North Atlantic SST mainly resides in the ensemble mean signals after three to four forecast years. Analysis suggests that in the CCSM3 the subpolar gyre SST anomalies associated with the AMOC variability can influence the atmosphere and produce surface climate predictability that goes beyond the ENSO time scale. However, the resulting initial-value predictability in the atmosphere is very weak.

scholarly journals Calculating the Meridional Volume, Heat, and Freshwater Transports from an Observing System in the Subpolar North Atlantic: Observing System Simulation Experiment

2017 ◽  
Vol 34 (7) ◽  
pp. 1483-1500 ◽  
Author(s):  
Feili Li ◽  
M. Susan Lozier ◽  
William E. Johns
Keyword(s):  
North Atlantic ◽  
System Simulation ◽  
Meridional Overturning Circulation ◽  
Continuous Measure ◽  
Simulation Experiments ◽  
Overturning Circulation ◽  
Latitudinal Dependence ◽  
Meridional Overturning ◽  
The Mean ◽  
Observing System Simulation Experiment

AbstractA transbasin monitoring array from Labrador to Scotland was deployed in the summer of 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). The aim of the observing system is to provide a multiyear continuous measure of the Atlantic meridional overturning circulation (AMOC) and the associated meridional heat and freshwater transports in the subpolar North Atlantic. Results from the array are expected to improve the understanding of the variability of the subpolar transports and the nature and degree of the AMOC’s latitudinal dependence. In this present work, the measurements of the OSNAP array are described and a suite of observing system simulation experiments in an eddy-permitting numerical model are used to assess how well these measurements will estimate the fluxes across the OSNAP section. The simulation experiments indicate that the OSNAP array and calculation methods will adequately capture the mean and temporal variability of the overturning circulation and of the heat and freshwater transports across the subpolar North Atlantic.

scholarly journals Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation

2016 ◽  
Vol 29 (10) ◽  
pp. 3767-3785 ◽  
Author(s):  
G. Gastineau ◽  
B. L’Hévéder ◽  
F. Codron ◽  
C. Frankignoul
Keyword(s):  
Heat Flux ◽  
Sea Ice ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Late Winter ◽  
Atmospheric Response ◽  
Overturning Circulation ◽  
The North ◽  
Eddy Heat Flux ◽  
The North Atlantic

Abstract In climate models, an intensification of the Atlantic meridional overturning circulation (AMOC) precedes a warming in the North Atlantic subpolar basin by a few years. In the IPSL-CM5A-LR model, this warming may explain the atmospheric response to the AMOC observed in winter, which resembles a negative phase of the North Atlantic Oscillation (NAO). To firmly establish the causality links between the ocean and the atmosphere and illustrate the underlying mechanisms in this model, ensembles of atmosphere-only simulations are conducted, prescribing the SST and sea ice anomalies that follow an AMOC intensification. In late winter, the North Atlantic SST and sea ice anomalies drive atmospheric circulation anomalies similar to those found in the coupled model. Simulations only driven by the SST anomalies related to the AMOC show that the largest oceanic influence is due to the warm subpolar SST anomaly, which enhances the oceanic heat release and decreases the lower-tropospheric baroclinicity in the region of maximum eddy growth, resulting in a weaker meridional eddy heat flux in the atmosphere. The transient eddy feedback leads to a negative NAO-like response. An AMOC intensification is also followed by less sea ice over the Labrador Sea and more sea ice over the Nordic seas. The simulations with full boundary forcing suggest that such anomalies act to strengthen both the poleward momentum flux and the upward heat flux into the polar stratosphere and lead to a stratospheric warming, which then reinforces the negative NAO signal in late winter.

Increased ocean heat transport into the Nordic Seas and Arctic Ocean over the period 1993-2016

2021 ◽  
Author(s):  
Takamasa Tsubouchi ◽  
Kjetil Våge ◽  
Bogi Hansen ◽  
Karin Larsen ◽  
Svein Østerhus ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Arctic Ocean ◽  
Heat Transport ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Ocean Heat Transport ◽  
Overturning Circulation ◽  
Sea Ice Extent ◽  
Nordic Seas ◽  
Poleward Heat Transport

<div> <p>Warm water of subtropical-origin flows northward in the Atlantic Ocean and transports heat to high latitudes. This poleward heat transport has been implicated as one possible cause of the declining sea ice extent and increasing ocean temperatures across the Nordic Seas and Arctic Ocean, but robust estimates are still lacking. Here we use a box inverse model and over 20 years of volume transport measurements to show that the mean ocean heat transport was 305±26 TW for 1993-2016. A significant increase of 21 TW occurred after 2001, which is sufficient to account for the recent accumulation of heat in the northern seas. Therefore, ocean heat transport may have been a major contributor to climate change since the late 1990s. This increased heat transport contrasts with the Atlantic Meridional Overturning Circulation (AMOC) slowdown at mid-latitudes and indicates a discontinuity of the overturning circulation measured at different latitudes in the Atlantic Ocean.</p> </div>

Potential Predictability of the North Atlantic Heat Transport Based on an Oceanic State Estimate

2012 ◽  
Vol 25 (24) ◽  
pp. 8475-8486 ◽  
Author(s):  
Bente Tiedje ◽  
Armin Köhl ◽  
Johanna Baehr
Keyword(s):  
Heat Transport ◽  
North Atlantic ◽  
Initial Conditions ◽  
Meridional Overturning Circulation ◽  
Lead Times ◽  
Meridional Heat Transport ◽  
Potential Predictability ◽  
Latitude Dependence ◽  
The North ◽  
The North Atlantic

Abstract This paper investigates the potential predictability of the meridional heat transport (MHT) in the North Atlantic on interannual time scales using hindcast ensembles based on an oceanic data assimilation product. The work analyzes the prognostic potential predictability (PPP), using the ocean synthesis of the German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) as initial conditions and as boundary conditions. The PPP of the MHT varies with latitude: local maxima are apparent within the subpolar and the subtropical gyres, and a minimum is apparent at the boundary between the gyres. This PPP minimum can also be seen in the PPP structure of the Atlantic meridional overturning circulation (AMOC), although it is considerably less pronounced. The decomposition of the MHT shows that within the subpolar gyre, the gyre component of the MHT influences the PPP structure of the MHT. Within the subtropical gyre, the overturning component of the MHT characterizes the PPP structure of the MHT. At the boundary between the subpolar and the subtropical gyres, the dynamics of the Ekman heat transport limit the predictable lead times of the MHT. At most latitudes, variations in the velocity field control the PPP structure of the MHT. The PPP structure of the AMOC can also be classified into gyre and gyre-boundary regimes, but the predictable lead times within the gyres are only similar to those of the overturning component of the MHT. Overall, the analysis provides a reference point for the latitude dependence of the MHT’s PPP structure and relates it to the latitude dependence of the AMOC’s PPP structure.

Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic

2021 ◽  
Author(s):  
D. Gwyn Evans ◽  
N. Penny Holliday ◽  
Marilena Oltmanns
Keyword(s):  
North Atlantic ◽  
Water Mass ◽  
Meridional Overturning Circulation ◽  
Mass Analysis ◽  
Overturning Circulation ◽  
Water Mass Analysis ◽  
Meridional Overturning ◽  
The Mean ◽  
Water Mass Transformation ◽  
Water Mass Transformations

<p>The OSNAP (Overturning in the Subpolar North Atlantic Program) array at ~60°N has provided new and unprecedented insight into the strength and variability of the meridional overturning circulation in the subpolar North Atlantic. OSNAP has identified the region of the subpolar North Atlantic east of Greenland as a key region for the water mass transformation and densification that sets the strength and variability of the overturning circulation. Here, we will investigate the drivers of this water mass transformation and their roles in driving the overturning circulation at OSNAP. Using a water mass analysis on both model-based and observational-based datasets, we isolate diathermal (across surfaces of constant temperature) and diahaline (across surfaces of constant salinity) transformations due to air-sea buoyancy fluxes, and mixing. We show that the time-mean overturning strength is set by both the air-sea buoyancy fluxes and the strength of subsurface mixing. This balance is apparent on a seasonal timescale, where we resolve large seasonal fluctuations in the both the air-sea buoyancy fluxes and mixing. The residual of this seasonal cycle then corresponds to the mean overturning strength. On interannual timescales, mixing becomes the dominant driver of variability in the overturning circulation. To determine the location of these water mass transformations and the dynamical processes responsible for the mixing-driven variability, our water mass analysis is projected onto geographical coordinates.</p>

The Atlantic Meridional Heat Transport at 26.5°N and Its Relationship with the MOC in the RAPID Array and the GFDL and NCAR Coupled Models

2013 ◽  
Vol 26 (12) ◽  
pp. 4335-4356 ◽  
Author(s):  
Rym Msadek ◽  
William E. Johns ◽  
Stephen G. Yeager ◽  
Gokhan Danabasoglu ◽  
Thomas L. Delworth ◽  
...  
Keyword(s):  
Heat Transport ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Boundary ◽  
Meridional Heat Transport ◽  
Coupled Models ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Model Version ◽  
Meridional Overturning

Abstract The link at 26.5°N between the Atlantic meridional heat transport (MHT) and the Atlantic meridional overturning circulation (MOC) is investigated in two climate models, the GFDL Climate Model version 2.1 (CM2.1) and the NCAR Community Climate System Model version 4 (CCSM4), and compared with the recent observational estimates from the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) array. Despite a stronger-than-observed MOC magnitude, both models underestimate the mean MHT at 26.5°N because of an overly diffuse thermocline. Biases result from errors in both overturning and gyre components of the MHT. The observed linear relationship between MHT and MOC at 26.5°N is realistically simulated by the two models and is mainly due to the overturning component of the MHT. Fluctuations in overturning MHT are dominated by Ekman transport variability in CM2.1 and CCSM4, whereas baroclinic geostrophic transport variability plays a larger role in RAPID. CCSM4, which has a parameterization of Nordic Sea overflows and thus a more realistic North Atlantic Deep Water (NADW) penetration, shows smaller biases in the overturning heat transport than CM2.1 owing to deeper NADW at colder temperatures. The horizontal gyre heat transport and its sensitivity to the MOC are poorly represented in both models. The wind-driven gyre heat transport is northward in observations at 26.5°N, whereas it is weakly southward in both models, reducing the total MHT. This study emphasizes model biases that are responsible for the too-weak MHT, particularly at the western boundary. The use of direct MHT observations through RAPID allows for identification of the source of the too-weak MHT in the two models, a bias shared by a number of Coupled Model Intercomparison Project phase 5 (CMIP5) coupled models.

scholarly journals Influence of the 26°N RAPID–MOCHA Array and Florida Current Cable Observations on the ECCO–GODAE State Estimate

2010 ◽  
Vol 40 (5) ◽  
pp. 865-879 ◽  
Author(s):  
Johanna Baehr
Keyword(s):  
Heat Transport ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Florida Current ◽  
Monthly Means ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Estimation System

Abstract The incorporation of local temperature and salinity observations from the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA), as well as the cable estimates of volume transport in the Florida Current (FC), is tested in the Estimating the Circulation and Climate of the Ocean–Global Ocean Data Assimilation Experiment (ECCO–GODAE) estimation system for their impact on the estimate of the meridional overturning circulation (MOC) and the meridional heat transport in the Atlantic. An experimental setup covering the first deployment period of RAPID–MOCHA from March 2004 to March 2005 is used to test different strategies for incorporating these datasets. Incorporating both monthly means of the FC data and monthly means of the RAPID–MOCHA temperature and salinity measurements at the eastern and western boundaries of the basin as an observational constraint in a 1-yr experiment results in an adjustment to the reference estimate, which does not include these datasets, of approximately 1 Sv (1 Sv ≡ 106 m3 s−1) in the MOC at 26°N and the adjacent latitudes (approximately ±15°), with a larger northward branch of the MOC above 1000 m, compensated by a larger flow in the southward branch of the MOC between approximately 2000 and 3000 m. The meridional heat transport from 26°N to near 40°N is approximately 0.05 PW larger than in the reference experiment.

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