Link between transformation rate and overturning in the Iceland Basin and Irminger Sea

2020 ◽  
Author(s):  
Tillys Petit ◽  
Susan Lozier ◽  
Simon A. Josey ◽  
Stuart A. Cunningham
Keyword(s):  
North Atlantic ◽  
Water Mass ◽  
Meridional Overturning Circulation ◽  
Transformation Rate ◽  
Transformation Theory ◽  
Buoyancy Forcing ◽  
Iceland Basin ◽  
Irminger Sea ◽  
Meridional Overturning ◽  
The Mean

<p>The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, transforms warm and salty waters from the subtropical gyre into colder and fresher waters in the subpolar gyre and Nordic Seas. To measure the mean AMOC and its variability at subpolar latitudes, the Overturning in the Subpolar North Atlantic Program (OSNAP) array was deployed in the summer of 2014. Based on observations through May 2016, the majority of the light‐to‐dense water conversion takes place north of the OSNAP East line, which runs from the southeast tip of Greenland to the Scottish shelf. In this study, we assess the transformation of dense waters in the area located between the Greenland-Scotland Ridge and the OSNAP East section. From 2014 to 2016, the mean overturning within this area is estimated at 6.9 ± 1.3 Sv across σ<sub>0</sub> = 27.55 kg m<sup>-3</sup>, the isopycnal that separates the northward and southward flows. This mean overturning estimate is in close agreement with the value (6.5 ± 1 Sv) derived by applying water mass transformation theory to air-sea buoyancy fluxes from atmospheric reanalysis. However, the large monthly variability of the overturning (standard deviation of 4.1 Sv) cannot easily be attributed to the buoyancy forcing or to variability in the overflow through the Greenland-Scotland Ridge. We explore possible mechanisms that can account for this variability.  </p>

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>

scholarly journals Changes in the geometry and strength of the Atlantic Meridional Overturning Circulation during the last glacial (20–50 ka)

2016 ◽  
Author(s):  
Pierre Burckel ◽  
Claire Waelbroeck ◽  
Yiming Luo ◽  
Didier Roche ◽  
Sylvain Pichat ◽  
...  
Keyword(s):  
Flow Rate ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20–50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). At the onset of Heinrich Stadial 2, the structure of the AMOC significantly changed. The deep Atlantic was probably directly affected by a southern sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.

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 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.

OSNAP and water mass transport in CMIP6 models

2021 ◽  
Author(s):  
Laura Jackson
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Water Mass ◽  
Deep Convection ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
The West ◽  
Low Frequency Variability ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulation (AMOC) influences our climate by transporting heat northwards in the Atlantic ocean. The subpolar North Atlantic plays an important role in this circulation, with transformation of water to higher densities, deep convection and formation of deep water. Recent OSNAP observations have shown that the overturning is stronger to the east of Greenland than the west.</p><p>Here we analyse a CMIP6 climate model at two resolutions (HadGEM3 GC3.1 LL and MM) and show both compare well with the OSNAP observations. We explore the source of low frequency variability of the AMOC and how it is related to the surface water mass transformation in different regions. We also investigate time-mean and low frequency water mass transformations in other CMIP6 climate models.</p>

On Multidecadal Variability of the Atlantic Meridional Overturning Circulation in the Community Climate System Model Version 3

2008 ◽  
Vol 21 (21) ◽  
pp. 5524-5544 ◽  
Author(s):  
Gokhan Danabasoglu
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
System Model ◽  
Subpolar Gyre ◽  
Climate System Model ◽  
Multidecadal Variability ◽  
Meridional Overturning ◽  
The Mean

Abstract Multidecadal variability of the Atlantic meridional overturning circulation (MOC) is investigated diagnostically in the NCAR Community Climate System Model version 3 (CCSM3) present-day simulations, using the highest (T85 × 1) resolution version. This variability has a 21-yr period and is present in many other ocean fields in the North Atlantic. In MOC, the oscillation amplitude is about 4.5 Sv (1 Sv ≡ 106 m3 s−1), corresponding to 20% of the mean maximum MOC transport. The northward heat transport (NHT) variability has an amplitude of about 0.12 PW, representing 10% of the mean maximum NHT. In sea surface temperature (SST) and sea surface salinity (SSS), the peak-to-peak changes can be as large as 6°–7°C and 3 psu, respectively. The Labrador Sea region is identified as the deep-water formation (DWF) site associated with the MOC oscillations. In contrast with some previous studies, temperature and salinity contributions to the total density in this DWF region are almost equal and in phase. The heat and freshwater budget analyses performed for the DWF site indicate a complex relationship between the DWF, MOC, North Atlantic Oscillation (NAO), and subpolar gyre circulation anomalies. Their complicated interactions appear to be responsible for the maintenance of this multidecadal oscillation. In these interactions, the atmospheric variability associated with the model’s NAO plays a prominent role. In particular, the NAO modulates the subpolar gyre strength and contributes to the formation of the temperature and salinity anomalies that lead to positive/negative density anomalies at the DWF site. In addition, the wind stress curl anomalies occurring during the transition phase between the positive and negative NAO states produce fluctuations of the subtropical–subpolar gyre boundary, thus creating midlatitude SST and SSS anomalies. Comparisons with observations show that neither the pattern nor the magnitude of this dominant SST variability is realistic.

scholarly journals An Outsized Role for the Labrador Sea in the Multidecadal Variability of the Atlantic Overturning Circulation

2021 ◽  
Author(s):  
Stephen Yeager ◽  
Fred Castruccio ◽  
Ping Chang ◽  
Gokhan Danabasoglu ◽  
Elizabeth Maroon ◽  
...  
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Sea Water ◽  
Meridional Overturning Circulation ◽  
Climate Simulation ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
Realistic Representation ◽  
Meridional Overturning

Climate models are essential tools for investigating intrinsic North Atlantic variability related to variations in the Atlantic meridional overturning circulation (AMOC), but recent observations have called into question the fidelity of models that emphasize the importance ofLabrador Sea processes. A multi-century pre-industrial climate simulation that resolves ocean mesoscale eddies has a realistic representation of key observed subpolar Atlantic phenomena,including the dominance of density-space overturning in the eastern subpolar gyre, and thus provides uniquely credible context for interpreting short observational records. Despite weak mean surface diapycnal transformation in the Labrador Sea, multidecadal AMOC variability can be traced to anomalous production of dense Labrador Sea Water with local buoyancy forcing in the interior Labrador Sea playing a significant driving role.

scholarly journals Changes in the geometry and strength of the Atlantic meridional overturning circulation during the last glacial (20–50 ka)

2016 ◽  
Vol 12 (11) ◽  
pp. 2061-2075 ◽  
Author(s):  
Pierre Burckel ◽  
Claire Waelbroeck ◽  
Yiming Luo ◽  
Didier M. Roche ◽  
Sylvain Pichat ◽  
...  
Keyword(s):  
Flow Rate ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. We reconstruct the geometry and strength of the Atlantic meridional overturning circulation during the Heinrich stadial 2 and three Greenland interstadials of the 20–50 ka period based on the comparison of new and published sedimentary 231Pa / 230Th data with simulated sedimentary 231Pa / 230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present-day North Atlantic deep water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic bottom water (AABW). Our results further show that during Heinrich stadial 2, the deep Atlantic was probably directly affected by a southern-sourced water mass below 2500 m depth, while a slow, southward-flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.

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

2020 ◽  
Author(s):  
Ben Moat ◽  
David Smeed ◽  
Eleanor Frajka-Williams ◽  
Damien Desbruyeres ◽  
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

<div> <div> <div> <p>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 - 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.</p> <p>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.</p> </div> </div> </div>

Sign in / Sign up

Export Citation Format

Share Document