Submesoscale modulation of deep water formation in the Labrador Sea

Abstract Submesoscale structures fill the ocean surface, and recent numerical simulations and indirect observations suggest that they may extend to the ocean interior. It remains unclear, however, how far-reaching their impact may be—in both space and time, from weather to climate scales. Here transport pathways and the ultimate fate of the Irminger Current water from the continental slope to Labrador Sea interior are investigated through regional ocean simulations. Submesoscale processes modulate this transport and in turn the stratification of the Labrador Sea interior, by controlling the characteristics of the coherent vortices formed along West Greenland. Submesoscale circulations modify and control the Labrador Sea contribution to the global meridional overturning, with a linear relationship between time-averaged near surface vorticity and/or frontogenetic tendency along the west coast of Greenland, and volume of convected water. This research puts into contest the lesser role of the Labrador Sea in the overall control of the state of the MOC argued through the analysis of recent OSNAP (Overturning in the Subpolar North Atlantic Program) data with respect to estimates from climate models. It also confirms that submesoscale turbulence scales-up to climate relevance, pointing to the urgency of including its advective contribution in Earth systems models.

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Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

10.5194/egusphere-egu21-1164 ◽

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

The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting&#160; potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model&#160; 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.&#160;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.

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The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0629.1 ◽

2018 ◽

Vol 31 (13) ◽

pp. 5165-5188 ◽

Cited By ~ 6

Author(s):

He Wang ◽

Sonya Legg ◽

Robert Hallberg

Keyword(s):

North Atlantic ◽

Climate Models ◽

Deep Convection ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Labrador Sea ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning

This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate.

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The Role of Northern Sea Ice Cover for the Weakening of the Thermohaline Circulation under Global Warming

Journal of Climate ◽

10.1175/jcli4232.1 ◽

2007 ◽

Vol 20 (16) ◽

pp. 4160-4171 ◽

Cited By ~ 32

Author(s):

A. Levermann ◽

J. Mignot ◽

S. Nawrath ◽

S. Rahmstorf

Keyword(s):

Global Warming ◽

Sea Ice ◽

Heat Loss ◽

Climate Models ◽

Thermohaline Circulation ◽

Ice Cover ◽

Near Surface ◽

Ice Retreat ◽

Sea Ice Retreat

Abstract An increase in atmospheric CO2 concentration and the resulting global warming are typically associated with a weakening of the thermohaline circulation (THC) in model scenarios. For the models participating in the Coupled Model Intercomparison Project (CMIP), this weakening shows a significant (r = 0.62) dependence on the initial THC strength; it is stronger for initially strong overturning. The authors propose a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMBER-3α. The mechanism is based on the fact that sea ice cover greatly reduces heat loss from the ocean. The extent of sea ice is strongly influenced by the near-surface atmospheric temperature (SAT) in the North Atlantic but also by the strength of the THC itself, which transports heat to the convection sites. Consequently, sea ice tends to extend farther south for weaker THC. Initially larger sea ice cover responds more strongly to atmospheric warming; thus, sea ice retreats more strongly for an initially weaker THC. This sea ice retreat tends to strengthen (i.e., stabilize) the THC because the sea ice retreat allows more oceanic heat loss. This stabilizing effect is stronger for runs with weak initial THC and extensive sea ice cover. Therefore, an initially weak THC weakens less under global warming. In contrast to preindustrial climate, sea ice melting presently plays the role of an external forcing with respect to THC stability.

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Transport and modeling of stratospheric inorganic chlorine

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-8597-2007 ◽

2007 ◽

Vol 7 (3) ◽

pp. 8597-8616 ◽

Cited By ~ 1

Author(s):

D. W. Waugh ◽

S. E. Strahan ◽

P. A. Newman

Keyword(s):

Climate Models ◽

Transport Model ◽

Stratospheric Ozone ◽

Chemical Transport Model ◽

Transport Pathways ◽

Future Evolution ◽

Organic Chlorine ◽

Stringent Test ◽

The Relationship

Abstract. Correctly modeling stratospheric inorganic chlorine (Cly) is crucial for modeling the past and future evolution of stratospheric ozone. However, comparisons of the chemistry climate models used in the latest international assessment of stratospheric ozone depletion have shown large differences in the modeled Cly, with these differences explaining differences in the simulated evolution of ozone over the next century. Here in, we examine the role of transport in determining the simulated Cly using three simulations from the same off-line chemical transport model that have the same lower tropospheric boundary conditions and the same chemical solver, but differing resolution and/or meteorological fields. These simulations show that transport plays a key role in determining the Cly distribution, and that Cly depends on both the time scales and pathways of transport. The time air spends in the stratosphere (e.g., the mean age) is an important transport factor determining stratospheric Cly, but the relationship between mean age and Cly is not simple. Lower stratospheric Cly depends on the fraction of air that has been in the upper stratosphere, and transport differences between models having the same mean age can result in differences in the fraction of organic chlorine converted into Cly. Differences in transport pathways result in differences in vertical profiles of CFCs, and comparisons of observed and modeled CFC profiles provides a stringent test of transport pathways in models.

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Sensitivity of stratospheric inorganic chlorine to differences in transport

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-4935-2007 ◽

2007 ◽

Vol 7 (18) ◽

pp. 4935-4941 ◽

Cited By ~ 22

Author(s):

D. W. Waugh ◽

S. E. Strahan ◽

P. A. Newman

Keyword(s):

Climate Models ◽

Transport Model ◽

Stratospheric Ozone ◽

Chemical Transport Model ◽

Transport Pathways ◽

Future Evolution ◽

Organic Chlorine ◽

Stringent Test ◽

The Relationship

Abstract. Correctly modeling stratospheric inorganic chlorine (Cly) is crucial for modeling the past and future evolution of stratospheric ozone. However, comparisons of the chemistry climate models used in the latest international assessment of stratospheric ozone depletion have shown large differences in the modeled Cly, with these differences explaining many of the differences in the simulated evolution of ozone over the next century. Here in, we examine the role of transport in determining the simulated Cly using three simulations from the same off-line chemical transport model that have the same lower tropospheric boundary conditions and the same chemical solver, but differing resolution and/or meteorological fields. These simulations show that transport plays a key role in determining the Cly distribution, and that Cly depends on both the time scales and pathways of transport. The time air spends in the stratosphere (e.g., the mean age) is an important transport factor determining stratospheric Cly, but the relationship between mean age and Cly is not simple. Lower stratospheric Cly depends on the fraction of air that has been in the upper stratosphere, and transport differences between models having the same mean age can result in differences in the fraction of organic chlorine converted into Cly. Differences in transport pathways result in differences in vertical profiles of CFCs, and comparisons of observed and modeled CFC profiles provide a stringent test of transport pathways in models.

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Transport of Antarctic stratospheric strongly dehydrated air into the troposphere observed during the HALO-ESMVal campaign 2012

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-7895-2015 ◽

2015 ◽

Vol 15 (6) ◽

pp. 7895-7932 ◽

Cited By ~ 2

Author(s):

C. Rolf ◽

A. Afchine ◽

H. Bozem ◽

B. Buchholz ◽

V. Ebert ◽

...

Keyword(s):

Water Vapor ◽

Exchange Process ◽

Satellite Measurements ◽

Transport Barrier ◽

Transport Pathways ◽

Near Surface ◽

Air Masses ◽

The Antarctic

Abstract. Dehydration in the Antarctic winter stratosphere is a well-known phenomenon that is occasionally observed by balloon-borne and satellite measurements. However, in-situ measurements of dehydration in the Antarctic vortex are very rare. Here, we present detailed observations with the in-situ and GLORIA remote sensing instrument payload aboard the new German aircraft HALO. Strongly dehydrated air masses down to 1.6 ppmv of water vapor were observed as far north as 47° S and between 12 and 13 km in altitude, which has never been observed by satellites. The dehydration can be traced back to individual ice formation events, where ice crystals sedimented out and water vapor was irreversibly removed. Within these dehydrated stratospheric air masses, filaments of moister air reaching down to the tropopause are detected with the high resolution limb sounder, GLORIA. Furthermore, dehydrated air masses are observed with GLORIA in the Antarctic troposphere down to 7 km. With the help of a backward trajectory analysis, a tropospheric origin of the moist filaments in the vortex can be identified, while the dry air masses in the troposphere have stratospheric origins. The transport pathways of Antarctic stratosphere/troposphere exchange are investigated and the irrelevant role of the Antarctic thermal tropopause as a transport barrier is confirmed. Further, it is shown that the exchange process can be attributed to several successive Rossby wave events in combination with an isentropic interchange of air masses across the weak tropopause and subsequent subsidence due to radiative cooling. Once transported to the troposphere, air masses with stratospheric origin are able to reach near-surface levels within 1–2 months.

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

10.31223/x5zp68 ◽

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.

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Assessment of near-surface air pollutants at an urban station over the central Indo-Gangetic Basin: Role of pollution transport pathways

Meteorology and Atmospheric Physics ◽

10.1007/s00703-021-00798-x ◽

2021 ◽

Author(s):

Amarendra Singh ◽

Atul Kumar Srivastava ◽

V. Varaprasad ◽

Sunil Kumar ◽

Virendra Pathak ◽

...

Keyword(s):

Air Pollutants ◽

Pollution Transport ◽

Urban Station ◽

Transport Pathways ◽

Near Surface

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Assessing the probability of rare climate events

10.1093/oxfordhb/9780198703174.013.16 ◽

2018 ◽

Author(s):

James S. Clark ◽

Dave Bell ◽

Michael Dietze ◽

Michelle Hersh ◽

Ines Ibanez ◽

...

Keyword(s):

Climate Models ◽

Analysis Data ◽

Meridional Overturning Circulation ◽

First Century ◽

Collapse Probability ◽

Climate Simulations ◽

Complex Models ◽

Meridional Overturning ◽

Climate Events

This article focuses on the use of Bayesian methods in assessing the probability of rare climate events, and more specifically the potential collapse of the meridional overturning circulation (MOC) in the Atlantic Ocean. It first provides an overview of climate models and their use to perform climate simulations, drawing attention to uncertainty in climate simulators and the role of data in climate prediction, before describing an experiment that simulates the evolution of the MOC through the twenty-first century. MOC collapse is predicted by the GENIE-1 (Grid Enabled Integrated Earth system model) for some values of the model inputs, and Bayesian emulation is used for collapse probability analysis. Data comprising a sparse time series of five measurements of the MOC from 1957 to 2004 are analysed. The results demonstrate the utility of Bayesian analysis in dealing with uncertainty in complex models, and in particular in quantifying the risk of extreme outcomes.

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Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models

10.5194/egusphere-egu2020-2981 ◽

2020 ◽

Author(s):

Susan Lozier ◽

Matthew Menary ◽

Laura Jackson

Keyword(s):

North Atlantic ◽

Climate Models ◽

Climate Model ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Overturning Circulation ◽

Important Indicator ◽

The North ◽

North Atlantic Subpolar Gyre ◽

Meridional Overturning

The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.

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