Preconditioning of the Weddell Sea Polynya by the Ocean Mesoscale and Dense Water Overflows

Abstract The Weddell Sea polynya is a large opening in the open-ocean sea ice cover associated with intense deep convection in the ocean. A necessary condition to form and maintain a polynya is the presence of a strong subsurface heat reservoir. This study investigates the processes that control the stratification and hence the buildup of the subsurface heat reservoir in the Weddell Sea. To do so, a climate model run for 200 years under preindustrial forcing with two eddying resolutions in the ocean (0.25° CM2.5 and 0.10° CM2.6) is investigated. Over the course of the simulation, CM2.6 develops two polynyas in the Weddell Sea, while CM2.5 exhibits quasi-continuous deep convection but no polynyas, exemplifying that deep convection is not a sufficient condition for a polynya to occur. CM2.5 features a weaker subsurface heat reservoir than CM2.6 owing to weak stratification associated with episodes of gravitational instability and enhanced vertical mixing of heat, resulting in an erosion of the reservoir. In contrast, in CM2.6, the water column is more stably stratified, allowing the subsurface heat reservoir to build up. The enhanced stratification in CM2.6 arises from its refined horizontal grid spacing and resolution of topography, which allows, in particular, a better representation of the restratifying effect by transient mesoscale eddies and of the overflows of dense waters along the continental slope.

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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO3.4

Geoscientific Model Development ◽

10.5194/gmd-8-3119-2015 ◽

2015 ◽

Vol 8 (10) ◽

pp. 3119-3130 ◽

Cited By ~ 18

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Climate Model ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Antarctic Bottom Water

Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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Internal Ocean Dynamics Control the Long-Term Evolution of Weddell Sea Polynya Activity

Frontiers in Climate ◽

10.3389/fclim.2021.718016 ◽

2021 ◽

Vol 3 ◽

Author(s):

Jonathan W. Rheinlænder ◽

Lars H. Smedsrud ◽

Kerim H. Nisanciouglu

Keyword(s):

Sea Ice ◽

Deep Convection ◽

Vertical Mixing ◽

Open Ocean ◽

Weddell Sea ◽

Ocean Dynamics ◽

Heat Reservoir ◽

Ice Dynamics

Open-ocean polynyas effectively couple the ocean and atmosphere through large ice-free areas within the sea-ice cover, release vast quantities of oceanic heat, and impact deep ocean ventilation. Changes in polynya activity, particularly in the Weddell Sea, may be key to longer time-scale climate fluctuations, feedbacks and abrupt change. While changes in the occurrence of Weddell Sea polynyas are generally attributed to changes in the atmospheric surface forcing, the role of internal ocean dynamics for polynya variability is not well-resolved. In this study we employ a global coupled ocean-sea ice model with a repeating annual atmospheric cycle to explore changes in Weddell Sea water mass properties, stratification and ocean circulation driven by open-ocean polynyas. During the 1300-year long simulation, two large polynyas occur in the central Weddell Sea. Our results suggest that Weddell polynyas may be triggered without inter-annual changes in the atmospheric forcing. This highlights the role of ocean processes in preconditioning and triggering open-ocean polynyas on multi-centennial time-scales. The simulated polynyas form due to internal ocean-sea ice dynamics associated with a slow build-up and subsequent release of subsurface heat. A strong stratification and weak vertical mixing is necessary for building the subsurface heat reservoir. Once the water column turns unstable, enhanced vertical mixing of warm and saline waters into the surface layer causes efficient sea ice melt and the polynya appears. Subsequent, vigorous deep convection is maintained through upwelling of warm deep water leading to enhanced bottom water formation. We find a cessation of simulated deep convection and polynya activity due to long-term cooling and freshening of the subsurface heat reservoir. As subsurface waters in the Southern Ocean are now becoming warmer and saltier, we speculate that larger and more persistent Weddell polynyas could become more frequent in the future.

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Relationship between Ocean Carbon and Heat Multidecadal Variability

Journal of Climate ◽

10.1175/jcli-d-17-0134.1 ◽

2018 ◽

Vol 31 (4) ◽

pp. 1467-1482 ◽

Cited By ~ 5

Author(s):

Jordan Thomas ◽

Darryn Waugh ◽

Anand Gnanadesikan

Keyword(s):

Southern Ocean ◽

Carbon Content ◽

Climate Model ◽

Deep Convection ◽

Heat Content ◽

Weddell Sea ◽

Global Ocean ◽

Multidecadal Variability ◽

Model Simulations ◽

Ocean Carbon

The global ocean serves as a critical sink for anthropogenic carbon and heat. While significant effort has been dedicated to quantifying the oceanic uptake of these quantities, less research has been conducted on the mechanisms underlying decadal-to-centennial variability in oceanic heat and carbon. Therefore, little is understood about how much such variability may have obscured or reinforced anthropogenic change. Here the relationship between oceanic heat and carbon content is examined in a suite of coupled climate model simulations that use different parameterization settings for mesoscale mixing. The differences in mesoscale mixing result in very different multidecadal variability, especially in the Weddell Sea where the characteristics of deep convection are drastically changed. Although the magnitude and frequency of variability in global heat and carbon content is different across the model simulations, there is a robust anticorrelation between global heat and carbon content in all simulations. Global carbon content variability is primarily driven by Southern Ocean carbon variability. This contrasts with global heat content variability. Global heat content is primarily driven by variability in the southern midlatitudes and tropics, which opposes the Southern Ocean variability.

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Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model

Geophysical Research Letters ◽

10.1002/2014gl061313 ◽

2014 ◽

Vol 41 (20) ◽

pp. 7262-7269 ◽

Cited By ~ 21

Author(s):

Raffaele Bernardello ◽

Irina Marinov ◽

Jaime B. Palter ◽

Eric D. Galbraith ◽

Jorge L. Sarmiento

Keyword(s):

Climate Model ◽

Deep Convection ◽

Weddell Sea ◽

Anthropogenic Carbon

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Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends

Journal of Climate ◽

10.1175/jcli-d-12-00281.1 ◽

2013 ◽

Vol 26 (19) ◽

pp. 7767-7782 ◽

Cited By ~ 61

Author(s):

Mojib Latif ◽

Torge Martin ◽

Wonsun Park

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Atmospheric Circulation ◽

Climate Model ◽

Deep Convection ◽

Surface Air Temperature ◽

Weddell Sea ◽

Sea Ice Extent ◽

Low Level ◽

Antarctic Sea Ice

Abstract Evidence is presented for the notion that some contribution to the recent decadal trends observed in the Southern Hemisphere, including the lack of a strong Southern Ocean surface warming, may have originated from longer-term internal centennial variability originating in the Southern Ocean. The existence of such centennial variability is supported by the instrumental sea surface temperatures (SSTs), a multimillennial reconstruction of Tasmanian summer temperatures from tree rings, and a millennial control integration of the Kiel Climate Model (KCM). The model variability was previously shown to be linked to changes in Weddell Sea deep convection. During phases of deep convection the surface Southern Ocean warms, the abyssal Southern Ocean cools, Antarctic sea ice extent retreats, and the low-level atmospheric circulation over the Southern Ocean weakens. After the halt of deep convection the surface Southern Ocean cools, the abyssal Southern Ocean warms, Antarctic sea ice expands, and the low-level atmospheric circulation over the Southern Ocean intensifies, consistent with what has been observed during the recent decades. A strong sensitivity of the time scale to model formulation is noted. In the KCM, the centennial variability is associated with global-average surface air temperature (SAT) changes of the order of a few tenths of a degree per century. The model results thus suggest that internal centennial variability originating in the Southern Ocean should be considered in addition to other internal variability and external forcing when discussing the climate of the twentieth century and projecting that of the twenty-first century.

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Transient Response of the Southern Ocean to Changing Ozone: Regional Responses and Physical Mechanisms

Journal of Climate ◽

10.1175/jcli-d-16-0474.1 ◽

2017 ◽

Vol 30 (7) ◽

pp. 2463-2480 ◽

Cited By ~ 13

Author(s):

William J. M. Seviour ◽

Anand Gnanadesikan ◽

Darryn Waugh ◽

Marie-Aude Pradal

Keyword(s):

Southern Ocean ◽

Climate Model ◽

Cold Water ◽

Deep Convection ◽

Weddell Sea ◽

Freshwater Flux ◽

Surface Salinity ◽

Sea Ice Concentration ◽

Climate Model Simulations ◽

The Impact

The impact of changing ozone on the climate of the Southern Ocean is evaluated using an ensemble of coupled climate model simulations. By imposing a step change from 1860 to 2000 conditions, response functions associated with this change are estimated. The physical processes that drive this response are different across time periods and locations, as is the sign of the response itself. Initial cooling in the Pacific sector is driven not only by the increased winds pushing cold water northward, but also by the southward shift of storms associated with the jet stream. This shift drives both an increase in cloudiness (resulting in less absorption of solar radiation) and an increase in net freshwater flux to the ocean (resulting in a decrease in surface salinity that cuts off mixing of warm water from below). A subsurface increase in temperature associated with this reduction in mixing then upwells along the Antarctic coast, producing a subsequent warming. Similar changes in convective activity occur in the Weddell Sea but are offset in time. Changes in sea ice concentration also play a role in modulating solar heating of the ocean near the continent. The time scale for the initial cooling is much longer than that seen in NCAR CCSM3.5, possibly reflecting differences in natural convective variability between that model (which has essentially no Southern Ocean deep convection) and the one used here (which has a large and possibly unrealistically regular mode of convection) or to differences in cloud feedbacks or in the location of the anomalous winds.

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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-2949-2015 ◽

2015 ◽

Vol 8 (3) ◽

pp. 2949-2972 ◽

Cited By ~ 2

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Cmip5 Models ◽

Antarctic Bottom Water

Abstract. Most CMIP5 models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross Seas. To identify the triggering mechanisms leading to Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing are warmer, open larger polynyas and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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On the Generation of Weddell Sea Polynyas in a High-Resolution Earth System Model

Journal of Climate ◽

10.1175/jcli-d-20-0229.1 ◽

2021 ◽

pp. 1-58

Author(s):

Prajvala Kurtakoti ◽

Milena Veneziani ◽

Achim Stössel ◽

Wilbert Weijer ◽

Mathew Maltrud

Keyword(s):

High Resolution ◽

Wind Stress ◽

Earth System Model ◽

Weddell Sea ◽

System Model ◽

Necessary Condition ◽

Earth System ◽

Heat Reservoir ◽

Wind Stress Curl ◽

Maud Rise

AbstractLarger Weddell Sea polynyas (WSPs), differentiated in this study from the smaller Maud Rise Polynyas (MRPs), forming to the east of the prime meridian in the proximity of the Maud Rise seamount, have last been observed in the 1970s. We investigate WSPs that grow realistically out of MRPs in a high-resolution (HR) preindustrial simulation with the Energy Exascale Earth System Model version 0.1. The formation of MRPs requires HR to simulate the detailed flow around Maud Rise, while the realistic formation of WSPs requires a model to produce MRPs. Furthermore, WSPs tend to follow periods of a prolonged build-up of a heat reservoir at depth and weakly negative wind-stress curl in association with the core of the southern hemisphere westerlies at an anomalously northern position. While this scenario also leads to drier conditions over the central Weddell Sea, which some literature claims to be a necessary condition for the formation of WSPs, our model results indicate that open-ocean polynyas do not occur during periods of weakly negative wind-stress curl despite drier atmospheric conditions. Our study supports the hypothesis noted in earlier studies that a shift from a weakly negative to a strongly negative wind-stress curl over the Weddell Sea is a prerequisite for WSPs to form, together with a large heat reservoir at depth. However, the ultimate trigger is a pronounced MRP; whose associated convection creates high surface salinity anomalies that propagate westward with the flow of the Weddell Gyre. If large enough, these anomalies trigger the formation of a WSP and a pulse of newly formed Antarctic Bottom Water.

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Towards European-Scale Convection-Resolving Climate Simulations

10.5194/gmd-2016-119 ◽

2016 ◽

Cited By ~ 4

Author(s):

David Leutwyler ◽

Oliver Fuhrer ◽

Xavier Lapillonne ◽

Daniel Lüthi ◽

Christoph Schär

Keyword(s):

Graphics Processing Units ◽

Climate Models ◽

Climate Model ◽

Deep Convection ◽

Moist Convection ◽

Cold Pools ◽

Cosmo Model ◽

Climate Simulations ◽

Graphics Processing ◽

Horizontal Grid

Abstract. The representation of moist convection in climate models represents a major challenge, due to the small scales involved. Using horizontal grid spacings of O(1km), convection-resolving weather and climate models allow to explicitly resolve deep convection. However, due to their extremely demanding computational requirements, they have so far been limited to short simulations and/or small computational domains. Innovations in supercomputing have led to new hybrid node designs, mixing conventional multicore CPUs and accelerators such as graphics processing units (GPUs). One of the first atmospheric models that has been fully ported to these architectures is the COSMO model. Here we demonstrate the convection-resolving COSMO model on continental scales using a version of the model capable of using GPU accelerators. The verification of a week-long simulation containing winter storm Kyrill shows that, for this case, convection-parameterizing simulations and convection-resolving simulations agree well. Furthermore we demonstrate the applicability of the approach to longer simulations by conducting a three-month long simulation of the summer season 2006. Its results corroborate the findings found on smaller domains such as more credible representation of the diurnal cycle of precipitation in convection-resolving models and a tendency to produce more intensive hourly precipitation events. Both simulations also show how the approach allows for the representation of interactions between synoptic-scale and meso-scale atmospheric circulations at scales ranging from 1000 to 10 km. This includes the formation of sharp cold frontal structures, convection embedded in fronts and small eddies, or the formation and organization of propagating cold pools. Finally we assess the performance gain from using heterogeneous hardware equipped with GPUs with respect to multi-core hardware. With the COSMO model, we now use a climate model that has all the necessary modules required for real-case convection-resolving climate simulations on GPUs.

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A product form for the general stochastic matching model

Journal of Applied Probability ◽

10.1017/jpr.2020.100 ◽

2021 ◽

Vol 58 (2) ◽

pp. 449-468

Author(s):

Pascal Moyal ◽

Ana Bušić ◽

Jean Mairesse

Keyword(s):

Stationary Distribution ◽

Product Form ◽

Necessary Condition ◽

Matching Model ◽

Bipartite Matching ◽

Compatibility Graph ◽

Stochastic Matching ◽

Dynamic Reversibility ◽

Original Dynamic ◽

Do So

AbstractWe consider a stochastic matching model with a general compatibility graph, as introduced by Mairesse and Moyal (2016). We show that the natural necessary condition of stability of the system is also sufficient for the natural ‘first-come, first-matched’ matching policy. To do so, we derive the stationary distribution under a remarkable product form, by using an original dynamic reversibility property related to that of Adan, Bušić, Mairesse, and Weiss (2018) for the bipartite matching model.

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