Comparison of the atmospheric forcing and oceanographic responses between the Labrador Sea and the Norwegian and Barents seas

Deep oceanic convection occurs in few locations around the globe. One such location is found in the Labrador Sea where dense waters can subside to depths in excess of 2km below the surface. The weak stratification preconditions the water column for deep convection, triggered by wintertime surface cooling associated with high wind speed events. The convected water brings with it dissolved gases, such as Carbon Dioxide, which are in constant flux between ocean and atmosphere. It is thought that this process of turbulent boundary layer interactions coupled with deep convection is responsible for mixing these gases into the deep ocean, making the ocean the largest sink of anthropogenic carbon.The convective overturning process depends on the temperature and salinity profiles which, together dictate density and thus the static stability of the water column. We have adapted a widely used one-dimensional mixed-layer model, referred to as PWP, to include a parameterization of the air-sea flux of gases such as Oxygen and Carbon Dioxide. &#160;The model is forced with surface meteorological fields from the ERA5 reanalysis as well as the higher resolution operational reanalysis from the ECMWF.With the model, we investigate the sensitivity of deep-water formation and the vertical profile of these gases to various atmospheric forcing scenarios. Overturning in the Labrador Sea is most active during the winter months when heat flux out of the ocean is at its maximum. It is found that overturning is far more sensitive to thermal forcing than it is to freshwater forcing within the range of forcings typical to the Labrador Sea. We explore the impact of this sensitivity, including the dependence of the atmospheric forcing on modes of climate variability such as the NAO, &#160;has on the role that the Labrador Sea plays as a marine sink for anthropogenic carbon.

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Atmospheric Forcing of Ocean Convection in the Labrador Sea

10.21236/ada630656 ◽

1999 ◽

Author(s):

Peter S. Guest

Keyword(s):

Labrador Sea ◽

Atmospheric Forcing ◽

Ocean Convection

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Characteristics and variability of ocean ventilation in the high-latitude North Atlantic in an eddy-permitting ocean model

10.5194/egusphere-egu21-9128 ◽

2021 ◽

Author(s):

Helen L. Johnson ◽

Graeme MacGilchrist ◽

David P. Marshall ◽

Camille Lique ◽

Matthew Thomas ◽

...

Keyword(s):

Ocean Circulation ◽

Mixed Layer ◽

North Atlantic ◽

High Latitude ◽

Circulation Model ◽

Open Ocean ◽

Labrador Sea ◽

Surface Mixed Layer ◽

Atmospheric Forcing ◽

Boundary Current

A substantial fraction of the deep ocean is ventilated in the high latitude North Atlantic. As a result, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a set of comprehensive Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined &#8216;North Atlantic Deep Water&#8217; (NADW) reveal the times and locations of subduction from the surface mixed layer at high temporal and spatial resolution. The major fraction (&#8764;60%) of NADW ventilation results from subduction directly into the Labrador Sea boundary current, with a smaller fraction (&#8764;25%) arising from open ocean deep convection in the Labrador Sea. There is a notable absence of ventilation arising from subduction in the Greenland&#8211;Iceland&#8211;Norwegian Seas, due to the re-entrainment of those waters as they move southward. Temporal variability in ventilation arises both from changes in subduction &#8212; driven by large-scale atmospheric forcing &#8212; and from year-to-year changes in the subsurface retention of newly subducted water, the result of an inter-annual equivalent of Stommel&#8217;s mixed layer demon. This interannual demon operates most effectively in the open ocean where newly subducted water is slow to escape its region of subduction. Thus, while subduction in the boundary current dominates NADW ventilation, processes in the open ocean set the variability, mediating the translation of inter-annual variations in atmospheric forcing to the ocean interior.

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Atmospheric forcing during active convection in the Labrador Sea and its impact on mixed-layer depth

Journal of Geophysical Research Oceans ◽

10.1002/2015jc011607 ◽

2016 ◽

Vol 121 (9) ◽

pp. 6978-6992 ◽

Cited By ~ 5

Author(s):

Lena M. Schulze ◽

Robert S. Pickart ◽

G. W. K. Moore

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Labrador Sea ◽

Atmospheric Forcing ◽

Layer Depth ◽

Active Convection

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The Influence of High-Frequency Atmospheric Forcing on the Circulation and Deep Convection of the Labrador Sea

Journal of Climate ◽

10.1175/jcli-d-14-00564.1 ◽

2015 ◽

Vol 28 (12) ◽

pp. 4980-4996 ◽

Cited By ~ 26

Author(s):

Amber M. Holdsworth ◽

Paul G. Myers

Keyword(s):

Mixed Layer ◽

High Frequency ◽

North Atlantic Ocean ◽

Deep Convection ◽

Mixed Layer Depth ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Atmospheric Forcing ◽

The North ◽

Average Maximum

Abstract The influence of high-frequency atmospheric forcing on the circulation of the North Atlantic Ocean with emphasis on the deep convection of the Labrador Sea was investigated by comparing simulations of a coupled ocean–ice model with hourly atmospheric data to simulations in which the high-frequency phenomena were filtered from the air temperature and wind fields. In the absence of high-frequency atmospheric forcing, the strength of the Atlantic meridional overturning circulation and subpolar gyres was found to decrease by 25%. In the Labrador Sea, the eddy kinetic energy decreased by 75% and the average maximum mixed layer depth decreased by between 20% and 110% depending on the climatology. In particular, high-frequency forcing was found to have a greater impact on mixed layer deepening in moderate to warm years whereas in relatively cold years the temperatures alone were enough to facilitate deep convection. Additional simulations in which either the wind or temperature was filtered revealed that the wind, through its impact on the bulk formulas for latent and sensible heat, had a greater impact on deep convection than the temperature.

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Mechanisms behind the Temporary Shutdown of Deep Convection in the Labrador Sea: Lessons from the Great Salinity Anomaly Years 1968–71

Journal of Climate ◽

10.1175/jcli-d-11-00549.1 ◽

2012 ◽

Vol 25 (19) ◽

pp. 6743-6755 ◽

Cited By ~ 39

Author(s):

Renske Gelderloos ◽

Fiammetta Straneo ◽

Caroline A. Katsman

Keyword(s):

Greenland Ice Sheet ◽

The Arctic ◽

Subsurface Water ◽

Labrador Sea ◽

Freshwater Flux ◽

Atmospheric Forcing ◽

Tight Coupling ◽

Positive Feedbacks ◽

Salinity Anomaly ◽

Great Salinity Anomaly

Abstract From 1969 to 1971 convection in the Labrador Sea shut down, thus interrupting the formation of the intermediate/dense water masses. The shutdown has been attributed to the surface freshening induced by the Great Salinity Anomaly (GSA), a freshwater anomaly in the subpolar North Atlantic. The abrupt resumption of convection in 1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here oceanic and atmospheric data collected in the Labrador Sea at Ocean Weather Station Bravo and a one-dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail. These results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet. The analysis herein shows that the shutdown was initiated by the GSA-induced freshening as well as the mild 1968/69 winter. After the shutdown had begun, however, the continuing lateral freshwater flux as well as two positive feedbacks [both associated with the sea surface temperature (SST) decrease due to lack of convective mixing with warmer subsurface water] further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air–sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surface water. In 1972 convection resumed because of both the extreme atmospheric forcing and advection of saltier waters into the convection region.

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Atmospheric vorticity sets the basin-scale circulation in Hudson Bay

Elem Sci Anth ◽

10.1525/elementa.049 ◽

2020 ◽

Vol 8 (1) ◽

Author(s):

Igor A. Dmitrenko ◽

Paul G. Myers ◽

Sergei A. Kirillov ◽

David G. Babb ◽

Denis L. Volkov ◽

...

Keyword(s):

Water Circulation ◽

Sea Surface ◽

Water Dynamics ◽

Labrador Sea ◽

Hudson Bay ◽

Atmospheric Forcing ◽

Riverine Water ◽

Freshwater Transport ◽

Basin Scale ◽

Western Hudson Bay

Hudson Bay of northern Canada receives upward of 700 km3 of river discharge annually. Cyclonic water circulation in Hudson Bay transports this massive volume of riverine water along the coast toward Hudson Strait and into the Labrador Sea. However, synoptic, seasonal and interannual variability of the freshwater transport in Hudson Bay remains unclear. Using yearlong observations of current velocity profiles, collected from oceanographic moorings deployed in western Hudson Bay from September 2016 to September/October 2017, we examined the role of atmospheric forcing on circulation and freshwater transport in the Bay. Our analysis reveals that the along-shore southeastward current through western Hudson Bay was amplified through the entire water column in response to winds generated by cyclones passing over Hudson Bay toward Baffin Bay and/or the Labrador Sea. An atmospheric vorticity index was used to describe the atmospheric forcing and found to correlate with sea surface height and along-shore currents. We showed that a surface Ekman on-shore transport increases sea surface heights along the coast, producing a cross-slope pressure gradient that drives an along-shore southeastward flow, in the same direction as the wind. Expanding our observations to the bay-wide scale, we confirmed this process of wind-driven water dynamics with (1) satellite altimetry measurements and (2) ocean model simulations. Ultimately, we find that cyclonic wind forcing amplifies cyclonic water circulation in Hudson Bay facilitating the along-shore freshwater transport to Hudson Strait. During periods of positive atmospheric vorticity, this forcing can reduce the residence time of riverine water in Hudson Bay.

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