Atmospheric Forcing of Ocean Convection in the Labrador Sea

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

Progress In Oceanography ◽

10.1016/j.pocean.2013.03.007 ◽

2013 ◽

Vol 114 ◽

pp. 11-25 ◽

Cited By ~ 19

Author(s):

K. Drinkwater ◽

E. Colbourne ◽

H. Loeng ◽

S. Sundby ◽

T. Kristiansen

Keyword(s):

Labrador Sea ◽

Atmospheric Forcing

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Early to mid-Holocene sea ice changes on the Labrador Shelf - biomarker evidence.

10.5194/egusphere-egu2020-13231 ◽

2020 ◽

Author(s):

Henriette Kolling ◽

Ralph Schneider ◽

Annalena Lochte ◽

Kirsten Fahl ◽

Ruediger Stein

Keyword(s):

Sea Ice ◽

Deep Ocean ◽

Ice Cover ◽

Freshwater Inflow ◽

Labrador Sea ◽

Time Interval ◽

Hudson Bay ◽

Phytoplankton Productivity ◽

Deep Ocean Convection ◽

Ocean Convection

Understanding the Earth&#8217;s climate system and by that improving predictions of future changes are of utmost importance. A key player in this context is the global thermohaline ocean circulation, of which North Atlantic deep ocean convection is an essential component. Hence, one important region for deep ocean convection is the Labrador Sea, where the warm Gulf Stream meets cold polar waters in the Subpolar Gyre. Sea surface temperature and salinity play a major role in this convective process; two factors that influence these parameters are seasonal sea ice cover and freshwater inflow. During the early Holocene a major freshening in the Labrador Sea at 8.5 ka BP has been associated with the collapse of the Hudson Bay Ice Saddle (Lochte et al., 2019a). This collapse was triggered by a subsurface warming of the western Labrador Sea, linked to the strengthening of the Irminger and West Greenland Current that could have accelerated the ice saddle collapse. However, the role of sea ice in this process is yet unknown.&#160;Here, we present a reconstruction of sea ice cover during the respective time interval, based on the organic biomarker IP25, a highly branched isoprenoid that is considered as a reliable proxy for past sea ice conditions. Actually, we apply the more advanced PIP25 sea ice index, together with other biomarkers for phytoplankton productivity, to reconstruct sea ice changes at centennial scale for the early to mid Holocene from a Labrador Shelf sediment core.&#160;Based on this approach we infer that nearly perennial sea ice cover opened towards more seasonally, extremely fluctuating, conditions around 8.5 ka, parallel to the strengthening of Atlantic warm water inflow towards the Labrador Shelf. The shift to more seasonal sea ice cover may have favoured the advance of Atlantic water into Hudson Bay and could have accelerated the collapse and subsequent drainage of the Hudson Bay Ice Saddle. The opening of the sea ice triggered phytoplankton productivity and we find evidence for the establishment of a stable ice edge in the vicinity of the core location between 8.1 and 7.6 ka. With the establishment of the Labrador Sea Water formation around 7.4 ka (Lochte et al., 2019b) sea ice continued to fluctuate seasonally and reduced freshwater inflow favoured enhanced phytoplankton productivity.&#160;References:Lochte, A. A., Repschl&#228;ger, J., Kienast, M.,Garbe-Sch&#246;nberg, D., Andersen, N., Hamann, C., Schneider, R., 2019a. Labrador Sea freshening at 8.5 ka BP caused by Hudson Bay Ice Saddle collapse. Nature Communications, 10-586Lochte, A. A., Repschl&#228;ger, J., Seidenkrantz, M-S., Kienast, M., Blanz, T., Schneider, R.R., 2019b. Holocene water mass changes in the Labrador Current. The Holocene 1-15

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