Mean structure and dynamics of the shelfbreak jet in the Middle Atlantic Bight during fall and winter

Using a collection of high-resolution shipboard acoustic Doppler current profiler (ADCP) velocity sections that cross the Middle Atlantic Bight shelfbreak jet near 70°W, the mean structure of the frontal jet is described and the dominant modes of variability of the jet are examined. A mean section is constructed in a translating coordinate frame whose origin tracks the instantaneous position of the core of the jet, thereby minimizing variability associated with the lateral meandering of the current. The mean jet so constructed extends to the bottom, tilting onshore with depth, with near-bottom flow exceeding 0.10 m s-1. The corresponding cross-stream flow reveals a clear pattern of convergence that extends along the tilted axis of the jet, with enhanced convergence both near the surface and near the bottom. This convergence is largely attributed to the locally convergent topography and is shown to drive an ageostrophic circulation dominated by downwelling at, and offshore of, the jet core. The collection of ADCP sections also suggests a previously undetedted mode of variability, whereby the jet systematically fluctuates between a convergent, bottom-reaching state and a surface-trapped state with weaker cross-stream velocities. This variability is associated with significant variations in the southwestward transport of the jet and does not seem to be related to simple meandering of the current

The annual salinity cycle of the Denmark Strait Overflow

The Denmark Strait Overflow (DSO) is an important source of dense water input to the deep limb of the Atlantic Meridional Overturning Circulation. It is fed by separate currents from the north that advect dense water masses formed in the Nordic Seas and Arctic Ocean which then converge at Denmark Strait. Here we identify an annual salinity cycle of the DSO, characterized by freshening in winter and spring. The freshening is linked to freshening of the Shelfbreak East Greenland Current in the Blosseville Basin north of the Denmark Strait. We demonstrate that the East Greenland Current advects fresh pycnocline water above the recirculating Atlantic Water, which forms a low salinity lid for the overflow in Denmark Strait and in the Irminger Basin. This concept is supported by intensified freshening of the DSO in lighter density classes on the Greenland side of the overflow. The salinity of the DSO in the Irminger Basin is significantly correlated with northerly/northeasterly winds in the Blosseville Basin at a lag of 3-4 months, consistent with estimated transit times. This suggests that wind driven variability of DSO source water exerts an important influence on the salinity variability of the downstream DSO, and hence the composition of the deep limb of the Atlantic Meridional Overturning Circulation

What causes the location of the air-sea turbulent heat flux maximum over the Labrador Sea?

The Labrador Sea is a region of climatic importance as a result of the occurrence of oceanic wintertime convection, a process that is integral to the Atlantic Meridional Overturning Circulation. This process requires large air-sea heat fluxes that result in a loss of surface buoyancy, triggering convective overturning of the water column. The Labrador Sea wintertime turbulent heat flux maximum is situated downstream of the ice edge, a location previously thought to be causal. Here we show that there is considerable similarity in the characteristics of the regional mean atmospheric circulation and high heat flux events over the Labrador Sea during early winter, when the ice is situated to the north, and midwinter, when it is near the region of maximum heat loss. This suggests that other factors, including the topography of the nearby upstream and downstream landmasses, contribute to the location of the heat flux maximum

Liquid freshwater transport estimates from the East Greenland Current based on continuous measurements north of Denmark Strait

Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11 month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012 to 2014. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of −131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s

Eddies and the Distribution of Eddy Kinetic Energy in the Arctic Ocean

Mesoscale eddies are important to many aspects of the dynamics of the Arctic Ocean. Among others, they maintain the halocline and interact with the Atlantic Water circumpolar boundary current through lateral eddy fluxes and shelf-basin exchanges. Mesoscale eddies are also important for transporting biological material and for modifying sea ice distribution. Here, we review what is known about eddies and their impacts in the Arctic Ocean in the context of rapid climate change. Eddy kinetic energy (EKE) is a proxy for mesoscale variability in the ocean due to eddies. We present the first quantification of EKE from moored observations across the entire Arctic Ocean and compare those results to output from an eddy resolving numerical model. We show that EKE is largest in the northern Nordic Seas/Fram Strait and it is also elevated along the shelf break of the Arctic Circumpolar Boundary Current, especially in the Beaufort Sea. In the central basins, EKE is 100–1,000 times lower. Generally, EKE is stronger when sea ice concentration is low versus times of dense ice cover. As sea ice declines, we anticipate that areas in the Arctic Ocean where conditions typical of the North Atlantic and North Pacific prevail will increase. We conclude that the future Arctic Ocean will feature more energetic mesoscale variability

The Greenland flow distortion experiment

Author Posting. ©American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 89 (2008): 1307-1324, doi:10.1175/2008BAMS2508.1.Greenland has a major influence on the atmospheric circulation of the North Atlantic–western European region, dictating the location and strength of mesoscale weather systems around the coastal seas of Greenland and directly influencing synoptic-scale weather systems both locally and downstream over Europe. High winds associated with the local weather systems can induce large air–sea fluxes of heat, moisture, and momentum in a region that is critical to the overturning of the thermohaline circulation, and thus play a key role in controlling the coupled atmosphere–ocean climate system. The Greenland Flow Distortion Experiment (GFDex) is investigating the role of Greenland in defining the structure and predictability of both local and downstream weather systems through a program of aircraft-based observation and numerical modeling. The GFDex observational program is centered upon an aircraft-based field campaign in February and March 2007, at the dawn of the International Polar Year. Twelve missions were flown with the Facility for Airborne Atmospheric Measurements' BAe-146, based out of the Keflavik, Iceland. These included the first aircraft-based observations of a reverse tip jet event, the first aircraft-based observations of barrier winds off of southeast Greenland, two polar mesoscale cyclones, a dramatic case of lee cyclogenesis, and several targeted observation missions into areas where additional observations were predicted to improve forecasts. In this overview of GFDex the background, aims and objectives, and facilities and logistics are described. A summary of the campaign is provided, along with some of the highlights of the experiment.The GFDex would not have been possible without the dedication and flexibility shown by all at the FAAM, DirectFlight, and Avalon. GFDex was funded by the Natural Environmental Research Council (NE/C003365/1), the Canadian Foundation for Climate and Atmospheric Sciences (GR-641), and the European Union Fleet for Airborne Research (EUFAR) and European Union Coordinated Observing System (EUCOS) schemes

Meridional Heat and Salinity Transports and the Surface Freshwater Exchange Derived from the OSNAP (Overturning in the Subpolar North Atlantic Program) Array between August 2014 and May 2018

Data from the full OSNAP array for the first 4 years between 2014 and 2018 have been used to produce the 30-day mean meridional heat (MHT) and salinity transports (MST), and the derived surface freshwater (FW) exchange time series.Related article: Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the ArcticAn international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the subpolar overturning circulation and its associated property exchanges. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin velocity, temperature and salinity in the subpolar North Atlantic. Data from the full OSNAP array for the first 4 years between 2014 and 2018 have been used to produce the 30-day mean meridional heat (MHT) and salinity transports (MST), and the derived surface freshwater (FW) exchange time series.National Science Foundation Award Number 194833

Meridional Overturning Circulation Observed by the OSNAP (Overturning in the Subpolar North Atlantic Program) Array from August 2014 to May 2018 - Originally published data

Strength of the Meridional Overturning Circulation across the OSNAP array, defined as the maximum of the streamfunction in density space.The project scientists would appreciate it if you would use the data DOI https://doi.org/10.35090/wa93-m688 and add the following acknowledgement to any publication that use this data: “OSNAP data were collected and made freely available by the OSNAP (Overturning in the Subpolar North Atlantic Program) project and all the national programs that contribute to it (www.o-snap.org).”NOTE: This item is replaced by http://hdl.handle.net/1853/65537. An error was detected in this original dataset.An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (MOC) and its variability. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin volume transports in the subpolar North Atlantic. The first 4 years of data (August 2014 - May 2018) from the full OSNAP array has been used to produce the 30-day mean estimates of the MOC at OSNAP. All data are freely available from www.o-snap.org. The corrected version of this dataset is available at https://doi.org/10.35090/gatech/65537National Science Foundation Award Number 194833