Monday, 09 April
Session: OS1.5/AS1.29/CL2.14, Climate variability of the Atlantic and Europe (co-organized), Room L3
First Results from OSNAP: Overturning in the Subpolar North Atlantic Program EGU2018-10132
Susan Lozier and Feili Li and the OSNAP Observations Team
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 (AMOC) and its variability. With high resolution mooring arrays from the Labrador coast to the Scottish shelf, OSNAP provides a continuous record of the full water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic and has been operational since 2014. Data from the first 21 months of the full OSNAP observing system has been used to produce the first continuous time series of these variables. In addition to these time series, time mean estimates for all fluxes and attendant uncertainties will be presented, along with comparisons with other contemporaneous AMOC measurements and a discussion of subpolar overturning variability.
Circulation in the Eastern Subpolar North Atlantic Based on Three Years of Mooring Measurements EGU2018-14519
Stuart Cunningham, Loïc Houpert, Clare Johnson, Neil Fraser, Stefan Gary, Mark Inall, and Penny Holliday
The subpolar North Atlantic is a globally important region for climate, where ocean-atmosphere exchanges are intense. Around two thirds of the Atlantic Meridional Overturning Circulation (AMOC) deep limb is formed through deep winter mixing in the Nordic Seas, and entrainment as overflows cross the Greenland-Scotland Ridge. The desire to understand the variability of volume, heat and freshwater fluxes and their relationship with water mass transformations, led to the establishment of a purposefully designed international observing system. The Overturning in the Subpolar North Atlantic Program (OSNAP) was deployed in July 2014 and consists of multiple transatlantic mooring arrays supplemented by hydrographic and glider observations. Here, we present the first three years of results from the eastern boundary of the array which comprises of four moorings within the Rockall Trough. These moorings capture the warmest and saltiest branch of Atlantic Water on the OSNAP section: transporting heat and salt to the Nordic Seas. Transports at the boundaries of the Rockall Trough are estimated from direct current measurements, whilst fluxes in the interior are calculated from end-point density moorings referenced to altimetry. The resulting transports are the first continuous, direct measurements for the warm water path through the Rockall Trough. Using the OSNAP data we are able to quantify the mean flow and short-term variability year-round for the first time. Finally, we place our results in context by comparison with multi-decadal length hydrographic time-series and discuss the implications for both the regional and large-scale circulation in the subpolar North Atlantic.
US AMOC Meeting
July 24-27, 2018 in Coconut Grove, FL
Circulation in the Eastern Subpolar North Atlantic Based on Three Years of Mooring Measurements
Stuart Cunningham, Loic Houpert, Clare Johnson, Stefan Gary, Mark Inall, Neil Fraser, Penny Holliday
The principal objective of the Overturning in the Subpolar North Atlantic Program (OSNAP) is to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic and relate those to water-mass transformations and AMOC variability. The UK led Eastern Boundary (EB) array is designed to (i) quantify the flux of northward-flowing warm and saline water through the Rockall Trough and, (ii) across the Rockall-Hatton Plateau (Houpert et al. 2018 submitted), and (iii) determine the magnitude and variability of the cold overflow across the Wyville-Thomson Ridge (Johnson et al. 2017). Here, we present the first ever continuous, direct measurements for the warm water path through the Rockall Trough. The array consists of four moorings within the Rockall Trough. Transports at the boundaries are estimated from direct current measurements, whilst fluxes in the interior are calculated from end-point density moorings referenced to altimetry (though the majority of transport is contained in the baroclinic shear). The 3-year mean transport is 7.7±4.9 Sv, which is ~100% larger than canonical hydrographic estimates assuming a mid-depth reference level (based on several decades of observations). The meridional flow structure has -1.9±2.4 Sv in a western boundary flow with a typical speed of -0.1 m/s surface-to-seabed, but with a velocity maximum near 1400 m. In the mid-basin we find 6.7±4.1 Sv is the main transport of warm and salty North Atlantic Current and the transport is mostly explained by the baroclinic shear. There is a shelf-edge current of 2.9±1.8 Sv centred at 200m depth and with a width of about 15 km. Comparing our observations to the Met Office 7 km Atlantic Margin Model we find a close correspondence between the observed and modelled variability of the shelf-edge current. We use the model to fill in data gaps caused by mooring losses in this highly fished boundary zone. Some of the higher than anticipated net transport appears, from Argo float trajectories, to circulate back to the Iceland Basin south of the Wyville Thomson Ridge. We will discuss the implications of our new results for our understanding both the regional and large-scale circulation and fluxes in the subpolar North Atlantic
Structure and transport of the North Atlantic Current in the eastern subpolar gyre from sustained glider observations
L. Houpert, M. Inall, E. Dumont, S. Gary, C. Johnson, M. Porter, W. Johns, S. A. Cunningham
There is mounting evidence of the importance of the transports of heat and freshwater by the North Atlantic Subpolar Gyre (NASPG) for impacts on European and global climate. To adequately measure the NASPG and understand its dynamics, an international transoceanic observing system was set up in the subpolar North Atlantic in the framework of the Overturning in the Subpolar North Atlantic Program (OSNAP). One of the objectives of the UK led Eastern Boundary (EB) array is to quantify the circulation and transport of the North Atlantic Current (NAC) through Rockall Hatton Plateau (RHP). The RHP is an extensive region in the Iceland Basin, with typical depths of 1000-1500m unobserved by Argo float. Uncertainties over the net circulation led to the design of a glider endurance line across the RHP to a deep dynamic height endpoint mooring in the Iceland Basin at 21°W.
Using sixteen gliders sections collected along 58°N and between 21°W and 15°W, absolute geostrophic velocity are calculated and subsequently characterized the horizontal and vertical structure of the transport. The mean northward transport (+/- standard deviation) is 4.9 +/-3.4 Sv over the RHP. In summer (May to October), the northward transport appear to be stronger (6.7 +/- 2.6 Sv) and accounts for 50\% of the total NAC transport of upper-ocean waters (?0 < 27.55 kg.m-3) estimated by Sarafanov et al.  along 59.5°N between the Reykjanes Ridge and Scotland. Two quasi-permanent northward-flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 +/- 2.1Sv) over the eastern flank of the Iceland Basin (20.5°W to 18.5°W); and (ii) the Rockall Bank Jet (1.5 +/- 0.7Sv) over the eastern flank of the Hatton-Rockall Basin (16°W to 15°W). In addition, a southward flow of 1.1 +/- 1.4 Sv is observed over the western flank of the Hatton-Rockall Basin (18.5°W to 16.0°W)
Uncertainties are estimated on each individual glider section using a Monte Carlo approach and the mean uncertainty on the absolute transport is less than 0.5Sv. Although comparisons with altimetry-based estimates indicate similar circulation patterns over the RHP, altimetry data are unable to resolve the small structure of the topographically-defined mean circulation of the Hatton-Rockall Basin.