Variability and prcesses of the Denmark Strait overflow

The Atlantic Meridional Overturning Circulation (AMOC) is considered as an important part of the global climate system. The densest component of the AMOC is the Denmark Strait Overflow Water, entering the deep Atlantic across the sill between Greenland and Iceland. Here, four years of overflow measurements in Denmark Strait are analyzed. The data suggest, that the overflow consists of a density driven, hydraulically controlled part, and a barotropic, wind stress forced component. The observed overflow transport reduction of 20% from 1999 (3.7 Sv) to 2003 (3.1 Sv) is likely caused by both a dense water reservoir height decrease in the Iceland Sea, and a reduction of the local wind stress forcing. The interannual fluctuations are consistent with a reduction of the North Atlantic Oscillation (NAO). Interannual temperature variability of 0.5 °C is linked to variable upstream entrainment rates and/or variable percentages of different water masses rather than changes of the individual sources. Further, an anticorrelation with the Faroe Bank Channel Overflow is found, with the total dense water outflow from the Nordic Seas being almost constant at 5.5 Sv from 1999 to 2003

Upstream sources of the Denmark Strait Overflow : observations from a high-resolution mooring array

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 112 (2016): 94-112, doi:10.1016/j.dsr.2016.02.007.We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kgm-3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport is more consistent than the transport in either branch; compensation takes place among the pathways that maintains a stable total overflow transport. This is especially true for the two East Greenland Current branches whose transports vary out of phase with each other on weekly and longer time scales. We argue that wind forcing plays a role in this partitioning.The mooring and analysis work was supported by NSF OCE research grants OCE-0959381 and OCE-1433958, by the European Union 7th Framework Programme (FP7 2007-2013) under grant agreement n. 308299 NACLIM, and and by the Research Council of Norway through the Fram Centre Flaggship project 6606-299.2017-03-2

Upstream sources of the Denmark Strait Overflow : observations from a high-resolution mooring array

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 112 (2016): 94-112, doi:10.1016/j.dsr.2016.02.007.We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kgm-3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport is more consistent than the transport in either branch; compensation takes place among the pathways that maintains a stable total overflow transport. This is especially true for the two East Greenland Current branches whose transports vary out of phase with each other on weekly and longer time scales. We argue that wind forcing plays a role in this partitioning.The mooring and analysis work was supported by NSF OCE research grants OCE-0959381 and OCE-1433958, by the European Union 7th Framework Programme (FP7 2007-2013) under grant agreement n. 308299 NACLIM, and and by the Research Council of Norway through the Fram Centre Flaggship project 6606-299.2017-03-2

Ocean bottom pressure variability: Which part can be reliably modeled?

Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability. A question how well it can modeled by the present general ocean circulation models on time scales of 1 day and more is addressed. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed. The simulated variability is compared to a new data set from an array of PIES (Pressure-Inverted Echo Sounder) gauges deployed along a transect in the Southern Ocean. We show that while the STD of monthly averaged variability agrees well with observations except for the locations with high eddy activity, models lose a significant part of variability on shorter time scales. Furthermore, despite good agreement in the amplitude of variability, the OBP from the PIES and simulation show almost no correlation. Our findings point to limitations in geophysical background models required for space geodetic applications. We argue that major improvements in OBP modelling require data assimilation in order to increase the coherence between modelled and observed signals

Climate-Relevant Ocean Transport Measurements in the Atlantic and Arctic Oceans

Ocean circulation redistributes heat, freshwater, carbon, and nutrients all around the globe. Because of their importance in regulating climate, weather, extreme events, sea level, fisheries, and ecosystems, large-scale ocean currents should be monitored continuously. The Atlantic is unique as the only ocean basin where heat is, on average, transported northward in both hemispheres as part of the Atlantic Meridional Overturning Circulation (AMOC). The largely unrestricted connection with the Arctic and Southern Oceans allows ocean currents to exchange heat, freshwater, and other properties with polar latitudes

Transport variability of the ACC and teleconnection with the Southern Annular Mode (SAM) south of Africa.

To study the Antarctic Circumpolar Current (ACC) volume transport several cruises have taken place. The results of these cruises show snapshots without information about the time variability. To investigate the time variability of the ACC the Alfred Wegener Institute operates an array of Pressure Inverted Echo Sounders (PIES) along a satellite altimeter ground track south of Africa. PIES monitor ocean bottom pressure and acoustic travel time across the water column. A Gravest Empirical Mode (GEM, Meinen and Watts 1998) was applied to determine the geostrophic transport between the PIES. These time series were used to compute a transfer function between satellite Sderived transport and geostrophic transport. Satellite altimetry offers the possibility to calculate ACC transport between 1992 and 2010. A mean transport of 115 Sv and a variability of 7 Sv were derived for the Topex/Poseidon, Jason 1 and Jason 2 time period. A wavelet analysis shows that the ACC transport highly correlates with the winter and spring SAM index, whereas a direct correlation on a monthly scale could not be shown

Ocean Bottom Pressure Variability: Can It Be Reliably Modeled?

Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability, the knowledge of which is needed in geophysical applications. The question of how well it can be modeled by the present general ocean circulation models on time scales in excess of 1 day is addressed here by comparing the simulated OBP variability with the observed one. To this end, a new multiyear data set is used, obtained with an array of bottom pressure gauges deployed deeply along a transect across the Southern Ocean. We present a brief description of OBP data and show large‐scale correlations over several thousand kilometers at all time scales using daily and monthly averaged data. Annual and semiannual cycles are weak. Close to the Agulhas Retroflection, signals of up to 30 cm equivalent water height are detected. Further south, signals are mostly intermittent and noisy. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed to capture the variability due to eddies. Furthermore, despite good agreement in the amplitude of variability, the in situ and simulated OBP show only modest correlation

Fabrication and efficiency measurement of a Mo/C/Si/C three material system multilayer Laue lens

In this letter, we report on the manufacturing of a multilayer Laue lens (MLL) consisting of a multilayer stack with three materials: molybdenum and silicon as the absorber and spacer layer, respectively, and carbon as transition layers. The design has four layers per period: Mo/C/Si/C. It yields 6000 zones and provides an aperture of 50 μm. This allows the MLL structure to accept a large portion of the coherent part of the beam and to achieve a small spot size. The MLL deposition was made by magnetron sputtering at the Fraunhofer IWS, and the sectioning was done by laser cutting and subsequent focused ion beam milling to a thickness that provides a good efficiency for a photon energy of 12 keV. The diffraction efficiency as a function of the tilting angle has been measured at beamline 1-BM of the Advanced Photon Source. An efficiency of almost 40% has been achieved. This shows that the material system performs well compared to MLLs made of two-materials and that it is in excellent agreement with the numerically calculated efficiency for a comparable molybdenum/silicon bilayer system lens. We conclude that the three material system offers high efficiencies and is advantageous for stress reduction in MLLs