Enhanced turbulence driven by mesoscale motions and flow‐topography interaction in the Denmark Strait Overflow plume

The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012, a multiplatform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study, we report on two high-dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O( math formula) W kg−1 was associated with enhanced small-scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast-response thermistor. Isotherm displacement slope spectra reveal a wave number-dependence characteristic of turbulence in the inertial-convective subrange ( math formula) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom-intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy-driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow-topography interaction is suggested to drive the intense mixing at this site.

[1]  T. Fischer,et al.  Turbulence and finestructure in a deep ocean channel with sill overflow on the mid-Atlantic ridge , 2015 .

[2]  S. Dye,et al.  On the origin and propagation of Denmark Strait overflow water anomalies in the Irminger Basin , 2015 .

[3]  C. Mertens,et al.  Intra-seasonal variability of the DWBC in the western subpolar North Atlantic , 2015 .

[4]  T. Haine,et al.  Hydrostatic and non-hydrostatic simulations of dense waters cascading off a shelf: The East Greenland case , 2015 .

[5]  S. Legg.  Scattering of Low-Mode Internal Waves at Finite Isolated Topography , 2014 .

[6]  Thomas W. N. Haine,et al.  Fates and Travel Times of Denmark Strait Overflow Water in the Irminger Basin , 2013 .

[7]  V. Paka,et al.  Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume , 2013 .

[8]  M. Alford,et al.  Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage , 2013 .

[9]  K. Brink,et al.  Water column structure and statistics of Denmark Strait Overflow Water cyclones , 2013 .

[10]  R. Käse,et al.  Present-Day Manifestation of the Nordic Seas Overflows , 2013 .

[11]  P. Lherminier,et al.  On the Cascading of Dense Shelf Waters in the Irminger Sea , 2012 .

[12]  D. Quadfasel,et al.  Variability of the Denmark Strait overflow: Moored time series from 1996–2011 , 2012 .

[13]  R. Schmitt,et al.  The East Greenland boundary current system south of Denmark Strait , 2012 .

[14]  S. Dye,et al.  Wind forcing of salinity anomalies in the Denmark Strait overflow , 2011 .

[15]  M. Spall,et al.  Significant role of the North Icelandic Jet in the formation of Denmark Strait overflow water , 2011 .

[16]  I. Fer,et al.  Mixing in the stratified interface of the Faroe Bank Channel overflow: The role of transverse circulation and internal waves , 2011 .

[17]  P. M. Saunders,et al.  The Deep Western Boundary Current at Cape Farewell: Results from a Moored Current Meter Array , 2010 .

[18]  K. Polzin.  Mesoscale Eddy–Internal Wave Coupling. Part II: Energetics and Results from PolyMode , 2010 .

[19]  I. Fer,et al.  Intense mixing of the Faroe Bank Channel overflow , 2010 .

[20]  D. Quadfasel,et al.  Entrainment in the Denmark Strait overflow plume by meso-scale eddies , 2009 .

[21]  J. Swift,et al.  Sources to the East Greenland Current and its contribution to the Denmark Strait Overflow , 2008 .

[22]  H. Burchard,et al.  Gravity Current Dynamics and Entrainment—A Process Study Based on Observations in the Arkona Basin , 2007 .

[23]  J. Toole,et al.  Sensor Corrections for Sea-Bird SBE-41CP and SBE-41 CTDs* , 2007 .

[24]  J. Klymak,et al.  Oceanic Isopycnal Slope Spectra. Part II: Turbulence , 2007 .

[25]  J. Klymak,et al.  Oceanic Isopycnal Slope Spectra. Part I: Internal Waves , 2007 .

[26]  Uwe Send,et al.  Spatial and temporal structure of the Denmark Strait Overflow revealed by acoustic observations , 2007 .

[27]  Edward R. Levine,et al.  On Measuring the Terms of the Turbulent Kinetic Energy Budget from an AUV , 2006 .

[28]  S. Østerhus,et al.  CLIMATE SCIENCE: Enhanced: Already the Day After Tomorrow? , 2004 .

[29]  D. Farmer,et al.  Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf , 2003 .

[30]  J. Girton,et al.  Descent and Modification of the Overflow Plume in the Denmark Strait , 2003 .

[31]  D. Farmer,et al.  Stratified flow over topography: upstream influence and generation of nonlinear internal waves , 2003, Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[32]  R. Käse,et al.  Structure and variability of the Denmark Strait Overflow: Model and observations , 2003 .

[33]  Martin Visbeck,et al.  Deep Velocity Profiling Using Lowered Acoustic Doppler Current Profilers: Bottom Track and Inverse Solutions* , 2002 .

[34]  Douglas R. Caldwell,et al.  The Efficiency of Mixing in Turbulent Patches: Inferences from Direct Simulations and Microstructure Observations , 2001 .

[35]  P. Eriksson,et al.  Hydrographic observations in Denmark Strait in fall 1997, and their implications for the entrainment into the overflow plume , 1999 .

[36]  M. Spall,et al.  Mesoscale Variability in Denmark Strait: The PV Outflow Hypothesis* , 1998 .

[37]  S. Bacon.  Circulation and Fluxes in the North Atlantic between Greenland and Ireland , 1997 .

[38]  H. Seim,et al.  The importance of aspiration and channel curvature in producing strong vertical mixing over a sill , 1997 .

[39]  R. Dickson,et al.  The production of North Atlantic Deep Water: Sources, rates, and pathways , 1994 .

[40]  J. Jouzel,et al.  Evidence for general instability of past climate from a 250-kyr ice-core record , 1993, Nature.

[41]  R. Lueck,et al.  Thermal Inertia of Conductivity Cells: Theory , 1990 .

[42]  T. Osborn,et al.  Dissipation estimates for stratified turbulence , 1990 .

[43]  A. Gargett,et al.  Local isotropy and the decay of turbulence in a stratified fluid , 1984, Journal of Fluid Mechanics.

[44]  N. Oakey,et al.  Determination of the Rate of Dissipation of Turbulent Energy from Simultaneous Temperature and Velocity Shear Microstructure Measurements , 1982 .

[45]  Geoffrey Ingram Taylor,et al.  Statistical theory of turbulenc , 1935, Proceedings of the Royal Society of London. Series A - Mathematical and Physical Sciences.

[46]  J. Schaffer.  Small-scale variability associated with mixing in the Denmark Strait Overflow plume based on horizontally profiling observations , 2013 .

[47]  Andreas Schmittner,et al.  Ocean circulation : mechanisms and impacts : past and future changes of meridional overturning , 2007 .

[48]  C. Torrence,et al.  A Practical Guide to Wavelet Analysis. , 1998 .

[49]  J. G. Bruce.  Eddies southwest of the Denmark Strait , 1995 .

[50]  T. Osborn,et al.  Estimates of the Local Rate of Vertical Diffusion from Dissipation Measurements , 1980 .

[51]  T. von Karman,et al.  On the Statistical Theory of Turbulence. , 1937, Proceedings of the National Academy of Sciences of the United States of America.