Characteristics and causes of Deep Western Boundary Current transport variability at 34.5° S during 2009–2014

Abstract. The Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic carries a significant fraction of the cold deep limb of the Meridional Overturning Circulation (MOC), and therefore its variability affects the meridional heat transport and consequently the regional and global climate. Nearly 6 years of observations from a line of pressure-equipped inverted echo sounders (PIESs) have yielded an unprecedented data set for studying the characteristics of the time-varying DWBC volume transport at 34.5° S. Furthermore, the horizontal resolution of the observing array was greatly improved in December 2012 with the addition of two current-and-pressure-equipped inverted echo sounders (CPIESs) at the midpoints of the two westernmost pairs of PIES moorings. Regular hydrographic sections along the PIES/CPIES line confirm the presence of recently ventilated North Atlantic Deep Water carried by the DWBC. The time-mean absolute geostrophic transport integrated within the DWBC layer, defined between 800–4800 dbar and within longitude bounds of 51.5 to 44.5° W, is −15 Sv (1 Sv  =  106 m3 s−1; negative indicates southward flow). The observed peak-to-peak range in volume transport using these integration limits is from −89 to +50 Sv, and the temporal standard deviation is 23 Sv. Testing different vertical integration limits based on time-mean water-mass property levels yields small changes to these values, but no significant alteration to the character of the transport time series. The time-mean southward DWBC flow at this latitude is confined west of 49.5° W, with recirculations dominating the flow further offshore. As with other latitudes where the DWBC has been observed for multiple years, the time variability greatly exceeds the time mean, suggesting the presence of strong coherent vortices and/or Rossby Wave-like signals propagating to the boundary from the interior.

[1]  A. Ríos,et al.  The water masses along the western boundary of the south and equatorial Atlantic , 2000 .

[2]  John Marshall,et al.  Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review , 2016 .

[3]  K. Speer,et al.  Global Ocean Meridional Overturning , 2007 .

[4]  A. Piola,et al.  Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5 S , 2012 .

[5]  Gustavo Goni,et al.  Measuring the Atlantic Meridional Overturning Circulation , 2015 .

[6]  C. Schmid,et al.  An observations and model‐based analysis of meridional transports in the South Atlantic , 2016 .

[7]  D. Randolph Watts,et al.  Measuring Dynamic Heights with Inverted Echo Sounders: Results from MODE , 1977 .

[8]  H. Bryden,et al.  Variability of Antarctic Bottom Water at 24.5°N in the Atlantic , 2011 .

[9]  S. Krastel,et al.  Morphosedimentary and hydrographic features of the northern Argentine margin: The interplay between erosive, depositional and gravitational processes and its conceptual implications , 2013 .

[10]  Robert F. Cahalan,et al.  Sampling Errors in the Estimation of Empirical Orthogonal Functions , 1982 .

[11]  S. Dong,et al.  Decadal Modulations of Interhemispheric Global Atmospheric Circulations and Monsoons by the South Atlantic Meridional Overturning Circulation , 2016 .

[12]  J. Sprintall,et al.  Basin-wide oceanographic array bridges the South Atlantic , 2014 .

[13]  Y. Masumoto.  A fifty-year eddy-resolving simulation of the world ocean : Preliminary outcomes of OFES (OGCM for the Earth Simulator) , 2004 .

[14]  Nelson G. Hogg,et al.  Direct measurement of the deep circulation within the Brazil Basin , 1999 .

[15]  Karen L. Tracey,et al.  On Gulf Stream Meander Characteristics Near Cape Hatteras , 1986 .

[16]  Gustavo Goni,et al.  Dynamics of the Brazil‐Malvinas Confluence based on inverted echo sounders and altimetry , 1996 .

[17]  T. Rossby,et al.  On monitoring depth variations of the main thermocline acoustically , 1969 .

[18]  T. M. Chin,et al.  Basin-Scale, High-Wavenumber Sea Surface Wind Fields from a Multiresolution Analysis of Scatterometer Data , 1998 .

[19]  J. Toole,et al.  Transport of the North Atlantic Deep Western Boundary Current about 39°N, 70°W: 2004–2008 , 2011 .

[20]  F. Schott,et al.  Break-up of the Atlantic deep western boundary current into eddies at 8° S , 2004, Nature.

[21]  Hitoshi Uehara,et al.  An Eddy-Resolving Hindcast Simulation of the Quasiglobal Ocean from 1950 to 2003 on the Earth Simulator , 2008 .

[22]  Andrew D. Greene,et al.  Mapping Circulation in the Kuroshio Extension with an Array of Current and Pressure Recording Inverted Echo Sounders , 2010 .

[23]  Characterization of the multiple equilibria regime in a global ocean model , 2007 .

[24]  G. Siedler,et al.  Path of the North Atlantic Deep Water in the Brazil Basin , 1998 .

[25]  D. Chelton,et al.  Global observations of large oceanic eddies , 2007 .

[26]  H. Bryden,et al.  South Atlantic overturning circulation at 24°S , 2011 .

[27]  S. Dong,et al.  Interannual variations in the Atlantic meridional overturning circulation and its relationship with the net northward heat transport in the South Atlantic , 2009 .

[28]  W. Johns,et al.  Transport variability of the Deep Western Boundary Current and the Antilles Current off Abaco Island, Bahamas , 2004 .

[29]  D. Olson,et al.  Changes in Antarctic Bottom Water properties in the western , 1996 .

[30]  Stuart A. Cunningham,et al.  The Atlantic Meridional Overturning Circulation , 2013 .

[31]  W. Zenk,et al.  Circulation and Variability at the Southern Boundary of the Brazil Basin , 1999 .

[32]  P. DiNezio,et al.  Observed low frequency variability of the Brazil Current front , 2011 .

[33]  P. Polito,et al.  Do eddies ride on Rossby waves , 2015 .

[34]  A. Thurnherr,et al.  A Zonal Pathway for NADW in the South Atlantic , 2005 .

[35]  Richard A. Wood,et al.  Global Climatic Impacts of a Collapse of the Atlantic Thermohaline Circulation , 2002 .

[36]  P. Cornillon,et al.  Properties of Rossby Waves in the North Atlantic Estimated from Satellite Data , 2004 .

[37]  R. Evans,et al.  Temporal variations in the separation of Brazil and Malvinas Currents , 1988 .

[38]  William E. Johns,et al.  Continuous, Array-Based Estimates of Atlantic Ocean Heat Transport at 26.5°N , 2011 .

[39]  S. Garzoli,et al.  The South Atlantic and the Atlantic Meridional Overturning Circulation , 2011 .

[40]  Susanne L. Weber,et al.  The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates , 2011 .

[41]  C. Meinen,et al.  Vertical structure and transport on a transect across the North Atlantic Current near 42°N: Time series and mean , 2000 .

[42]  H. Bryden,et al.  Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises , 2015, Science.

[43]  Walter H. F. Smith,et al.  Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings , 1997 .

[44]  R. Matano.  On the Separation of the Brazil Current from the Coast , 1993 .

[45]  S. Dong,et al.  Temporal variability of the meridional overturning circulation at 34.5°S: Results from two pilot boundary arrays in the South Atlantic , 2013 .

[46]  S. Dong,et al.  The fate of the Deep Western Boundary Current in the South Atlantic , 2015 .

[47]  S. Dong,et al.  Temporal variability of the South Atlantic Meridional Overturning Circulation between 20°S and 35°S , 2015 .

[48]  W. Johns,et al.  Variability of the Deep Western Boundary Current at 26.5°N during 2004–2009 , 2013 .

[49]  A. Mariano,et al.  A Consistent Theory for Linear Waves of the Shallow-Water Equations on a Rotating Plane in Midlatitudes , 2007 .

[50]  S. Garzoli,et al.  Variability in Deep Western Boundary Current transports: Preliminary results from 26.5°N in the Atlantic , 2006 .

[51]  U. Send,et al.  Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data , 2011 .

[52]  C. Meinen,et al.  Calibrating Inverted Echo Sounders Equipped with Pressure Sensors , 1998 .

[53]  William E. Johns,et al.  Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic , 2012 .

[54]  J. Fischer,et al.  The shallow and deep western boundary circulation of the South Atlantic at 5-11°S , 2005 .

[55]  D. Smeed,et al.  Compensation between meridional flow components of the Atlantic MOC at 26° N , 2016 .

[56]  K. Speer,et al.  Large-Scale Vertical and Horizontal Circulation in the North Atlantic Ocean , 2003 .

[57]  Jonathan V. Durgadoo,et al.  Interannual to decadal changes in the western boundary circulation in the Atlantic at 11°S , 2015 .

[58]  Timothy W. Liu,et al.  Global characterization of Rossby waves at several spectral bands , 2003 .

[59]  Noel Keenlyside,et al.  Tropical sea surface temperature, vertical wind shear, and hurricane development , 2007 .

[60]  S. Garzoli,et al.  Baroclinic instabilities and forced oscillations in the Brazil/Malvinas confluence front , 1990 .

[61]  F. Schott,et al.  Circulation and Deep-Water Export at the Western Exit of the Subpolar , 2004 .

[62]  S. Garzoli,et al.  Geostrophic Velocity Measurement Techniques for the Meridional Overturning Circulation and Meridional Heat Transport in the South Atlantic , 2011 .

[63]  A. Gordon,et al.  Geostrophic circulation of the Brazil-Falkland confluence , 1986 .

[64]  S. Garzoli,et al.  Meridional heat transport determined with expandable bathythermographs—Part II: South Atlantic transport , 2007 .

[65]  S. Dong,et al.  South Atlantic meridional fluxes , 2013 .

[66]  D. Chelton,et al.  Global Observations of Oceanic Rossby Waves , 1996, Science.

[67]  C. Wunsch,et al.  Large-Scale Ocean Heat and Freshwater Transports during the World Ocean Circulation Experiment , 2003 .

[68]  M. Arhan,et al.  On the deep water circulation of the eastern South Atlantic Ocean , 2003 .

[69]  W. Dewar,et al.  Subsurface Energetics of the Gulf Stream near the Charleston Bump , 1985 .

[70]  Linear Waves in Midlatitudes on the Rotating Spherical Earth , 2009 .

[71]  William E. Johns,et al.  Variability of shallow and deep western boundary currents off the Bahamas during 2004–05: results from the 26°N RAPID–MOC Array , 2008 .

[72]  S. Dong,et al.  Seasonal variations in the South Atlantic Meridional Overturning Circulation from observations and numerical models , 2014 .

[73]  Robert E. Synovec,et al.  Data Analysis Methods , 2012 .

[74]  S. Garzoli,et al.  Interannual to decadal changes in the western South Atlantic's surface circulation , 2011 .

[75]  S. Garzoli.  Geostrophic velocity and transport variability in the Brazil-Malvinas Confluence , 1993 .

[76]  S. Dong,et al.  The Role of Interocean Exchanges on Decadal Variations of the Meridional Heat Transport in the South Atlantic , 2011 .

[77]  C. Schmid.  Mean vertical and horizontal structure of the subtropical circulation in the South Atlantic from three-dimensional observed velocity fields , 2014 .

[78]  C. Provost,et al.  Brazil Malvinas Frontal System as seen from 9 years of advanced very high resolution radiometer data , 2004 .

[79]  D. Smeed,et al.  Ocean impact on decadal Atlantic climate variability revealed by sea-level observations , 2015, Nature.

[80]  K. Trenberth,et al.  The atmospheric energy budget and implications for surface fluxes and ocean heat transports , 2001 .

[81]  Andrei P. Sokolov,et al.  Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes , 2006 .

[82]  D. Randolph Watts,et al.  Deep-Ocean Bottom Pressure Measurement: Drift Removal and Performance , 1990 .