Icebreakers Continually Measure ADCP Transects Across Drake Passage
Despite its remote location in high austral latitudes, the Southern Ocean has far-reaching influence on Earth’s climate and global ocean circulation. Recent climate studies clarified the Southern Ocean’s role in absorbing large amounts of CO2 and heat that would otherwise reside in the atmosphere.
Oceanographic studies found widespread intense mixing and large uptake of wind energy that reinforce the region’s importance for the global overturning circulation. Furthermore, the Southern Ocean is a sensitive indicator of the global ocean’s response to changing climate—especially shifting winds and warming.
Geographic location of long-term repeated ADCP transects across Drake Passage. This program emblem identifies US Antarctic supply ship R/V L.M. Gould. Image credit: T. Chereskin, Scripps Institution of Oceanography.
Deep, strong ocean currents circle the globe in the Southern Ocean. This influential flow, known as the Antarctic Circumpolar Current, connects all three major oceans. Its narrowest constriction is Drake Passage, a location notorious for its challenging seas. Despite the difficulty, Drake Passage has seen several enduring studies by US, UK, and French researchers. For two decades, ADCPs carried by US Antarctic supply ships have continually measured currents during crossings of Drake Passage.
Various forcing mechanisms produce upper-ocean currents in Drake Passage. The repetition and depth of the ADCP transects are helpful for sorting out the different types of currents. These well-resolved ADCP transects have been used to explore features of the Antarctic Circumpolar Current and its energetic eddy field. A range of scientific papers followed. Some described the spatial distribution and vertical structure of currents and plankton. Others examined dynamical properties.
Since 1999, two supply ships of the US Antarctic Program have repeated ADCP transects while crossing Drake Passage about twice per month. The ships move at 5 m/s so that each crossing of Drake Passage takes about two days.
NSF Polar Programs provided continuous funding for this program. Researchers from Scripps Institution of Oceanography and University of Hawaii drove this effort, measuring sustained changes in the speed, transport, and distribution of the prevailing currents.
Initially, a hull‐mounted 150 kHz ADCP profiled currents to a depth of 300 m. Five years later, a second 38 kHz ADCP was added to reach 1000 m. The two frequencies offer complementary range / resolution capabilities, with data sets having vertical resolution of 8 m and 24 m respectively. Unique aspects of these installations included the ADCP’s immersion in anti-freeze solution, and their protection during ice breaking operations.
A variety of scientific studies have used these ADCP transects to explore features of the Southern Ocean, particularly the Antarctic Circumpolar Current. Most work has considered water currents. Two reports focused on ADCP echo intensity data, describing patterns in plankton distribution—especially the near- surface scattering layer.
Separating Antarctica from South America, Drake Passage constricts the flow to a width of 800 km. Farther upstream, many frontal filaments demark boundaries between different water masses. At Drake Passage, these filaments converge into three predominant fronts: Subantarctic (SAF), Polar (PF), and Southern Antarctic Circumpolar Current (SACCF).
These frontal regions are distinguished by clear changes in water properties and strong narrow currents, called “jets.” The current field is enriched with motions due to eddies, frontal meandering, and strong winds. Some zonation is observed; for example, eddy energy is greater northward of the Polar Front.
Teledyne RDI 38 kHz phased-array ADCP mounted in US Antarctic icebreaker R/V Palmer. Photo credit: T. Chereskin, Scripps Institution of Oceanography.
From the mid-1980s, ADCPs on research ships provided a new 2-D view of upper-ocean currents: along-track and through depth. Two different types of measurements are merged to observe currents from a ship underway. The first is the apparent velocity of the water when seen from the moving ship; the second is the motion of the ship. Away from coastal regions, a GPS or GNSS generally supplies the latter.
Over two decades, the scientific value of this data type was boosted by two technological improvements: advanced GPS capability, which more accurately recorded the ship’s heading; and far deeper current profiling, provided by Teledyne RDI’s phased-array ADCPs operating at much lower frequency.
Researchers at the University of Hawaii and other institutions developed best practices for extracting high-quality ADCP sections (https://goo.gl/zxB5Pk). They also improved understanding of the environmental influences on the vertical reach of shipboard ADCPs.
Accurately mapping the velocity of deep ocean currents from a ship moving faster than 3 m/s is challenging. It demands that shipboard ADCPs operate at low frequency yet keep narrow acoustic beams. This pairing resulted in large-sized transducer assemblies.
Teledyne RDI developed a more compact phased-array design. It emits four narrow beams of sound from a single transducer face. This advance made it feasible to install 38 kHz ADCPs on ships. In turn, this frequency permitted profiling to 1000 m depth consistently.
The observed profiling range can vary with acoustic backscattering conditions. The deepest profiling range recorded for a Teledyne RDI ADCP is 1600 m.
Several recent additions to the US research fleet carry 38 kHz phased-array DCPs, including R/Vs Neil Armstrong and Sally Ride. Profiling to depths beyond 1000 m, these ADCPs have provided some impressive transects. One cruise running lines across the Gulf Stream recorded detailed sections o f t h e c e l e b r a t e d d e e p undercurrent that flows counter to the overlying waters.
Long time-series measurements in oceanography are uncommon because their collection and funding are difficult. Yet such series are prized for assessing how the ocean is responding to a changing climate. A standout feature of the ADCP transects across Drake Passage is their 20-year duration. Other distinctions are their high spatial resolution (5 km), depth (1000 m), and accuracy (1 cm/s).
Under the leadership of Dr. Teresa Chereskin, scientists at Scripps Institution of Oceanography used the ADCP transects to study dynamical features. One was persistent structure of the Antarctic Circumpolar Current. By examining vertical structure of the upper kilometer, the researchers identified distinctive aspects of the current field.
For example, the energy levels of eddy motions showed marked changes in depth whereas average current conditions did not. In fact, speeds to 20 cm/s persisted at a depth of one kilometer.
The dominant vertical direction of energy propagation flipped between upper ocean and greater depths. This is consistent with internal wave sources: storms at the surface and submarine ridges at depth.
Due to the sharply defined spatial structure in the ADCP transects, scientists could quantify reliably the frontal regions and energetic eddy fields. In particular, current speeds of narrow frontal jets appear more accurate rather than smeared, and quiet regions near frontal jets are unmasked.
The longevity of the ADCP program permits more reliable statistical calculations and has produced notable results. One was to discern characteristics of the mean Ekman layer and its associated heat flux. These features tend to be masked in mid-latitude oceans.
Eddies were seen to produce a momentum convergence that reinforces the frontal zones. And, by combining ADCP data with XBT temperature profiles, scientists found a large poleward heat flux in the near‐surface layer.
With the upsurge of scientific interest in the Southern Ocean, the enduring ADCP transects across Drake Passage will likely see more attention.
Average currents in upper 300 m of Drake Passage. Processing merged ADCP data from more than 250 transects spanning a dozen years. Gray lines show frontal zones (see text). Image credit: J. Sprintall, et al., 2012, Scripps Institution of Oceanography.
Sprintall, J., T.K. Chereskin, and C. Sweeney. 2012. High-resolution underway upper ocean and surface atmospheric observations in Drake Passage: Synergistic measurements for climate science. Oceanography 25(3):70–81, http://dx.doi.org/10.5670/oceanog.2012.77. https://goo.gl/bnLD3Q
Firing, E., J.M. Hummon, and T.K. Chereskin. 2012. Improving the quality and accessibility of current profile measurements in the Southern Ocean. Oceanography 25(3):164–165, http://dx.doi.org/10.5670/ oceanog.2012.91. https://goo.gl/zxB5Pk
Polarstern Expedition Team Departs for t
Subsea 7 S.A. Awarded Two Contracts Offs
Ashtead Technology Announces Two Key App
ABS to Class Two Newbuild Offshore Const
Northern and Southern Hemisphere Clim
Two Global Leaders Join Forces in Brazil
TidalStream Triton Platform Chosen for B
InterMoor UK Further Expands its Managem
SMD Supply Bibby With Two Quasar Work Cl
Nakilat Damen Launches First Two Mooring
Two International Commissions Select HYP
Polarcus Receives Letter of Award for 3D
iSURVEY Awarded Two Year Contract with I
Sound Ocean Systems, Inc. Delivers Two M
Livermore Scientists Suggest Ocean Warmi
Licences granted for two new interconnec
KVH Wins Two Prestigious Product Awards
NOAA Team Discovers two Vessels from WWI
Global Efforts to Protect Southern Ocean
Makai’s OTEC Turbine and Two New Heat Ex
iXBlue Delivers two OCEANO Heavy Duty Ac
ASI MARINE WINS ON TWO CONTINENTS WITH F
AXYS Deploys two Dual-LiDAR WindSentinel
PARTRAC Deploys Two AXYS Technologies Wi
Bibby Subsea Contracts Two New HD ROVs f
Two New Environmental Observation System
Global Establishes Southern California L
Providence Resources P.l.c Provides Tec
Harkand Achieves Two Survey Firsts in Me
EdgeTech 6205 MPES Highlighted in Two Re
EdgeTech 6205 MPES Highlighted in Two Re
IMCA Publishes Revisions of Two Importan
Blue Ocean Monitoring Opens Two New Offi
The Southern Ocean Is Breathing in Carbo
DNV GL Launches Two New JIPs with Potent
Swire Seabed Strengthens Its Capabilitie
2G Robotics Delivers Two Deep-rated Lase
Flying Lab to Investigate Southern Ocean
Louis Dreyfus Travocean Order Two Saab S
Two Chinese Research Institutes Orders T
In the Southern Ocean, a Carbon-Dioxide
NASA Takes Part in Airborne Study of Sou
Antarctic Coastline Images Reveal Four D
Unique Group's Diving & Life Support Div
FSI Announces Two Products for Relocatio
Three Decades of Underwater Excellence
Polarcus Awarded Two Projects Offshore S
BP Announces Start of Drilling on New Pr
Intertek Appointed for Specialist Study
Veolia and Peterson Partnership Awarded
Pioneer Consulting to Provide Procuremen
DeepSea Power & Light Adds Two New Camer
Wood Group Awarded Two Mad Dog Phase 2 C
Prysmian Awarded Two New Contracts for O
CTF-150 Reports Second Seizure of Heroin
Statoil Awards Transocean Two Rig Contra
Forum Sells Two Comanche ROVs to Interna
Ocean Infinity Purchases Two Additional
KONGSBERG to Market Two New Underwater G
Maryland PSC Awards ORECS to Two Offshor
Southern Cross NEXT Cable Making Pacific
Ocean Infinity Purchases Two Additional
MacArtney Opens New Office in Southern U
iSURVEY Boosts Business Development Team
Turkey Takes Delivery of First Two Damen
ENGIE Announces First Gas from Cygnus Br
Southern Ocean Wave Buoy Heading for Chi
HRI Appoints Two International Chairs fo
Statoil Achieves Climate Target Two Year
Future USS Little Rock to be Commissione
Fugro Wins Two Offshore Wind Contracts f
Danos Announces Two New Executive Leader
The EIC Appoints Two New Board Members
Secretary of the Navy Names Two Littoral
Two Global Diving VP’s Elected to Promin
Unique System Tales Delivery of Two Hydr
CGG Appoints New CEO & the Cooptation of
BP Starts Two New Developments in the No
EnerMech Appoints Two Senior Executives
Algeria to be Connected to Two New Subse
CMHI and DNV GL Ink LoI for Two Multi-Ac
MetOcean Solutions Measures Record Wave
SeaRobotics Delivers Two USV’s to the Ca
BP’s World Energy Review 2018: Two Steps
Rock Seven and Blue Sky Network in Partn
Network Innovations and Rock Seven in Te
Rotech Expands to the Middle East with T
Neptune Energy Group Announces Two Senio
Rotech Subsea Wraps Up Two Major Work Sc
Siemens Gamesa Awards First Two Contract
Offshore Southern Greece Test Site for S
Two MVP30-350 Systems for Geological Sur
New Research Expedition to Study the Sou
JFD Delivers the 2nd of Two Third Genera
Navy Awards Contract for Construction of
Researchers Define a Distinct Mantle Dom
Arctic Sea Ice Has Been in Decline for D
U.S. Sends Two Carrier Strike Groups to
MacArtney to Deploy Two Unmanned Winch S
Seaway 7 Awarded Major Contract for Horn