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Sampling the Southern Ocean: technology for observing the marine system

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Information Summary

Version: 1

Published: 21/06/2018 GMT

Reviewed: 21/06/2018 GMT

Authors

Oscar Schofield
Josh Kohut

Center for Ocean Observing Leadership
School of Environmental and Biological Sciences,
Rutgers University, New Jersey, USA

oscar@marine.rutgers.edu

DOI: 10.18124/D4863P


Synopsis

The Southern Ocean around Antarctica is complex and especially difficult to investigate as harsh conditions often limit oceanographic sampling. Given the range of physical, chemical, and biological processes that need to be investigated over a wide range of spatial and temporal scales, a broad suite of technologies is needed to collect data. New observational technologies are continually becoming available to oceanographers. Satellites and ship sampling are core approaches used to map ocean properties and are complemented by fixed assets include moorings and shore-based systems. Mobile platforms include profiling floats, gliders and autonomous underwater vehicles can provide spatial maps of subsurface data. Numerical models are a further crucial tool in building our understanding of marine ecosystems and processes as well as the linkages between sea and atmosphere. The coupled observational and modeling networks together provide a critical basis for improving our understanding of the ocean and predicting its likely future state.

 


Summary

Understanding the dynamics of the Southern Ocean is critical to developing our understanding of the Earth System as it plays a disproportionately large role, relative to its size, in the global climate and biogeochemistry. Sampling the Southern Ocean is difficult because the conditions are harsh due to strong winds, very rough seas, ice, extensive cloud cover and prolonged darkness in winter.  Additionally, many of the key processes span a wide range of spatial and temporal scales (Fig. 1) which no single technology is capable of sampling (Fig. 2).

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Figure 1. A Stommel diagram showing the range of spatial and temporal scales over which ocean processes operate (Source: Kavanaugh et al. [17]).

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Figure 2. The time and space sampling capabilities of different ocean platforms. The different colors represent different platforms.

 

A multiplatform approach is needed utilizing a range of technologies to sample the appropriate time and space domains [1].  Many platforms are available for making aquatic measurements and while the descriptions below are not exhaustive, they provide a snapshot of the major approaches.  The platforms carry sensors for physical, chemical, and biological properties and although most new sensors are usually shipborne some can be deployed on autonomous platforms.

Sensors

The most developed sensors are those that measure physical and geophysical variables such as temperature, salinity, pressure, currents, waves, and seismic activity.  Except for seismic variables, most of the physical sensors can be deployed on most of the platforms listed below.  Many of the physical variables are the key measurements used in ocean numerical models.  Currently chemical sensors can measure dissolved gases (primarily oxygen) and dissolved organic material, and recently sensors capable of measuring nutrients (primarily nitrogen) are becoming commercially available.  Biological sensors currently consist of optical and acoustic sensors.  The optical sensors are used to provide information on the concentration, composition and physiological state of the phytoplankton.  Acoustic sensors can provide information on the presence and amounts of organisms from zooplankton to fish, depending on the frequency band that is chosen. There is active development of new chemical sensors to measure a wide range of nutrients, salinity, pH, gases and pigments continually rather than just from water samples. Measurements, when sustained, allow researchers to document the trends in the physical, chemical and biological systems and this provides the basis for developing conceptual and numerical models to test our understanding of how the systems operate. By combining datasets in various ways, it is possible to address spatial dimensions from local to regional and then use validated numerical models to help fill in the gaps. This article provides a minimal introduction and for more information on specific sensors please look under Resources.

Platforms

Ships:  The primary platform for oceanographers has been ships and these are likely to remain a central piece of infrastructure for the foreseeable future [2]. For the Southern Ocean ice strengthened ships are required to allow year round sampling. Ships are ideal platforms, carrying the widest array of sensors but they are also expensive to operate and are often limited by sea conditions and ice. The ships currently provide the only means for collecting discrete samples that can be analyzed by advanced instrumentation, for example using CTD rosettes (Fig. 3).

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Figure 3. CTD rosette being deployed in pack ice to make measurements of the vertical profile of temperature and salinity as well as collecting water samples at various depths for later analysis (Photo: M.Hoppmann)

Expendable Bathythermographs (XBTs) allow ships to make water profile measurements whilst under way and in virtually any sea conditions. Ships can tow systems behind them collecting material ideal for mapping distributions as demonstrated by the Continuous Plankton Recorder that currently provides increasing information on the Southern Ocean (https://www.scar.org/science/cpr/home/) (Fig. 4). Sound based systems can record ocean temperature over long transects, profiles of currents using the acoustic Doppler technique whilst sidescan sonar can map the sea floor.

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Figure 4. Longhurst-Hardy continuous plankton recorder that samples the surface plankton swarms as it is towed along behind the ship and preserves the specimens on a continuous filter roll. (Photo: P. Koubbi)

Field Stations:  Many countries maintain coastal stations around the Antarctic, which provide long-term measurements on inshore marine, terrestrial and atmospheric information relevant to understanding the Southern Ocean.  A good example of this is the Palmer Long Term Ecological Research site along the western coast of the Antarctic Peninsula.

Satellites: Satellites are the most important oceanographic technology of modern times [3].  Satellite observations have resulted in numerous advances in our fundamental understanding of the oceans [4] by resolving both regional and global features associated with the mesoscale circulation of physical and biological variables such as currents and phytoplankton distribution. Satellite data is fundamental to weather and ocean state prediction. Physical parameters available from space-based sensors provide information on ocean surface temperature, wind speed and direction, sea surface topography, sea ice distribution and thickness, whilst biological and chemical parameters can be derived from ocean colour radiometers. New technology can provide sea ice thickness monitoring The advent of unmanned airborne vehicles (UAV) now offer lower altitude systems capable of collecting regional scale data.

Shore-Based Radar: High frequency radar measures ocean surface current velocities over hundreds of square miles simultaneously. Each site measures the radial components of the ocean surface velocity directed toward or away from the site [5, 6] and the estimated velocity components allow surface currents (upper metre of water column) to be estimated [7].

Ocean Moorings:  Since the 1960s, modern buoys have enabled a wide range of studies to address the ocean’s role in climate, as well as providing insights into the biogeochemistry of the sea especially through sediment traps (Fig. 5).

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Figure 5. Deployment of sediment trap that will collect particles raining down from the surface over periods of months (Photo: Skylar Beyer)

Moorings provide the backbone to many of the global ocean networks studying ocean-atmosphere interactions as well as ocean currents and strengths (Fig. 6) and are the foundation for the global tsunami warning system network.  They will continue to be a key element of ocean observing infrastructure. Many of these approaches are also used to study ocean-ice interactions by using the ice as the fixed anchor with sensors hanging in and below the ice.

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Figure 6. Acoustic Doppler Current Profiler that can be moored anywhere in the water column to measure water currents continually. (Photo: ASCA)

Drifters and Floats:  Passive drifting platforms are important tools for creating surface and subsurface maps of ocean properties. These platforms are relatively inexpensive and can thus be deployed in large numbers. Drifters carry numerous sensors to create global maps of surface circulation.  The first neutrally buoyant floats were designed to observe subsurface currents [8].  The subsurface floats were enabled in the early 1990s with communication capabilities [9] and now comprise the international ARGO programme, which has over 3800 floats deployed throughout the world oceans (Fig.7).

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Figure 7. Global deployment of ARGO floats on 1 May 2018 (http://www.argo.ucsd.edu/).

Gliders: a type of autonomous underwater vehicle that use small changes in buoyancy in conjunction with wings to convert vertical motion to horizontal motion (Fig. 8), and thereby propel themselves forward with very low power consumption [10]. Gliders follow a saw-tooth path though the water, providing data on large temporal and spatial scales. They navigate with the help of periodic surface Global Positioning System fixes, pressure sensors, tilt sensors, and magnetic compasses. Using buoyancy-based propulsion, gliders have a significant range and duration, with missions lasting up to a year and covering over 10,000 km [11, 12, 13, 14].

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Figure 8. Battery powered glider which will dive repeatedly for days on a predetermined course to collect physical and chemical data (Photo: Bluefin Robotics)

Propeller-driven autonomous underwater vehicles (AUVs):  AUVs are powered by batteries or fuel cells and can operate in water as deep as 6000 meters (Fig. 9). Like gliders, AUVs relay data to shore via satellite. Between position fixes and for precise maneuvering, inertial navigation systems are often used onboard the AUV to measure the acceleration of the vehicle and, combined with Doppler velocity technology, measures rate of travel. A pressure sensor measures the vertical position. AUVs, unlike gliders, can move against most currents nominally at 3-5 kts, and therefore can systematically survey a particular line, area, and/or volume [15].

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Figure 9. Lowering an autonomous underwater vehicle down to the seabed to investigate undamaged benthic communities (Photo: D.W.H. Walton)

Animal tags: sensor sizes have decreased dramatically allowing sensors to be attached to marine fauna (whales, birds, seals, and fish).  The animals collect data during their normal movements, providing extremely useful datasets on the physics, chemistry and biology of the areas where they live and feed [16].

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Figure 10. Camera and electronic sensors which can be fitted to elephant seals and to penguins to record diving and feeding activity also provide detailed data on Southern Ocean temperature and salinity profiles (Photo: Christophe Guinet CEBC-SNO-MEMO)