The role of gliders in sustained
observations of the ocean
Deliverable 4.1 or WP 4
Outline
• Sustained observations of the ocean
• Opportunities and challenges for glider
technology
• Examples of sustained observations fomr
gliders
• Conclusions
Sustained observations
• The third imperative of the CLIVAR program is to
“Maintain over many decades a sustained ocean
observing system capable of detecting and
documenting global climate change”.
• Sustained observations that provide the long-term
perspective for climate research.
• Human activities impact the ocean and so long term
monitoring is also important to quantify changes in the
coastal environment and is a fundamental requirement
of the European Union (EU) Marine Strategy
Framework Directive (MSFD).
Sustained sub-surface observations in
the ocean
• Repeated ocean sections. Either:
hydrographic sections completed Research
vessels or XBT sections from vessels of
opportunity
• Fixed observatory sites. E.g. hydrographic data
at Ocean Weather Ship Station (OWS) Mike in
the Norwegian Sea since 1948.
• The ARGO network of profiling floats is a new
form of sustained observation.
GO-SHIP reference sections
Map of recent and planned GO-SHIP Reference Sections. These are repeat
hydrographic sections that are coast-to-coast or coast-to-ice, follow standard WOCE
lines with small modifications as necessary for territorial waters, ice coverage, etc.,
and maintain the standard WOCE sampling strategy.
EuroSITES fixed point observatories
EuroSITES map showing sites. EuroSITE observatories are shown with
blue circles and associated sites are shown in green.
The Argo program
The distribution of Argo profiles in the North Atlantic in 2012. The 1000 and
3000m isobaths are also shown. There are no profiles in water depth less
than 1000m and sampling close to the continental slope is generally poor.
Opportunities and challenges for glider
technology
• Gliders are able to support a growing number of
oceanographic sensors and there is considerable
interest in the use of gliders as a platform for
sustained observations.
• Argo profiling floats provide a great deal of data
in the deep ocean but cannot be relied upon to
sample at specific locations and little data is
obtained on the continental slope
• GRROM partners have made a number of testing
the capability of gliders to make sustained
obsrvations.
Why use gliders?
• To provide more frequent sampling than is
possible with available ship-time or budget.
• To obtain data with higher spatial resolution.
• To obtain data in real-time for data
assimilation and for increased data security.
Sustained observations by glider in
GROOM
• There were 11 projects run by GROOM
partners in which gliders were trialed or are
regularly used for sustained observations.
• In some cases gliders have replaced other
platforms or have enabled new programs to
start
• But there some cases where gliders were
found to not yet be suitable or cost effective.
Conclusions form GROOM
deployments
Advantages of using gliders were:
• To provide more frequent sampling than is possible
with available ship-time or budget.
• To obtain data with higher spatial resolution.
• To obtain data in real-time for data assimilation and for
increased data security.
Disadvantages were
• Navigation in strong currents.
• Reliability.
• Additional resources required.
• Risks of collision with vessels.
Example 1: The Balearic Channels in
the western Mediterranean
• When measurements were
made from ships
observations were biased to
the summer months (see
plot below).
• Three years of quasicontinuous glider
observations have been
obtained
Example 2: The RAPID program at
26°N
• Gliders are effective for
measuring near surface
variables. This is very
difficult with moored
instruments
• The project requires
continuous
measurements and
the reliability of
gliders was not
sufficient to replace
the moorings.
Conclusions 1
The use of gliders is most successful when:
• Gliders are able to completely replace other platforms. In the Balearic
Channels programs deployments and recoveries are made from small
vessels and there is no need for a research vessel. In contrast in the RAPID
program gliders can replace moorings in the upper 1000m but deeper
moorings, deployed from ocean going research vessels are still required.
• There is easy access to deployment and recovery sites close to shore. In
the case of the Fram Strait program the remote location means that
deployments and recoveries had to be made from ocean going vessels and
scheduled months in advance. Almost all glider losses by GROOM
partners have been in remote high-latitude locations (Brito et al., 2014a;
2014b).
• When it is only required to sample the upper 1000m. The shallow
thermocline in the Mediterranean Sea means that for the majority of
applications the depth range of gliders is more than sufficient (Ruiz et al.,
2012). However, the deeper thermocline of the Atlantic means that it is
often desirable to make measurements to 1500m or more.
Conclusions 2
There are several types of sustained observation for
which gliders are particularly well suited:
• When real-time data is required. Whilst there
other possibilities for the telemetry of data
gliders are very effective when data is needed in
real time.
• On the continental slope where Argo data is rare.
• When near-surface data is needed. Such
measurements are difficult to make with moored
instruments.
Conclusions 3
Glider technology has advanced greatly over the last decade. Future
developments that will enable the use of gliders in more sustained
observation programs are:
• Improved reliability. Profiling APEX floats deployed in the Argo
program were 5 times more likely to complete 100 profiles in 2006
than were those deployed in 2001 (Kobayashi et al., 2008). A
similar increase in reliability needs to be achieved for underwater
gliders.
• Increased depth capability would enable more applications.
However, increasing the depth of dives alone will mean reduced
frequency of sampling at fixed levels and so an increase in errors
arising from aliasing of high-frequency variability.
• Increased endurance. The greatest benefit from using gliders is
achieved when the need for ships is eliminated. Improved
endurance would enable a larger part of the ocean to be accessible
from individual glider ports.