Summary Document: Observing High Latitudes: extending the core Argo array
E. van Wijk1, S. Wijffels2, S. Riser3, S. Rintoul2, K. Speer4, O. Klatt5, O. Boebel5, B. Owens6, J.C. Gascard7, H. Freeland8, D. Roemmich9, A. Wong3.
The polar oceans of both hemispheres are important for deep ocean ventilation and the
sequestration of heat and gases (including carbon). The Southern Ocean connects the global ocean
basins and regulates the meridional overturning circulation. The exposed Arctic Ocean will affect
ocean and atmospheric circulation, moisture and heat fluxes. Therefore, both polar regions play a
critical role in setting the rate and nature of global climate variability through their moderation of
the earth’s heat, freshwater and carbon budgets.
Recent studies suggest that some of the strongest climate change signals are already underway in
the high latitudes. The Southern Ocean is warming more rapidly than the global average (Gille
2008, Boning et al., 2008). Increased precipitation and ice melt is linked to widespread freshening
of the upper and deep layers of the Southern Ocean (Aoki et al., 2005, Rintoul 2007). Small
increases in sea ice extent in the Ross Sea are partially offset by large decreases near the Antarctic
Peninsula (Meredith and King, 2005, Stammerjohn et al., 2008). In the Arctic, widespread
warming and changes in water mass properties and convection are observed (Polyakov et al.,
2005, Yashayaev et al. 2008). The rapid loss in Arctic summer sea ice extent as demonstrated by
the recent extreme 2007 sea ice minimum (Turner and Overland, 2009) is projected to increase
the seasonal ice zone and albedo under future climate scenarios (Lee et al., 2009). Routine,
sustained observations are required in order to detect, interpret and understand global climate
change (Rintoul et al., 2009, Lee et al., 2009).
Argo has revolutionised observational oceanography over the past decade by providing more
profiles than collected historically through ship-based hydrography. The original design of the
Argo mission specified nominal 3 x 3 degree spacing, with 10 day sampling interval, of the
oceans between 60 ˚N and 60 ˚S, excluding the high latitudes and marginal seas (Argo Steering
Team et al., 2009). The exclusion of the high latitudes was due to the inability of early floats to
sample under sea-ice. Technological advances in float design in recent years now give us this
capability. Advancements have come through re-design of hardware (i.e. armoured ice floats with
ice-hardened antennae), software (ice-avoidance algorithms and open-water test) and
communications (Iridium), allowing the transmission of stored winter profiles.
Observing the circulation in the seasonal ice zone is challenging and sampling at these higher
latitudes is less systematic than for the rest of the globe. To date, most observations have been
made during ice-free summer periods and consequently winter circulation beneath the sea-ice is
not well understood. Despite this, the Argo network has dramatically increased the total, and
importantly, the seasonal hydrographic coverage in the upper 2000 m of the water column (Figure
1). As of December 2008, over 100 floats had been deployed north of 60 ˚N and over 200 south of
60 ˚S. Approximately 60% of these are still active (failure rate of early floats was high as the icecapable technology was developed and tested). Mortality rates of newer ice floats are now
equivalent to those deployed in less demanding conditions. In fact, a number of floats deployed in
the Weddell Sea have survived for 7 years, equal to some of the longest-lived floats deployed
globally.
Figure 1. Data coverage from Argo (red dots) and hydrography (blue dots) over the past decade in the
Arctic (top panels) and Antarctic (bottom panels). The position of the late summer ice edge (light blue
line) and late winter ice edge (green line) during 2007 is shown. The number of Argo profiles in the
Antarctic (6586) far surpasses the number from hydrography (1977). In the Arctic, profiles from shipbased hydrography (19517) still outnumber those from Argo (8359).
Floats drifting in the seasonal ice zone require Iridium communications to store winter profiles
and software revisions to avoid ice. Several ice-avoidance algorithms are available; the Alfred
Wegener Institute and University of Washington, temperature-based, ice-sensing algorithms and
the Woods Hole Oceanographic Institute, communications-based, open-water test. Floats with the
latter algorithm also require an ice-hardened antenna (€ 850/$1,200 USD) to prevent damage
whilst attempting to surface under ice.
Argo float profiles can only be geolocated using satellite systems when the float surfaces. In
heavy ice, however, floats may not be able to find leads through which to surface and establish
communications. Under-ice geolocation can be achieved through an array of moored sound
sources (RAFOS) and floats carrying acoustic receivers. Developing basin-scale geopositioning
and communications has been highlighted as a key priority for observing systems in both the
Arctic and Southern Oceans (Calder et al., 2009, Lee et al., 2009 and Rintoul et al., 2009). Due to
logistical and cost constraints it is impractical to ensonify the entire Southern Ocean. Therefore
we recommend several regional arrays that will provide a more focused effort on basin scales.
An established array of sound-sources (RAFOS) and acoustically-tracked floats in the Weddell
Sea is already yielding valuable information on ocean circulation and structure beneath the sea-ice
(refer to Fig 6, Rintoul et al., 2009). The effective range of RAFOS acoustic tracking is at least
900 km with the sources presently in use (Figure 2). Reduced signal transmission occurs during
winter due to the reflection of sound underneath ice. To ensure coverage of the entire Weddell
Gyre, an array of 14 moored sound sources is required. Currently 10 active sources are in place,
with an expected lifespan of 6 years. A similar array should be established to sample the Ross
Gyre. The total cost of a single moored sound source is approx € 60,000 ($86,000 USD). Other
commercially available sound sources with greater range have not yet been tested in this context
but should also be considered. In addition, floats require an acoustic receiver which costs € 1,000
($1,430 USD). In the Arctic, an array of low frequency (< 100 Hz) sound sources could provide
geo-location across the entire basin (refer to Fig 5, Lee et al., 2009 for notional design of such an
array).
Figure 2. Transmission quality with time of year and distance from the sound source. The colour of the dot
represents signal quality, the higher the correlation height, the better the transmission. Black dots indicate
that the float was too far from the source to receive a useful signal.
In considering sampling strategies for the high latitudes we recommend extending the Argo
network beyond 60 ˚S and 60 ˚N through the deployment of ice-capable floats. At the nominal
design density, 360 floats would be required in the Southern Ocean and 300 floats in the Arctic.
Calculations are based on the number of floats required to populate the area between 60˚ of
latitude and the fast ice edge with an assumed profiling depth of 2000 m in the Antarctic and 900
m in the Arctic. Of the Southern Ocean floats, 85 floats require acoustic receivers in the Weddell
Gyre and 110 floats in the Ross Gyre. Deployment opportunities in the high latitudes will be
limited by reduced ship traffic and higher charter costs. Developing links with Antarctic tourist
vessels may open up additional deployment opportunities.
Summary
In the discussions above we’ve outlined the scope and requirements of a high latitude float array
that can be implemented now with proven technology. Investing in an array of ice-capable floats
and acoustic arrays will enable cost-effective, sustained autonomous measurements of the ocean
state in polar regions. Looking to the future, we envisage profiling floats that are able to penetrate
into the deep oceans (below 2000 m) and that will carry a suite of additional sensors (biological
and chemical) to measure primary productivity, nutrients, and perhaps even carbon - through the
proxy measurements of pH and nitrate.
Reviewing a decade of progress within Argo, we find there is considerable support and
justification for the official extension of the array into the high latitudes. Sustained,
comprehensive observations of the polar oceans are required to adequately monitor global climate
change signals. These can only be achieved in a broad-scale and cost-effective way by using
autonomous platforms like Argo profiling floats. It is thus imperative that a commitment is made
to enhance and maintain an array of ice-capable floats in the seasonal ice zone. In addition,
moored arrays of sound sources and floats equipped with RAFOS receivers will enable basinwide geolocation under-ice. The existing array in the Weddell Sea should be maintained and
completed, and similar arrays established in the Ross Sea Gyre and in the Arctic basin. The
extension of the core Argo network beyond 60 degrees in both hemispheres will ensure that it
remains one of the most important and truly global components of the ocean observing system.
Recommendations:
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Complete and maintain the existing Argo array at the original design density.
Extend Argo to the seasonal ice zone, through a combination of ice-capable floats and
acoustically-tracked floats.
Maintain and extend the array of sound sources and acoustically-tracked floats in the
Weddell Gyre. Establish a similar array in the Ross Sea Gyre.
Design and implement a low frequency (10-100 Hz) basin-scale array of sound sources
and acoustically-tracked floats in the Arctic.
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