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Background
In May
1999, the Arctic Ocean Science Board announced plans to study the two-way
oceanic exchanges that link the Arctic Ocean with subarctic seas (Dickson et
al., 1999, AOSB Newsletter 3(2)). The rationale is bound up with the fact
that most projections of greenhouse gas induced climate change anticipate a
weakening of the thermohaline circulation (THC) in the North Atlantic in
response to increased freshening and warming in the subpolar seas (Rahmstrof
1999, Nature 399, 523-524; Rahmstrof and Ganopolski 2000, Climatic Change 43,
353-367; Delworth and Dixon, 2000, J. Clim. 13, 3721-3727). Since the
overflow and descent of cold, dense waters across the Greenland-Scotland
Ridge is a principal means by which the deep ocean is ventilated and renewed,
the suggestion is that a reduction in upper-ocean density at high northern
latitudes will weaken the THC. Unfortunately, our models do not yet deal
adequately with many of the mechanisms believed to control the THC, and our
observations cannot yet supply many of the numbers they need. Though we have
a first indication that the coldest, densest part of the Faroe-Shetland
overflow may be decreasing (Hansen et al., 2001, Nature 411, 927-930), our
present observations of the large scale overturning circulation in the North
Atlantic (or anywhere else) are insufficient to detect whether or not it is
changing; we have no measurements of the freshwater flux between the Arctic
Ocean and Atlantic by either of its two main pathways; we have new
measurements (from the EC VEINS project) of the heat and salt flux to the
Arctic Ocean but not yet of its variability on any scale; we have a growing
knowledge of the long-term variability of dense overflows which “drive” the
THC but only embryonic ideas as to their causes, etc. Understandably then, we
would take the view that these key mechanisms and processes are too crudely
represented in the present generation of climate models. Palaeoclimate
records, however, show that massive and abrupt climate change has occurred in
the Northern Hemisphere, especially during and just after the last Ice Age
(Broecker, 1997, Science 278, 1582-1588; Broecker, 2000, PNAS 97, 1339-1342;
Marotzke, 2000, PNAS 97, 1347-1350), with THC change as the most plausible
driver, and both palaeoclimate records and models suggest that the changes in
the strength of the THC may occur rapidly, in a few decades. Further, in our
admittedly-short modern records of ocean variability, we have growing
evidence that hydrographic changes of decadal scale in the Arctic and
subarctic seas are able to feed south across the deep northern overflows to
cause hydrographic changes in the deep and abyssal layers of the Labrador
Sea. These variations are large and longsustained - e.g. the freshening of
both dense overflows by between 0.01 and 0.02 per decade for the past 3.5
decades (Dickson et al. 2002, Nature 416, 832-837) though
we don’t yet know enough about process to determine their climatic
significance. The high northern latitudes and the ocean fluxes that connect
them to adjacent seas are plainly not the only constituent parts of this
problem. The THC is driven globally by upwelling, downwelling and a strong
component of upper-ocean windforcing, and fluctuations in any one of these
components might affect the strength of the THC (see, for example, Toggweiler
and Samuels 1995, Deep-Sea Res. 42, 477-500, for the role of the Southern
Ocean windfield). Nonetheless buoyancy loss in the northern high latitudes
and the factors that control it are still of a fundamental importance, are
areas of continuing ignorance and are becoming tractable by modern observing
systems. These thermohaline controls and linkages, then, form the research
focus of ASOF.
On 6 April 2000 in Cambridge UK, as a joint
initiative of the Arctic Ocean Science Board and the International Arctic
Science Committee, a discussion meeting on the Sustained Monitoring of Arctic
Fluxes was held during Arctic Science Summit Week, with three main objectives.
First, to discuss the palaeo- and modelling evidence that THC slowdown or
shutdown has happened in the past and is likely to recur in the future.
Second, to begin to define the system of critical measurements that will be
needed to understand the role of the high-latitude oceans in decadal to
centennial climate variability. And third, to discuss ways of achieving the
co-ordinated long-term stamina in our funding that we will need if we are to
implement such a system across all the main gateways to/from the Arctic Ocean
over a period of a decade or more. The scientific planning of ASOF was later
advanced by means of a second discussion meeting and workshop, held at the
Norsk Polarinstitutt, Tromso on 21-24 September 2000, in conjunction with the
H. U. Sverdrup Symposium. Whereas the original meeting had focused on the
fundamental questions “Has THC shutdown happened before?” and “Are we right
to assume it can recur?” the Tromso workshop had the aim of providing a more
complete description of the required observing system, with preliminary
costs, and with some results in support (where these exist). The design of an
ASOF array was further refined at a National Academy of Sciences Workshop on
Abrupt Climate Change: Science and Public Policy, held at Lamont Doherty
Earth Observatory, Palisades NY, on October 30-31 2000. Throughout this
evolution, discussion has been guided by a sequence of so-called “Strawmen”
circulated in advance and intended to provide a concise, modern and expert
view of the issues discussed. The third and last of these Strawmen,
describing the present state and rationale for ASOF can be found in ASOF
Reports & Brochures.
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Objectives
The overall objective of ASOF is:
'To measure and model the variability
of fluxes between the Arctic Ocean and the Atlantic Ocean with a view to
implementing a longer term system of critical measurements needed to
understand the high-latitude ocean's steering role in decadal climate
variability.'
Several points are implicit in this
statement.
First, in keeping with a number of other
current research efforts, it is assumed that the role of the high latitude ocean
in decadal climate variability will take effect through its influence on the
Atlantic thermohaline circulation (THC or Meridional Overturning
Circulation). Most projections of greenhouse gas induced climate change
anticipate a weakened MOC in the North Atlantic due to increased freshening
and warming in subpolar seas and the supposition is that this climate signal
will be transferred to the deep ocean via the two overflows.
Second, it is an underlying assumption
of ASOF that in planning to make the first measurements of all the principal
oceanic fluxes that connect the Arctic Ocean and North Atlantic through
subarctic seas, the point of doing so is 'decadal'. The real objective would
not be met until our shorter term research "snapshots" can be set
into the context of decadal change.
Third, the ASOF aim suggests there is a
point to making these measurements in a coordinated way, so far as possible.
For example, a simulated increase in either of the main freshwater outputs
that connect the Arctic Ocean with the Atlantic (via the Canadian Arctic
Archipelago and along the East Greenland Shelf) seems capable of effecting a
slowdown of the MOC and advanced coupled models already indicate that these
two main freshwater inputs may have a shared time-dependence. It is
appropriate therefore to investigate this finding through simultaneous rather
than successive measurement, if at all possible.
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Implementation
The ASOF domain may be described in four
ways. The global figure of ongoing and
planned process studies in the Atlantic Sector as adopted by the International CLIVAR
Project Office (ICPO). It is designed to define the ASOF domain in broad
outline in order to set ASOF activities into context of ongoing and planned
studies in the Atlantic sector. It correctly defines the primary ASOF focus
to be the belt of subarctic seas that connect the Arctic Ocean with the North
Atlantic, and makes the point also that many other activities overlap with
ASOF in both motivation and location.
Alternately, the figure below defines
the ASOF domain in terms of its six main regional tasks, described in the
caption. Together, these are intended to meet the ASOF goal of measuring the
key ocean exchanges between the Arctic Ocean and subarctic seas, their
transformation on passing through the subarctic seas, and their arrival and
impact on the overturning circulation of the Northern North Atlantic. Many of
these tasks form a successor study to EC-VEINS (1997-2000), but now include
attempts to the measure both of the freshwater fluxes through northern seas
-- arguably the most important but least tractable components of exchange.
Implementing this distributed system of
measurements will require the funding, access and expertise of agencies and
scientists from both North America and Europe, and common funding calls on
both continents have been agreed for these sorts of tasks. In order to
organise research to meet the available funding on either side of the
Atlantic, the ASOF domain and its Steering Group are also organised into
"ASOF-East"
and "ASOF-West" groupings simply as a practical
measure.
The
fourth and final subdivision of ASOF, its tasks and domain, is to describe
progress towards implementation of the full range of activities -- region by
region -- that will contribute to each ASOF task. Some of these activities
will be directly funded as "ASOF" (for example the measurement of
the freshwater flux passing SE Greenland under ASOF-EC). Others will stem
from existing nationally or internationally funded efforts that happen to be
of central relevance to ASOF aims and objectives. One such is the recent bi-lateral
UK-Norway Initiative on Abrupt Climate Change proposed to address this issue
by the two Prime Ministers in 1998 which has been developed subsequently into
a new thematic programme Rapid Climate Change by the UK
Natural Environment Research Council and into the NOClim programme by the
Norwegian Research Council. As in the example just
given, it is important that we know of all of the main relevant activities in
planning the appropriate deployment of effort to meet a given ASOF task.
ASOF will aim to get most of this array into the
ocean by the end of 2003 and four factors suggest this is possible. First,
certain of the key measurements are already underway. The transport through
the Bering Strait has been estimated for decades (Coachman and Aagaard 1988,
J. Geophys. Res. 93, 15535-15539) and measured since 1990 (Weingartner et al.
1998, J. Geophys. Res. 103, 7647-7661); the core of the Denmark Strait
overflow has been measured with gaps since 1986. Second, in its initial form,
the ASOF ISSG is strong in the practical business of maintaining arrays of
equipment in these challenging waters. Third, almost all of the techniques
needed to make the necessary measurements now seem available or are in
prospect. The development at Bedford Institute of Oceanography of the Watson
compass for measuring flow directions close to the north magnetic pole, the
emergence of a range of cheap profiling CTD systems capable of sub-ice
hydrography, the successful trials of sea-glider systems in the past summer
giving the prospect of enriching our sparse moored arrays at realistic cost
mean that this is possibly the first time that much of ASOF might be
achieved.
 ASOF Implementation Plan Version 2 (pdf 36pp, 2MB)
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