Ola M. Johannessen - ola.johannesen@nersc.no, S. Sandven , H. Sagen, T. Hamre, H. Hobaek
Nansen Environmental and Remote Sensing Center (NERSC)
University of Bergen, Norway
K. Hasselmann - klaus.hasselmann@dkrz.de, E. Maier-Reimer, U. Mikolajewicz - Max Planck Institute for Meteorology, Hamburg, Germany
P. Wadhams - pw11@cam.ac.uk. and A. Kaletzky, Scott Polar Research Institute, University of Cambridge, England
L. Bobylev, E. Evert, V. Troyan, Nansen International Environmental and remote Sensing Center, St. Petersburg, Russia - nansen@online.ru
K.A.Naugolnykh knaugolnykh@etl.noaa.gov and I. Esipov, Nonlinear Acoustics Laboratory, N.Andreyev
Acoustics Institute, State Research Center, Moscow
Popular version of paper 3aAOb4
Presented Wednesday Morning, March 17, 1999
ASA/EAA/DAGA '99 Meeting, Berlin, Germany
The overall objective of AMOC is to develop and design an acoustic system that will monitor ocean temperature and ice thickness in the Arctic Ocean (including the Fram Strait), study long-term climate variability, and thus detect global warming. The acoustic monitoring system will be designed (using existing acoustic source/receiver technology) to fulfill the following requirements:
The source of energy which drives the climate on earth is radiation from the Sun. The global climate determines the energy budgets of the atmosphere, cryosphere, biosphere and other parts of the earth system. Usually when a climate change is discussed, one hears about increasing atmospheric temperatures - the Greenhouse effect. The importance of the ocean is often underestimated. Today the oceans cover 70% of the Earth's surface and their ability to store solar energy is high, which delays and reduces temperature changes on a daily, seasonal and regional scale. The oceans are also very important in the absorption of CO2, which is one of the main anthropogenic greenhouse gases.
Recent results from global climate models indicate that global warming in the next decades will be most pronounced in the Arctic region (Manabe et al., 1991, Cattle et al.,1995). Furthermore, time-dependent greenhouse warming simulations indicate that within the next decade the global warming signal should become detectable over the natural variability. This may cause a net melting of the sea ice in the Arctic due to advection of warmer Atlantic Water through the Fram Strait (Semtner, 1987, Johannessen et al., 1995 a, Johannessen et al., 1995 b) and St. Anna Trough.
Sound waves, however, propagate in water with very little absorption and have been used extensively in underwater defense systems for several decades. In oceanography, acoustical methods such as acoustic tomography and thermometry (Munk et al., 1995), have become increasingly important in studying physical processes such as surface waves, currents and temperature distribution. These methods employ configurations of sound sources and receivers in arrays.
Figure: Suggested acoustic tracks in a Arctic monitoring system.
In AMOC a system of well known acoustic source and receiver technology will be
established and mounted underwater. The source will send acoustic signals with
a given frequency content. The signals travel through the ocean and parts of it
will be received by a receiver positioned on, for example, the other side of the
Fram Strait (See Figure above). Usually the travel time between transmitter and
receiver is measured together with the transmission loss, showing how much the
signal is reduced compared to the source signal. The signal travels at a sound
speed modified by ocean temperature, sea water pressure and salinity. As
a rule of thumb, the speed of sound increases with around 4 m/s per degree
Celsius; 1.5 m/s per 100 m depth increase; and 1 m/s for a salinity increase of
1 %. The sound speed is most sensitive to ocean tempereature. If the travel time
for sound waves across the Arctic Basin changes over a period of 10-30 years the
change in mean ocean temperature can be estimated.
During a pilot Transarctic Acoustic Propagation experiment (TAP), conducted by
Mikhalevsky in the spring of 1994, long range propagation (2600 km) in the
Arctic Basin was performed. This experiment showed that transmission across the
Arctic basin is possible at frequencies around 20 Hz. Furthermore, the results of
this experiment showed that travel time and phase measurements are
sufficiently accurate to pick up changes caused by climate change in the Arctic
basin (Mikhalevsky et al. 1999). This demonstrates the feasibility of using
acoustic travel time measurements to monitor changes in average ocean
temperature. A new low frequency source has been deployed North-East of Svalbard,
transmitting sound at frequency close to 20 Hz every fourth day for 2.5 years
(Mikhalevsky et al. 1999).
Estimates have been made which combine hydrologic data from different times and locations, resulting in a large spread in values. Previous
estimates of current flux of the West Spitzbergen Current (WSC) across 79 N
vary from 1.9 Sv to 8 Sv (Simonsen and Haugan, 1996). This large variation in
water mass exchange introduces errors in ocean circulation models.
A promising method of monitoring temperature and current velocity through the
Fram Strait is acoustic signal travel time measurement at the cross-section of
the strait (which has a typical width of 300 km and depth of 2700 m - see
Figure). This method is based on the fact that the propagation time of acoustic
pulses along the paths connecting a source and a receiver (eigen rays) is
determined primarily by the distributions of temperature and longitudinal stream
velocity components. This allows the acquisition of appropriate data from
acoustic measurements within an acoustic tomography framework (K.A. Naugolnykh, et.
al., 1998a). In the sensitivity study of acoustic propagation to current
velocity changes the methods of reciprocal transmission, scintillation, and
Horizontal - Refraction Modal Tomography (HRTM) are used. It is shown that
HRMT potentially can be used in the Fram Strait for transverse current monitoring.
It was also found that the multi-year ice area has decreased more rapidly, about
9% per decade (i.e., 18%) over the observation period, which represents a reduction
of nearly 900,000 km2, of which about half is replaced by first-year ice. This
is the first time that quantitative changes in the character of the sea ice
cover have been derived from passive microwave satellite data. These findings
show an Arctic ice cover in apparent transformation, and suggest a thinner
winter ice cover, consistent with very recent analyses of two decades of
submarine sonar transect data (Wadhams, 1998). However, it must be noted that
even two decades of observations, whether satellite or in situ, are inadequate
to establish whether these represent long-term trends. Furthermore, the most
fundamental parameter in studies of the responses of the ice cover to global
warming - the ice thickness - cannot be measured by existing satellite techniques.
In the Arctic Ocean a cold surface layer, 80 - 200 m deep, is always present in
ice-covered areas. Depending on the thickness of the surface duct, sound above a given frequency will be trapped within the duct and repeatedly interact with the ice cover. After several bounces exclusively with the sea ice cover, the sound has been exposed to several reflection losses and scattering losses
due to the sea ice. The reflection loss is caused by conversion of acoustic energy into elastic
waves within the sea ice cover. The conversion of energy depends strongly on the
age of the sea ice (given by the elastic properties) and its thickness.
By considering the reflection loss a strong sensitivity to changes in ice
thickness and elastic properties of the ice cover is revealed at frequencies
above 100 Hz. Based on our simulations a thinning of the ice cover will make the
received signal from a broad band source less attenuated for a broader range of
frequencies. Below 100 Hz the reflection loss is insignificant and the sound
starts to leak out of the surface duct, causing it to be less and less sensitive
to the internal properties of the sea ice, including sea ice thickness. So, in
order to obtain information about the internal properties and ice thickness,
acoustic measurements must be made at frequencies or in frequency bands which
are sensible to the sea ice, not at low frequencies, which would cause total specular
reflection.
If the frequency is below 100 Hz the sound will not sense the
presence of the cold duct, and is attenuated and scattered from reflections off the rough underside of the sea ice. At these frequencies reflection loss caused by the rough sea floor is introduced at
shallow and intermediate water depths. Loss measurements at low frequencies will
provide information about the roughness of the sea ice and the reflectivity of
the sea floor. Thus the low frequency - 19.6 Hz - used in the TAP experiment can only provide limited information about sea ice roughness. As a consequence, a system for sea ice has to use a time-limited pulse or a coded pulse to obtain information about the internal properties of the sea ice. Since
it is the transmission loss caused by the sea ice cover that will be used, the
ranges have to be much shorter, in order to have a good enough signal to noise
ratio.
Ambient noise is defined as sound which is generated by many natural
processes within or beneath the sea-ice cover. The main sources in the Arctic
Ocean are related to building of ridges, thermal cracking and breakup of sea
ice. These noise generating processes reflect the dynamic processes of
the sea ice. On the other hand, long term changes in the Arctic Ocean climate will cause
changes in overall sea ice properties, such as the observed reduction of
more than one-year-old ice during winter time (Johannessen et. al., 1999). This
again will change the response of the sea ice to dynamic processes caused by
wind stress and current.
Finally, this
will cause a change in the ambient noise sources.
Similar to the acoustic energy generated by specific man-made sources, ambient noise is strongly affected by acoustic propagation conditions, which in the Arctic Ocean are characterized by strong
surface ducts and sea ice cover. The ambient noise recorded at a location
therefore contains combined information of the environment in which it was
generated and has propagated through, including the sea ice cover.
In 1974 Diachoke and Winikur concluded after a brief argumentation that
"Reflection and scattering losses at the interfaces affect transmission loss
and, consequently, the shapes of the ambient noise versus frequency curves".
After this study very little attention has been paid to the effect of
propagation loss on ambient noise characteristics.
The main effort in the present work was to analyze broad band ambient noise data obtained during the Marginal Ice Zone Experiment (MIZEX - 1985, 1987) and the Seasonal Ice Zone Experiment
(SIZEX - 1989, 1992) and relate it to the complex oceanographic conditions
found in the complicated region at the border of the sea ice-covered Arctic and
the open ocean. Ambient noise data from 82 different locations in the Barents
Sea and 72 locations in the Greenland Sea were analyzed. Environmental conditions
were observed both by in situ measurements and remote sensing data from satellites.
Briefly, our results show a correspondence between averaged ambient noise
observations in the frequency band from 20 Hz up to 5 kHz and ocean
stratification and sea ice properties. This correspondence is generally explained by sound propagation conditions that obtain under ocean stratification and sea ice reflrvtivity.
This makes the monitoring of changes in broad
band ambient noise characteristics very interesting as a component in future
Acoustic Monitoring System concepts in the Arctic Ocean. Ambient noise
measurements have several benefits. First of all, they produce no additional
man-made noise in the Arctic ocean. Secondly, receiver arrays are much cheaper
than configurations using large, low frequency acoustic sources. Thirdly,
ambient noise recording systems need much less energy supply and can also
easily be mounted at the sea bottom or under the ice. Finally, the ambient noise
can be used in a variety of ways, both to identify dynamic processes causing
break up, swell and ridging, and to retrieve changes in averaged ice parameters and
ocean stratification.
Remote sensing from space measures the ice extent efficiently, whereas no
synoptic means for measuring the average ocean temperature or ice thickness
distribution exists. Up to now average temperature of the Arctic ocean and sea
ice thickness distribution estimates have been based on point measurements, and
are thus very rough. Acoustic intensity tomography, with its
potential for obtaining sea ice thickness, will compliment satellite
data by correlating coverage and thickness changes. Acoustic thermometry will
monitor changes in average ocean temperature. The unique combination of underwater acoustic remote sensing (AMOC) with satellite remote sensing of the ice cover (including modelling and data
assimilation) in this sensitive Arctic region is key to monitoring and understanding global climate changes and early detection of global warming.
This research was supported by the European "Environment and climate programme"
under contract: ENV4-CT97-0463, the Norwegian Research Council project number
121290/720, 101798/410 and by NATO linkage grant: (E.NVIR.L G 960352)528 (96)
LVdC.
For more information, go to the AMOC Home Page.
Acoustic Monitoring of Temperature and Current Changes in the Fram Strait
The Fram Strait is the region where most of the heat and water exchange between
the Arctic Ocean and Atlantic Ocean takes place. To a first order approximation
it acts both as an entry and an exit port for the Arctic Ocean. The general
large scale ocean circulation in this region is dominated by the shallow,
southward-flowing, cold and low salinity East Greenland Current (which exports ice
and polar water out of the Arctic Ocean) and the northward-flowing, warm and
saline Atlantic water in the West Spitsbergen Current (WSC). The boundary conditions exhibited in this
large-scale water mass exchange are key to understanding large
scale ocean circulation and global climate models (Alendal, 1994).
Therefore, studies in the Fram Strait region have focused on measuring the
fluxes of mass and heat through the strait.
Monitoring Changes in Arctic Sea Ice
The sea ice cover is exposed both to climate changes in the atmosphere and
changes in the ocean. By monitoring the extent of sea ice and its properties
(such as average thickness and age),we can detect global warming.
Passive microwave satellite data provides the most consistent, reliable means of
deriving quantitative information on the global ice cover, such as sea ice
extent and ice classification maps. Recently, two decades of continuous
brightness temperature (TB) data - obtained from the satellite-borne Scanning
Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager
(SSMI) - have been analysed (Johannessen et al., 1999). It was found that the
total ice extent and area in the Arctic have decreased about 6% during two
decades, consistent with the reduction rate found in studies to 1996 (Bjrgo et al.,
1997). This represents a reduction of over 600,000 km2 in the total ice area.
Listen to the Climate Change?
The monitoring concepts above require both source and receiver
configurations. When analysing the received signal the signal to noise ratio can
be poor due to ambient noise generated by ice ridges, cooling processes and
break up of sea ice. Based on Ph.D. work performed by Sagen in 1998 we also
investigate the possibility of recording natural ambient noise and using it as a
source of additional information in a future acoustic motoring system in the
Arctic Ocean and in the Fram Strait.
Project Benefits
The most important benefit of the project will be improved methodology to
monitor climate change in the Arctic, which plays an important role for the
climate of Europe. Predictions of global circulation models show that climate
change in Arctic will be more rapid than at lower latitudes.
Recent data from Greenland ice cores (GRIP and GISP) indicates that that at times
in the past (before any anthropogenic input) rapid climatic changes happened,
with warmings or coolings of several degrees occurring within a period of a few
decades, or perhaps even more rapidly. We do not know what caused these
sudden changes, but we may suspect that they were associated with changes in
ocean currents, water structure or ice distribution, maybe due to the "turning
off" or "turning on" of some critical process. By understanding the present-day
Arctic Ocean system and its interactions with the global environment, we may be
able to understand what caused past rapid climate events in high northern
latitudes, and also whether similar rapid fluctuations are indeed in store for
us. A critical factor for those who live in northwest Europe (and who have a
warmer climate than we deserve, thanks to the North Atlantic Current), is that the
entire North Atlantic circulation may alter in strength, with serious
consequences for our well-being.
Related References