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First Year Project Results
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Table of Contents |
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Overview
During the first year of research we have reprocessed all available data
collected in the archives of the Arctic and Antarctic Research Institute (AARI),
St. Petersburg, Russia. In 1995-1997, the Arctic and Antarctic Research
Institute (AARI),
working with U.S. experts, have
compiled an Arctic oceanographic database and developed climate scale
hydrographic fields
for the winter and summer periods. The available Western data (Figure A) and
Russian data (Figure B) on temperature (T)
and salinity (S) were assembled into a combined Russian--American database for the Arctic
Ocean which is now located at AARI. The hydrographic database contains a total of 354,287
stations of which 138,553 are for the winter season and 215,734 for the summer season.
For the winter period analyses, March--May 1948--1993, 73,774 stations were used from the
joint hydrographic database. Some winter and summer Russian data are classified but
available for analysis by Russian scientists. A small portion of the western data remains
proprietary. For more details go to http://www.nnic.noaa.gov/atlas.
This is great new information but the problem is that the data for this
atlas were averaged mechanically without taking into account natural processes
and features of the climate variability in the Arctic. The data were averaged for
decades, for periods 1950--1959, 1960--1969, 1970--1979 and 1980--1989.
Analysis of the temperature and salinity fields obtained by this method
shows that there are no significant changes in the Arctic’s circulation
and freshwater content from decade to decade. See Figure C (pdf).
For our project, gridded water temperature and salinity fields from AARI archive
were
averaged for the years of cyclonic (1953-1957, 1964-1971, 1980-1983) and
anticyclonic (1950-1952, 1958-1963, 1972-1979, and 1984-1988) circulation
regimes (see Proshutinsky and Johnson, 1997). The method of data reconstruction
for each year is based on EOF analysis and is decribed in the EWG atlas
of the Arctic Ocean (see EWG, 1997). These fields averaged for the CCR
and ACCR regimes are available in the grid 200x200 km. Each file for each
depth (0, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500,
750, 1000, 1500, 2000, 2500, 3000, and 4000 meters) contains
grid point number, grid point coordinates
(latitude and longitude), water salinity, and water
temperature. Last five digits in the file name mean
a period of averaging, i.e. 50-52 means 1950-1952, 53-57 means 1953-1957, etc.)
Click here to this data.
Salinity distribution for each circulation regime
is shown in figures at depth 0, 10, 25, 50, 75, 100, 150,
200, 250, and 300m. Each figure shows mean salinity
distribution during an anticyclonic circulation, a
cyclonic circulation regime (based on the data analysis
of the current project) and as decadal mean
(based on EWG atlas data).
Differences in salinity
between corresponding anticyclonic and cyclonic
circulation regimes at depths 0, 10, 25, 50, and 75
meters are shown below:
Vertically averaged salinity in the upper 50 meters
for each year is shown below:
Distribution of water salinity at the section from Point
Barrow to Franz Josef Land is presented below:
and in the following figures:
Figure D - section-1-1950-1961 (pdf)
Figure E - section-1-1962-1973 (pdf)
Figure F - section-1-1974-1985 (pdf)
Figure G - section-1-1986-1989 (pdf)
Maps of freshwater storage in the ocean corresponding to the
anticyclonic and cyclonic climate regimes (based on gridded data)
are shown in Figure H (pdf).
Small crosses show locations of hydrographic stations taken
into account. Note that only period 1972-1979
is very well covered by observations. During other years there are no
enough data for a more or less good analyses of freshwater content in
the Canadian Basin and the Beaufort Gyre.
A conceptual model (described below) of the freshwater
accumulation and release during seasonal cycle and for a cyclonic
and anticyclonic climate regimes was formulated as the first
order approach. This is a new hypothesis along with
supporting evidence that the BG plays a significant role
in regulating the arctic climate variability. We propose
and demonstrate that the BG accumulates a significant
amount of fresh water during one climate regime (anticyclonic)
and releases this water to the North Atlantic during another
climate regime (cyclonic). This hypothesis can explain the origin
of the salinity anomaly periodically found in the North Atlantic
as well as its role in the decadal variability in the Arctic region.
A substantial release of the BG fresh water to the NA
in response to changing climate conditions can be a source for
a catastrophic salinity anomaly in the NA and consequently,
a source for an abrupt global cooling. The above perspectives
lead us to the conclusion that it is extremely important to
understand the structure of the BG water properties, its currents,
and their variability in space and time.
Specially designed "Beaufort Gyre Exploration
Program" (http://www.whoi.edu/science/PO/arcticgroup)
was proposed in response to the NOAA’s 2001 Ocean Exploration
Program announcement of opportunity in order to support
project’s goals and hypotheses and to explore one of
the most hostile and inaccessible areas of the globe.
The proposal was evaluated by two reviewers and both
recommended to fund the proposal but cost of the
project was too high and NOAA declined this proposal.
We have resubmitted the proposal to the NSF’s
program "Arctic Freshwater Cycle: Land/Upper-Ocean
Linkages. A contribution to the Study of
Environmental Arctic Change (SEARCH)" in June 2002.
Conceptual Model
Our research shows that the origin of the salinity minimum in the BG
can be inferred by a comparison
of the seasonal change in wind and sea ice motion. Fig. 2 shows the wind and ice
drift patterns seasonally averaged for the period 1979-1997. In winter (September-May),
the wind Figure 2A (pdf) drives the ice and ocean anticyclonically Figure 2C (pdf) and the ocean
accumulates potential energy through a deformation of the salinity field (Ekman
convergence and subsequent downwelling, see Figure 1C (pdf). The strength of the horizontal
salinity gradient and resultant geostrophic circulation depend on the intensity and
duration of the anticyclonic winds. During the winter season the wind-driven and
geostrophic currents coincide to set up a strong anticyclonic ice rotation Figure 2C (pdf).
In summer (June-August), the wind is weaker or it may even be cyclonic
Figure 2B (pdf) but in the mean the ice still rotates anticyclonically Figure 2D (pdf).
An obvious conclusion is that in summer the ocean geostrophic circulation
prevails and drives the ice against the wind motion. The salinity anomaly
and freshwater content (FC) in the BG Figure 1B (pdf) must decrease in summer,
because without wind support, the ocean loses potential energy, i.e., Ekman
pumping is reduced. During the following winter the ocean again accumulates
potential energy. Hence, the climatic structure of the salinity and dynamic
height distribution remain rather persistent (not shown) although exhibiting
some seasonal and interannual variability. When viewed on a seasonal scale,
the BG salinity anomaly stabilizes the circulation, remaining essentially
anticyclonic throughout the year, thus permitting the BG geostrophic
circulation cell to serve as a flywheel for the Arctic Ocean circulation.
Some modeling results confirming this mechanism are shown
in Figure 3 (pdf). An idealized situation has been tested using a 3-D Blumberg
and Mellor [1987] numerical model in a 2000x2000 km basin with 1500 m depth.
The basin was initially horizontally uniform but vertically stratified,
then it was forced for 9 months by symmetric anticyclonic winds followed
by 3 months of cyclonic symmetric winds. The anticyclonic winds generate
downwelling in the central basin and upwelling along the boundaries
Figures 3A-B (pdf). The results after anticyclonic forcing only are similar
to the winter Arctic conditions, and the salinity structure in Figure 3B (pdf)
resembles that in Figure 1C (pdf). The addition of cyclonic winds leads to
upwelling in the central basin and downwelling along the boundaries
and to a reduction in the anomaly in the salinity field generated by
anticyclonic winds. The distribution of salinity and currents after
3 months of cyclonic wind forcing are shown in Figures 3C-D (pdf). The circulation
pattern in Figure 3C (pdf) is similar to the ice drift pattern in Figure 2D (pdf), i.e.,
it is still anticyclonic but is weaker than in winter. The salinity
distribution in Figure 3D (pdf) resembles the summer salinity distribution in
Figure 1B (pdf) when the cyclonic wind forcing leads to the release of FW from
deep to upper layers. The seasonal variability of FW content in the
central part of the basin is about 10% (not shown).
Figure 1. (pdf): (A) The salinity distribution at 25m.
(B), (C) Salinity distribution along dashed line in summer
and winter. (D) Dynamic heights relative to 200 db and
direction of geostrophic currents.
Figure 2. (pdf): Winter (A) summer (B) sea level pressure (SLP, hPa)
and geostrophic wind. (C), (D) Season sea ice drift
Figure 3. (pdf): Results of numerical experiments in the ideal basin.
(A) Sea surface salinity (SSS) and surface currents.
(B) Salinity section along dashed line. Both figures
show results after 9 months of anticyclonic symmetric
wind forcing. (C), (D) The same characteristics as in
(A) and (B), respectively, but after an additional 3
months of symmetric cyclonic wind forcing.
Figure 4. (pdf): (A) The FC anomaly
(solid blue line) from observations and SSHG (red dashed line). (B) The FC anomaly
(solid blue line) from reconstruction and SSHG (red and yellow bars) as
defined by {\it P&J}. The thick black line depicts the sea ice volume
(km^3/year) anomalies from Hilmer and Lemke [2001]. Vertical
axes show units of SSHG (x10^-6), sea ice volume anomalies
(km^3/year), and the FC anomalies (km^3/year)
Figure 5. (pdf): Sea ice concentration (SIC) in the GIN Sea
averaged for the two CCRs: 1980-1983 and 1989-1997 (A),
and two ACCRs: 1984-1988 and 1998-2000 (B).
Hypothesis
A hypothetical chain of relationships among atmosphere, ice and
ocean in the Arctic at the decadal time scale has been proposed by Mysak
and Venegas [1998], Proshutinsky et al., [1999] (hereinafter P99) and
others but it is important to know what causes the variability. In order
to explain the relationship between the wind-driven and geostrophic
circulation and their influence on the accumulation and release of
FW we examine the interplay between the atmosphere, ice and ocean
in terms of the two circulation regimes identified by Proshutinsky
and Johnson [1997] (hereinafter P&J) and P99.
ACCR
During the anticyclonic circulation regime (ACCR),
when high atmospheric pressure prevails in the Arctic, the
Arctic Ocean accumulates FW through the increase of FW volume
in the BG (Ekman convergence and subsequent downwelling, see
Figure 1C (pdf)) and through the increase of ice thickness and area
due to enhanced ice growth (the Arctic is colder during an
ACCR than a cyclonic circulation regime (CCR) as shown in P99.
Ice is additionally accumulated in the BG during an ACCR due
to convergence and ridging under anticyclonic wind forcing.
River runoff is increased (trajectories of cyclones are shifted
toward land) (P&J; Johnson et al., 1999) and more FW accumulates
in the surface waters. When anticyclonic winds are prevalent,
the flow of Arctic waters towards Fram Strait is reduced (P&J;
Trembley and Mysak, 1998). Consequently, the ice and water flux
from the Arctic Ocean to the Greenland Sea and the transport of
Atlantic Water into the Arctic Ocean (as a compensation of outflow)
are weaker than usual. Deep convection in the Greenland Sea is
then enhanced because the vertical stratification is reduced
(less FW in the surface waters). This decoupling of the
Greenland, Iceland, and Norwegian Seas (GIN Sea) from
the Arctic leads to their eventual warming.
Transition to a CCR
All of the above processes lead (with some time lag)
to an increase in the gradient of dynamic height between the
BG and the NA. The resultant geostrophic circulation increases
as does the outflow of FW and ice from the Arctic. During warming
of the GIN Sea, the Icelandic Low intensifies and moves to the
north leading to an intensification of the transport of Atlantic
waters into the Arctic Ocean. This increase in warm water flux
to higher latitudes enhances the penetration of atmospheric cyclones
into the Arctic, and ultimately decreases the atmospheric pressure
in the Arctic. Warming of the Arctic establishes the CCR.
CCR
During the cyclonic circulation regime, when low atmospheric
pressure prevails in the Arctic (see table characterizing different
environmental features of CCR and ACCR in P99), the Arctic Ocean
releases FW to the NA through the passages in the Canadian Archipelago
and Fram Strait. Warming in the Arctic during the CCR increases ice
melting and releases additional FW to the central basin. The accumulation
and storage of FW in the BG is not favored by the CCR (even though the cyclonic
regime leads to increased ice melt, the FW is not accumulated in the BG
because of Ekman divergence and upwelling causing a decrease of freshwater
volume in the BG), and hence more FW is available for transport to the NA.
River runoff is lower during the CCR than during the ACCR but
precipitation over the ocean is increased and hence more fresh water
is available for immediate release to the NA from sea ice
and surface waters during the CCR.
The stronger surface winds of the CCR in the Fram Strait area
(P99) increase the transport of thick ice, and hence FW, to GIN Sea.
At the peak of these processes, when all of them coincide, we
observe low salinity anomalies in the GIN Sea.
Transition to ACCR
After several years of increased release of ice and FW to
the GIN Sea, the surface layer becomes cooler and fresher, and the
sea-ice extent increases in the Greenland Sea. Freshening associated
with melting of the increased ice volume and increased flux of fresher
surface waters leads to an increase in stratification and a decrease
in the interaction between the deep ocean and the atmosphere; deep-water
convection is consequently suppressed. After several years the dynamic
height gradient between the BG and the NA (and consequently the
geostrophic circulation) decreases, the Icelandic Low moves to the
south and the interactions between the GIN Sea and the Arctic Ocean
become weaker, reestablishing the anticyclonic circulation regime.
It is important to note that in this sequence of processes
the accumulation and release of FW and ice plays a fundamental role
in the interaction between the Arctic Basin and the GIN Sea.
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