|
|
A new analysis of 50 years of changes in freshwater inputs to the
Arctic Ocean and North Atlantic may help shed light on what's behind
the recently observed freshening of the North Atlantic Ocean. In a
report, published in the August 25, 2006 issue of the journal, Science,
MBL (Marine Biological Laboratory) senior scientist Bruce J. Peterson
and his colleagues describe a first-of-its-kind effort to create a
big-picture view of hydrologic trends in the Arctic. Their analysis
reveals that freshwater increases from Arctic Ocean sources appear to
be highly linked to a fresher North Atlantic.
"The high-latitude freshwater cycle is one of the most sensitive
barometers of the impact of changes in climate and broad-scale
atmospheric dynamics because of the polar amplification of the global
warming signal," says Peterson. "It's easiest to measure these changes
in the Arctic and the better we understand this system, the sooner we
will know what is happening to the global hydrologic cycle."
The multi-disciplinary team of scientists led by Peterson calculated
annual and cumulative freshwater input anomalies (deviations from
expected levels) from net precipitation on the ocean surface, river
discharge, net attrition of glaciers, and Arctic Ocean sea ice melt and
export for the latter half of the 20th century. The scientists compared
the fluxes to measured rates of freshwater accumulation in the North
Atlantic during the same time period.
Their analysis showed that increasing river discharge and excess net
precipitation on the ocean contributed the most freshwater (~20,000
cubic kilometers) to the Arctic and high-latitude North Atlantic. Sea
ice reduction provided another ~15,000 cubic kilometers of freshwater,
followed by ~2,000 cubic kilometers from melting glaciers. Together,
the sum of anomalous inputs from all of the freshwater sources analyzed
matched the amount and rate at which fresh water accumulated in the
North Atlantic during much of the period from 1965 through 1995.
"This synthesis allows us to judge which freshwater sources are the
largest, but more importantly shows how the significance of different
sources have changed over the past decades and what has caused the
changes," says Peterson. "It prompts us to realize that the relative
importance of different sources will change in future decades. Creating
a big-picture or synoptic view of the changes in various components of
the high-latitude freshwater cycle puts the parts in a perspective
where we can judge their individual and collective impact on ocean
freshening and circulation."
In recent years, much attention has been given to the observed
freshening of Arctic Ocean and North Atlantic and the potential impacts
it may have on the earth's climate. Scientists contend that a
significant increase of freshwater flow to the Arctic Ocean could slow
or halt the Atlantic Deep Water formation, a driving factor behind the
great "conveyor belt" current that is responsible for redistributing
salt and thermal energy around the globe, influencing the planet's
climate. One of the potential effects of altered global ocean
circulation could be a cooling of Northern Europe within this century.
The team's comparison of freshwater sources and ocean sink records
revealed that over the last half century changes in freshwater inputs
and ocean storage occurred not only in conjunction with one another,
but in synchrony with rising air temperatures and an amplifying North
Atlantic Oscillation (NAO), a climatic phenomenon that has strong
impacts on weather and climate in the North Atlantic region and
surrounding continents, and the associated Northern Annular Mode (NAM)
index.
Peterson and his colleagues contend that the interplay between the NAO
and NAM, and continued rising temperatures from global greenhouse
warming, will likely determine whether the Arctic and North Atlantic
Oceans will continue to freshen. But the scientists caution that the
difficultly in predicting fluctuations in atmospheric circulation makes
it impossible to know where we might be headed.
"Atmospheric modes of circulation such as the NAO and NAM exert a great
deal of control on net precipitation in the ocean and even on regional
temperatures, and hence ice melt as well," says Peterson. "But what
drives the NAO is the $64,000 question. Our inability to predict trends
in the NAO/NAM means that, even if we could predict global warming very
well, a large degree of uncertainty will remain in any forecasts of the
decadal-centennial trajectories of the Arctic freshwater balance."
Find more great stories in the OCEAN ARCHIVES >
|
|
|
