Observing Warm Water Pathways in the Subpolar North Atlantic
Martin Mork, Professor of Oceanography
University of Bergen
Paula Pérez-Brunius, Doctoral Candidate
Graduate School of Oceanography
Martin Mork is professor of physical oceanography at
the Geophysics Institute at the University of Bergen, Norway.
Paula Pérez-Brunius received a BS in physics from the Universidad
Nacional Autónoma de México in 1996. She is pursuing a PhD in
physical oceanography at GSO.
The temperate climate of Scandinavia and western Europe depends upon the warm
waters of the northern North Atlantic to warm the westerly winds before they
reach the European subcontinent. These waters, which originate in the Gulf Stream,
flow northeast across the ocean and enter the Nordic Seas between Scotland and
Iceland (see Fig. 1). There is considerable uncertainty
about the pathways by which these waters make their way north. For example,
the presence of a north-flowing, warm, salty current just west of Scotland was,
for many years, thought to be the principal path by which Gulf Stream waters
reached the Nordic Seas. This current can be seen in the famous 1909 Helland-Hansen
figure (see Fig. 1), that shows the inflow through the Shetland and Faroe Channel
and its continuation as the Norwegian Current, known popularly as the "Golf
Strom," in Scandinavia. It has, therefore, come as a bit of a surprise
to realize that the "Golf Strom" is not a simple extension north of
the flow off Scotland, but comes from the west along a front that extends southeast
from Iceland and passes just north of the Faroe Islands, also known as the Faroes.
The existence of this front has been known for a long time and is quite visible
in Figure 1, but its significance has largely gone unrecognized until just recently.
In 1988, Professor Bogi Hansen, from the Fisheries Institute, Faroes, wrote
a paper pointing out that a major part of the Atlantic inflow takes place over
the Iceland-Faroe Ridge. The total inflow is about 8x106m3/s
(roughly eight times the total outflow of all the world's rivers) with more
than half this flow entering between Iceland and the Faroes. These waters are
somewhat cooler (6°C to 7°C) than the Faroe-Scotland waters, so in terms
of heat transport, the two branches appear to contribute about the same amount
to the Nordic Seas. To put this into perspective, the heat flux, about 1014
watts, corresponds to the energy output of roughly 100,000 major electric utility
plants! The inflow appears to be highest in winter and lowest in summer. How
stable are these branches of warm water that flow into the Nordic Seas? What
happens if prevailing winds cause disturbances to the circulation in the northeastern
Atlantic?
While we know that much of the warm water must
originate in the Gulf Stream, there is a good deal of uncertainty as to how
it crosses the northeastern Atlantic. For example, Professor Wolfgang Krauss
from the University of Kiel, Germany, noted that none of 200 satellite-tracked
buoys deployed in the northern North Atlantic south of 55°N during 1981-89
made their way to the Nordic Seas. Why not? There is also debate about whether
the waters crossing the Iceland-Faroe Ridge come from south of Iceland or from
the Faroe Islands. Recent observations indicate that substantial amounts of
warm water move north along the Reykjanes Ridge, the section of the mid-Atlantic
Ridge south of Iceland. Some of these waters turn west toward Greenland, some
flow northwest of Iceland, and some are thought to flow northeast into the Norwegian
Sea (see Fig. 2). It is conceivable that part of our
uncertainty about the mean circulation is due to the fact that these numerous
branches vary, partly in response to changing wind patterns. Different studies
have found different patterns of flow, suggesting that they change over time.
We know, for example, that in the 1970s, the prevailing winds from Canada, south
of Greenland and Iceland, were stronger and colder than in the 1960s, and the
waters off Canada in the Labrador Sea were cooled and sank to great depths.
These waters had to be replaced, perhaps by the warm, salty waters from the
North Atlantic Current. We don't really know how, but perhaps when the winds
are weaker, more of the waters continue northeast into the Nordic Seas. There
is a compelling need for more information about the Atlantic currents, but it
is prohibitively expensive to use research vessels for such extensive investigations.
This is where a long-term program of current measurements from commercial vessels
can be of tremendous assistance.
Fortunately, there is a freighter, the MV Nuka
Arctica of the Greenland shipping company, The Royal Arctic Line, that operates
between Nuuk, on the west coast of Greenland, and Denmark, making one roundtrip
every three weeks. The transit line between Cape Farewell and the Shetlands
along 60°N (see Fig. 2) crosses all the waters that eventually move across
the Greenland-Iceland-Faroes-Scotland Ridge or turn west towards Greenland and
beyond. The ship owners have agreed to operate an acoustic Doppler current profiler
(ADCP) on the vessel. This instrument can measure currents as deep as 400m depending
upon the amount of backscattering zooplankton in the water. Fittings to accommodate
the instrument were installed in the ship's hull during its regular drydocking
in 1998, and the measurement program began in the spring of 1999.
The current measurements will be used for many
purposes. First, they will be used for statistical analyses to estimate the
mean flow and its variability with time (particularly on seasonal and longer,
interannual, time scales). This will provide basic descriptive information that
is needed about the circulation of the North Atlantic. Second, the observations
will be used for models and tests. For modelling studies of the Nordic Seas,
knowledge of the currents along this route will serve as a boundary condition
for numerical simulation of the currents in the Norwegian and Greenland Seas.
Third, the ADCP measurements will be helpful for studies of circulation (including
ice drift models) around Greenland. As our understanding of the upper ocean
circulation improves, so will our understanding of the ocean's role in climate
and climate change.
Although the MV Nuka Arctica program hasn't
started yet, the MV Nivi Ituk, another freighter operating along the
same line, was equipped with expendable bathythermographs (XBTs), instruments
that measure temperature to depths of about 450m. In the 1980s, the vessel took
some 500 profiles (Figure 2 shows the ship's track across the subpolar North
Atlantic). The mean temperature profiles for winter and summer, west and east
of the Reykjanes Ridge, are shown in Figure 3 (spring
and autumn profiles fall within these boundaries). The profiles measure the
variations near the surface between the two seasons: warm in the summer and
cold in the winter. The largest effect is confined to the first hundred meters.
In winter, the temperature does not vary with depth; this suggests that the
mixed layer in these two regions reaches to at least 400m. Even at those depths,
the temperature gets warmer in summer, indicating that seasonal influences reach
quite deep. These temperature changes could, in turn, affect the circulation
in the area. It is very interesting to note the difference in temperatures between
the two basins: the Irminger Sea is 2°C to 3°C colder than the Iceland
Basin. These temperature differences imply a northward flow of water, stronger
where the horizontal contrast is larger. Figure 4 shows
a transect obtained from one of the ship's cruises. One can see clearly the
front that separates the two different water masses on either side of the Reykjanes
Ridge; cold Irminger Sea water to the west of the ridge, warmer Iceland Basin
water to the east of it. This front, known as the Subarctic Front, is characterized
by a narrow region with a steeply inclined temperature field, and a northward
current flowing along the ridge. From the MV Nivi Ituk XBT data alone,
we can address the stability of this front; a glance at different transects
indicates that the front is generally found between 34°W and 26°W, more
likely on the steep slopes of the Reykjanes Ridge, and that its strength varies
on monthly time scales. This gives us some insight into changes in the flow
patterns across the region. However, we cannot estimate currents and transports
from this kind of information alone. This is why our knowledge of the circulation
remains so tentative. New measurement technologies offer exciting prospects
for major advancements in our understanding of the North Atlantic. When the
ADCP becomes fully operational later this year, the Nuka Arctica will
be equipped to measure currents down to depths of 300m to 400m, to obtain temperature
profiles to approximately 750m depth (with an XBT system), and to measure temperature
and salinity of the surface waters with a continuously recording thermosalinograph
(CRT) which is already installed and operational. In addition, the Danish Meteorological
Institute will take profiles of winds aloft every 12 hours by releasing weather
balloons. Technicians at Nuuk, Greenland, and Aalborg, Denmark, will collect
the data and service the instruments during port calls.