Having performed daily oceanographic measurements in the deep Norwegian Sea since 1 October 1948, Ocean Weather Ship Station (OWS) Mike, at 66N,02E, can present the longest existing homogeneous time series from the deep ocean. Station M is operating above the eastern margin of the Norwegian Sea deep basin where a branch of the Atlantic current is entering the area, Figure 1. The location proved to be strategic both for studying the Atlantic inflow and the Norwegian Sea Deep Water. The OWS M is operated by The Norwegian Meteorological Institute (DNMI) and the hydrographic programme is carried out by Geophysical Institute, the University of Bergen.
Fig. 1 The main current system (schematic) in the Nordic Seas with the position of the weather ship station MIKE. The open hatched arrows indicate the surface current patterns, and the black arrows indicate the deep/bottom current.
In 1946 a plan for a network of 13 ocean weather stations in the North Atlantic was set forth under the auspices of ICAO. The Stations were to supply meteorological services, search and rescue services, and navigational aids to aircraft. The USA, Canada and eighth European countries should be responsible for operating the stations, which were referred to by letters from A to M. Norway was to operate station M (phonetic name Mike) at 66N,02E, with financial backing from Sweden and Great Britain.
ICAO attempted to organize an international oceanographical research programme for the weather ships, but failed due to lack of interest, shortage of money and difficulties in procuring the necessary scientific equipment. In Norway, a country which held great traditions in oceanographical research, a small group of three scientists, led by the oceanographer Håkon Mosby , took upon themselves to implement an extensive research programme on station M (Hogstad and Østerhus, 1996).
Håkon Mosby implemented a routine programme
within physical oceanography, including serial observations of
temperature, salinity, and (since 1953) oxygen weekly at standard
depths to 2000 meters, and serial observations of temperature
and salinity at standard depths down to 1000 meters 3 or 4 times
a week. This programme has been running continuously since 1 October
1948 to this very day only hampered by occasionally extreme weather.
The method of obtaining temperature and salinity observations
(Nansen bottles with reversing thermometers) has not changed significantly
either so the time series are indeed homogeneous.
Fig. 2(left) Time series of temperature in the Atlantic Water (50,75,100,150,200,300,400,500,600,800 m)
Fig. 3(right) Time series of salinity in the Atlantic Water (50,75,100,150,200,300,400,500,600,800 m).
Figure 2 and 3 show that the Atlantic water has become cooler and fresher since 1991. For depth below 200 m the temperature and salinity are down to the levels observed during the late 70's, when the so-called “Great Salinity Anomaly” passed through (Dickson et al.,1988). Independent observations (Hansen and Kristiansen, 1994) indicate that the inflow of Atlantic Water to the Nordic Seas has been reduced. The long term trends are a cooling and freshening of the upper layer consistence with a accumulating of Arctic Surface Water in the Nordic Seas (Blindheim et al.,1996).
Smoothed monthly mean temperatures for the 3 deepest
standard depths (1200m, 1500m, 2000m) are shown in Figure 4 for
the period 1948-1995. Notice that a recent warming has occurred,
starting at 2000m in 1985, then gradually penetrating upwards
through 1500m in 1987 and reaching the 1200m level in 1990. The
temperature increase is about 0.07°C, and nearly constant
with depth.
Fig. 4 (left)
Time series of smoothed monthly mean temperature
at depth of 1200m, 1500m 2000m from the weather ship
station Mike.
Fig. 5 (right)
Time series of the mean temperature below
2000m in the central Greenland Sea.
The low temperature of the Norwegian Sea Deep Water
(NSDW) is maintained by the contribution of the Greenland Sea
Deep Water (GSDW). The bottom water in the Greenland Sea is renewed
locally by surface cooling of relative fresh water, resulting
in the coldest bottom water found in the deep ocean. NSDW is formed
by mixing GSDW and the deep water from the Arctic Ocean. The recent
warming of the NSDW has its forerunner in an even more markedly
warming of the GSDW, see figure 5, consonant with the idea that
the deep water formation in the Greenland Sea has ceased. The
Greenland Sea and the Norwegian Sea basins are separated by the
Mohn Ridge (Figure 1), and the exchange of water masses between
the two deep basins takes place through a channel which has a
threshold depth of 2200 m and is situated just north of Jan Mayen.
Since the warming of GSDW appears to have continued unchecked
to date, (Figure 5) the cessation of warming observed in the NSDW
since 1990 is certainly unexpected, (Figure 4) suggesting that
as GSDW production has (virtually) ceased, the transport through
the Jan Mayen Channel may have reduced or even reversed, see figure
6, cutting off the deep Norwegian Sea from the influence of the
GSDW and its changes, see Østerhus and Gammelsrød,
1996, and Østerhus et al., 1996.
Fig. 6
Results from the current measurements in the
Jan Mayen Channel from April to November 1991
(thin doted arrows), September 1983 to July 1984
(thin arrows) and from November 1992 to July 1993
(thick arrows). The numbers in parentheses indicate
height of current meter above the bottom.
The stability factor (defined as the absolute value
of the average current vector divided by the
average speed) was 0.91 in 83/84 and 0.18 in 92/93.
The mean temperature was -1.01°C in 83/84,
increasing to -0.94°C in 92/93.
The OSW M will continue its operation at least to
the year 2000. A CD containing quality controlled hydrographic
data (as stations data and time series) is planned for the 50
years anniversary in 1998. The history of OWS M is being written.
It is proposed that Polarfront should be equipped with an Acoustic
Doppler Current Profiler (ADCP). Twenty-four ADCP's and XBT's
sections a year across the Norwegian Atlantic Current will be
invaluable for monitoring the influx of Atlantic water to the
Arctic. Plans for extending the hydrographic, biological and geochemical
programmes exist. The future of M depends on us as researchers
being able to convince the powers of the purse that these data
are really necessary for climate research.
Svein Østerhus,
Nordic WOCE Project Office, Geophysical Institute,
University of Bergen,
Allégt. 70, N-5007 Bergen, Norway
Phone: +47 55 58 26 07, Fax: +47 55 58 96 68, E-mail: svein.osterhus@gfi.uib.no
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