Jökull - 01.12.1969, Blaðsíða 50

Fig. 2. Launching positions of drift bottles
recovered in Iceland. Ice limit I: Mean during
May 1919—1943, after Biidel (1950). Ice limit II
after Nansen (1924).
in salinity may be indicated by the air temp-
erature in the following way. In summer the
coldest winds at Jan Mayen are blowing from
the ice, i.e. from westerly direction. Low temp-
eratures will then indicate an eastward move-
ment of the cold and fresh polar waters, and
thus a lowering in the salinity. It is well
known that low salinity in the uppermost lay-
ers of the sea is a necessary condition for ice
formation and ice survival. Similarly, warni
east winds will bring high salinity inlo the
area, thus decreasing the ice risk in the future.
As mentioned before this will be particularly
true in summer. In winter the very cold wincls
may as well be northerly or even northeasterly.
Tliis will not expand the cold and fresh polar
waters towards the east. From the above con-
siderations it might be concluded that Jan
Mayen air temperature in summer is more in-
dicative of ice risk in the sea than is the winter
temperature. But here we rnust note an im-
portant fact. The thermocline formed near the
sea surface clue to summer heating may obscure
considerably the temperature of the important
underlaying layers, down to 50—100 meters
depth. After the cooling in fall and winter the
sea surface will again show approximatelv the
right temperature of these layers and thus be
a better indicator of the risk. It is therefore
difficult to judge in advance whether it is the
46 JÖKULL 19. ÁR
summer-, fall- or winter temperature at Jan
Mayen which will be most useful in forecast-
ing the drift ice in the following ice season at
Iceland. This has to be investigated and will
be discussed in the following.
MATERIAL STUDIED
The monthly temperature of Jan Mayen was
obtained from the Year Book of the Norweg-
ian Meteorological Institute (Norsk Meteoro-
logisk Arbok). These observations were started
in 1921. They were interrupted twice during
World War II. The annual ice incidence in
Iceland waters has been gathered front the
periodical Vedráttan, issued by the Icelandic
Meteorological Office. Some information was
also obtained from the Year Book of the Dan-
ish Meteorological Office (Meteorologisk Aar-
bog).
The ice year is here defined from the be-
ginning of October to the end of September
in the next year. The ice incidence is defined
as the sum of all days when drift ice is sight-
ed within 12 nautical miles from the Icelandic
coast. Icebergs have been omitted but other-
wise the size of ice fields has not been taken
into account. The annual ice incidence defin-
ed in this manner is evidently more prolonged
than the yearly period of heavy ice, obstruct-
ing navigation.
ANNUAL ICE FORECASTS
At first it was investigated how the annual
ice incidence was correlated with the Jan Mav-
en temperature of single months. The result
is shown in Fig. 3. It is the ice incidence in
the year N (Arið N) which is used in this
comparison. It is negatively correlated with
the Jan Mayen temperature of all but one of
the months from December in the year N-1
to May in the year N. Thus a low tempera-
ture in Jan Mayen indicates drift ice heavier
than usual in the Icelandic waters, as would
be expected. In the drawing, all the correla-
tion coefficients significant to the 0.01 level
are marked in black.
It is remarkable that all the summer and fall
months from June to Nov. show up with such
a significant correlation between Jan Mayen