Fróðskaparrit - 01.01.1998, Síða 168

174
LATE HOLOCENE CLIMATIC FORCING OF GEOMORPHIC ACTIVITY
IN THE FAROEISLANDS; NORTH ATLANTIC OCEAN
Climate during the instrumental period
Meteorological observations were initiated
in Tórshavn AD 1867 by the Danish Me-
teorological Institute, and the complete
temperature data series obtained is sum-
marised in Figure 2, showing the mean an-
nual air temperature (MAAT) as well as
seasonal values. In general, interannual
temperature variations in Tórshavn since
AD 1867 are seen to be substantial (1-2°C)
for the winter season, December-February,
and smaller (0.1-0.3°C) for the summer
season, June-August. The spring (March-
May) and the autumn (September-Novem-
ber) both display medium year-to-year
variations (0.2-0.8°C). Most of the regis-
tered interannual variation in MAAT is de-
rived from winter temperature variations,
and comparatively little is derived from
summer temperature variations.
The oldest observations (AD 1867-72)
indicate a substantial cooling, even though
summer temperatures then were on the rise,
leading up to a cold period lasting about 45
years, characterised by MAAT ranging
from 6 to 6.5 °C. This period represents the
last cold interval of the Little Ice Age
(LIA), and was terminated by a marked
warming from AD 1920 to 1930, clearly re-
flected in the MAAT, but primarily affect-
ing the winter season (Fig. 2). Since then,
air temperatures in Tórshavn have shown a
gradual falling trend. To illustrate this, the
Tórshavn AD 1931-60 MAAT was 7.0°C,
but only 6.5°C AD 1961-90, and in the late
20th century the Tórshavn climate is again
approaching the cold character experienced
AD 1872-1920. One may wonder if the Lit-
tle Ice Age really ended AD 1920 in the
Faroe Islands? Smaller variations of
±0.5°C with 10-15 yr periods are superim-
posed upon this overall climatic evolution.
These variations presumably reflect con-
temporary variations in water temperature
and intensity of the North Atlantic Drift
such as documented by Hansen and
Meincke (1984). The North Atlantic Drift
presently flows around the Faroe Islands
with atypical velocity of 0.1-0.3 ms"1 and a
mean SST of 8°C (Hansen, 1996). In the
absence of the North Atlantic Drift, consid-
ering the latitude, winter air temperatures
would probably be at least 6-7°C lower
than at present (Søgaard, 1996).
Precipitation in Tórshavn also varied sig-
nificantly during the instrumental period
(Fig. 2). The mean annual precipitation for
the whole period is 1471 mm water equiva-
lent (w.e.), but the annual precipitation has
varied significantly from about 900 to 2200
mm w.e. through a number of periods of
varying length, very much different from
contemporary variations of both MAAT
and seasonal temperatures. As an example,
the first half of the final Little Ice Age cold
interval was characterised by rather wet
conditions, while the latter half was com-
paratively dry. Also the temperature maxi-
ma around AD 1930 were associated with
dry conditions, while especially the late
20th century climate has been characterised
by comparatively high precipitation. It ap-
pears noteworthy that the marked rise in
MAAT AD 1920-30 is not reflected by
variations in the annual precipitation.
Projections of sea-level rise due to anti-
cipated modem greenhouse warming often
involves the assumption that increased wa-