carried out in Reykjavik and Gibraltar, allow scientists to
reconstruct past trends as far back as 1850. Comparison
between winter air temperatures measured in weather stations shows that during the positive phase of the NAO,
temperatures are higher than average in Western Europe and
the southeast of Northern America, but below average in
Greenland, the Labrador Sea, in northwestern Africa and the
Eastern Mediterranean. The opposite is true in the negative
phase of NAO.
These teleconnections between different sites are used by
paleoclimatologists to reconstruct changes in the NAO in the
pre-instrumental period. Tree rings provide a good record of
climate conditions, especially of periods of extreme heat or
drought (Chap. 16, Volume 1). Recordings obtained from
trees of North America and Europe show alternating phases,
some positive, some negative, since the beginning of the
eighteenth century, with short periodic elements (8 and
2.1 years) but also multi-decadal ones (70 and 24 years).
Snowfall and ice accumulation in the western part of
Greenland are abnormally low during the positive NAO
Storm frequency
anomaly (%)
Temperature
anomaly (°C)
Precipitation
anomaly (mm)
Fig. 1.7 The positive and
negative phases of the NAO and
their impacts on the distribution
of temperature, precipitation and
winds
Fig. 1.6 Impact of variations in the intensity of the monsoon between
the last glacial maximum and current times on the hydrology of the
North Indian Ocean. a The current salinity of the Bay of Bengal where
the rivers draining the Himalayas flow is much lower than that of the
Arabian Sea where evaporation is dominant. b The isotopic composition of oxygen in planktonic foraminifera in recent marine sediments
is a good record of these changes because there is little variation in the
temperature of the northern Indian Ocean. c The isotopic composition
of oxygen in planktonic foraminifera in marine sediments deposited
during the last glacial maximum dated by
14
C at around 18,000 years
BP shows a considerable decrease in the flow of rivers into the Bay of
Bengal and thus in the intensity of monsoon rains
b
1 The Climate System: Its Functioning and History
13
reconstruct past trends as far back as 1850. Comparison
between winter air temperatures measured in weather stations shows that during the positive phase of the NAO,
temperatures are higher than average in Western Europe and
the southeast of Northern America, but below average in
Greenland, the Labrador Sea, in northwestern Africa and the
Eastern Mediterranean. The opposite is true in the negative
phase of NAO.
These teleconnections between different sites are used by
paleoclimatologists to reconstruct changes in the NAO in the
pre-instrumental period. Tree rings provide a good record of
climate conditions, especially of periods of extreme heat or
drought (Chap. 16, Volume 1). Recordings obtained from
trees of North America and Europe show alternating phases,
some positive, some negative, since the beginning of the
eighteenth century, with short periodic elements (8 and
2.1 years) but also multi-decadal ones (70 and 24 years).
Snowfall and ice accumulation in the western part of
Greenland are abnormally low during the positive NAO
Storm frequency
anomaly (%)
Temperature
anomaly (°C)
Precipitation
anomaly (mm)
Fig. 1.7 The positive and
negative phases of the NAO and
their impacts on the distribution
of temperature, precipitation and
winds
Fig. 1.6 Impact of variations in the intensity of the monsoon between
the last glacial maximum and current times on the hydrology of the
North Indian Ocean. a The current salinity of the Bay of Bengal where
the rivers draining the Himalayas flow is much lower than that of the
Arabian Sea where evaporation is dominant. b The isotopic composition of oxygen in planktonic foraminifera in recent marine sediments
is a good record of these changes because there is little variation in the
temperature of the northern Indian Ocean. c The isotopic composition
of oxygen in planktonic foraminifera in marine sediments deposited
during the last glacial maximum dated by
14
C at around 18,000 years
BP shows a considerable decrease in the flow of rivers into the Bay of
Bengal and thus in the intensity of monsoon rains
b
1 The Climate System: Its Functioning and History
13