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2 Observations and Proxy Indicators
2.1 Direct Observations
Direct observations begin to exhibit large-scale changes in the climate system on the interdecadal time scale. From the comparison of systematic
temperature and salinity measurements of the last 40 years a distinct warming in depths between 700 m and 3000 m could be identified in the North
Atlantic (Roemmich and Wunsch 1984; Levitus 1989c). Moreover, Parilla
et al. (1994) find, superimposed on this warming, zonally alternating regions of cooling at 24°N and attribute this to large-scale decadal variability
in the North Atlantic. There is also evidence of long-term variability and
even step-like changes of properties in the marginal seas of the North Atlantic. Schlosser et al. (1991) found a significant reduction of ventilation
of the deepwater in the Greenland Sea on the basis of tracers, while Lazier
(1996) shows a notable decrease in salinity in the Labrador Sea over the
last thirty years. Although these latter two studies do not show cycles they
provide potentially important information on mechanisms and focal areas
of variability.
Variability on decadal-to-century time scales is also found in atmospheric
variables such as air temperature, wind speed and sea level pressure and
can be related to concurrent changes in the underlying ocean. Deser and
Blackmon (1993) find a cycle of 9-12 years in their analysis of 90 years of
winter time means of sea surface temperature (SST), air temperature, wind
speed and sea level pressure in the North Atlantic region. They identify
two modes in the North Atlantic. The first is a north-south aligned dipole
pattern with a negative anomaly of sea level pressure to the north. This
results in a westerly wind anomaly enhancing the prevailing wind field
and increasing the sea-air fluxes leading to negative SST anomalies. The
second pattern is associated with the Gulf Stream where strong positive
SST anomalies are found between the period 1939-1968 and 1900-1929. It
is probably a response to the global surface warming trend in the 1920s and
resulted in a general weakening of the basin-scale atmospheric circulation
over the North Atlantic.
Kushnir (1994) has also analysed the above data set with an emphasis
to identify the difference patterns between cold (1900-14, and 70-84) and
2 Observations and Proxy Indicators
2.1 Direct Observations
Direct observations begin to exhibit large-scale changes in the climate system on the interdecadal time scale. From the comparison of systematic
temperature and salinity measurements of the last 40 years a distinct warming in depths between 700 m and 3000 m could be identified in the North
Atlantic (Roemmich and Wunsch 1984; Levitus 1989c). Moreover, Parilla
et al. (1994) find, superimposed on this warming, zonally alternating regions of cooling at 24°N and attribute this to large-scale decadal variability
in the North Atlantic. There is also evidence of long-term variability and
even step-like changes of properties in the marginal seas of the North Atlantic. Schlosser et al. (1991) found a significant reduction of ventilation
of the deepwater in the Greenland Sea on the basis of tracers, while Lazier
(1996) shows a notable decrease in salinity in the Labrador Sea over the
last thirty years. Although these latter two studies do not show cycles they
provide potentially important information on mechanisms and focal areas
of variability.
Variability on decadal-to-century time scales is also found in atmospheric
variables such as air temperature, wind speed and sea level pressure and
can be related to concurrent changes in the underlying ocean. Deser and
Blackmon (1993) find a cycle of 9-12 years in their analysis of 90 years of
winter time means of sea surface temperature (SST), air temperature, wind
speed and sea level pressure in the North Atlantic region. They identify
two modes in the North Atlantic. The first is a north-south aligned dipole
pattern with a negative anomaly of sea level pressure to the north. This
results in a westerly wind anomaly enhancing the prevailing wind field
and increasing the sea-air fluxes leading to negative SST anomalies. The
second pattern is associated with the Gulf Stream where strong positive
SST anomalies are found between the period 1939-1968 and 1900-1929. It
is probably a response to the global surface warming trend in the 1920s and
resulted in a general weakening of the basin-scale atmospheric circulation
over the North Atlantic.
Kushnir (1994) has also analysed the above data set with an emphasis
to identify the difference patterns between cold (1900-14, and 70-84) and