Decadal to Interdecadal Variability
Decadal to Interdecadal Variability
It has recently been discovered that there exist significant
decadal-to-interdecadal climate variations in the atmosphere and
oceans. They have a lifetime much longer than that of the interannual
El Nino- Southern Oscillation (ENSO) phenomenon. While several types
of decadal-scale variations have so far been reported, the following
two have been dominant in recent decades: an interdecadal climate
change in the mid-1970s over the Pacific Ocean and a decadal change in
the mid- and high-latitude atmosphere around the winter of 1989. The
latter was also related to a distinct winter climate shift over Japan.
Coupling between the ocean and atmosphere appears crucial for the
former mode, and indeed the CCSR coupled atmosphere-ocean GCM has been
found capable of simulating Pacific interdecadal variations similar to
observations. On the other hand, an observational and modeling study
of the atmospheric change in 1989 has revealed that this change was
related to the variations in snow cover, sea surface temperature
(SST), and sea ice distributions.
Pacific Interdecadal Variations (Figure 1)
As shown in Figure 1 (left panels), the observed SST, sea-level
pressure (SLP), and surface winds in the tropical and extratropical
Pacific underwent a large shift around the years 1976-77. In the
decade after 1977, SSTs were higher in the eastern tropical Pacific,
while they were lower in the central North Pacific (Fig. 1, upper
left). Concurrently, lower SLP and intensified westerly winds were
found over the North Pacific (Fig. 1, lower left). Warmer SSTs in the
eastern tropical Pacific also accompanied westerly wind anomalies to
the west. These observed interdecadal changes were captured well by
the CCSR coupled GCM, which has simulated a decadal-to-bidecadal
variability in the Pacific. Spatial patterns of SST, SLP, and wind
anomalies in the simulation (Fig. 1, right panels) have a strong
resemblance to observations.
[Figure 1]: Decadal anomalies in the Pacific SST (upper panels),
sea level pressure and surface winds (lower panels) in
the observational (left) and simulated (right) fields.
Observations are based on 10-year difference
maps centered in 1977, while the results of a regression
analysis are shown for the simulation by the CCSR coupled
GCM.
Decadal Change in the Extratropical Atmosphere (Figure 2)
Another decadal-scale shift in the northern extratropical atmosphere
was detected in the winter of 1989 by observational data. Five-year
differences in surface temperature and mid-tropospheric pressure
fields show, respectively, strong warmings and positive anomalies over
Europe, eastern Eurasia, and the North Pacific (Fig. 2). Tropospheric
pressure also indicates negative anomalies in higher latitudes and
thus forms a north-south seesaw between mid-latitudes and polar
regions.
[Figure 2]: Difference maps for the 500hPa geopotential height
(upper panel) and the surface temperature (lower panel)
over 5 winters, 1989-93 and 1984-88.
Role of Snow and SST Anomalies (Figure 3)
An observational analysis suggests the importance of snow anomalies
over eastern Eurasia for the atmospheric change in the winter of 1989.
An atmospheric GCM experiment further supports this hypothesis;
mid-tropospheric pressure simulated by the model in which the Eurasian
snow has been artificially decreased shows an anomaly pattern similar
to observations made in 1989 (Fig. 3, upper panel). In addition, SST
anomalies during the period also had a significant impact on the
atmospheric changes (Fig. 3, lower panel). These results imply that
both snow and SST anomalies played important roles in generating and
maintaining the atmospheric decadal anomalies in the late 1980s.
[Figure 3]: Wintertime 500hPa height anomalies in the CCSR AGCM
forced with reduced Eurasian snow anomalies (upper panel)
and global SST anomalies (lower panel). Both panels bear
resemblances to observations during the winter of 1988-89.
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Last revised: Tuesday, 30-Jul-2002 15:03:20 JST