Inter-annual Variation
Inter-annual Variation
Studies of climate variability, such as anomalous weather, is
important not only for reducing natural disasters, but also for
increasing our knowledge of the climate system. CCSR makes efforts to
elucidate the mechanisms of natural climate variability using climate
models in combination with observational data analysis.
Monsoon
The Indian monsoon exerts a significant influence on weather, not only
in Japan and other Asian countries but also in Africa and Oceania. The
atmospheric general circulation model (AGCM) developed by CCSR
simulates the interannual variability of monsoons in the 1980s very
well. The dominant factor influencing the year-to-year variation is
the sea surface temperature (SST) changes in the equatorial eastern
Pacific related to the El Nino/La Nina phenomena, which are prescribed
in the model. On the other hand, it is found that land
surface-atmosphere interaction, such as the abundance or lack of snow
in the Himalayas, also plays a positive role, showing consistency with
limited observational evidence.
[Figure 1]: Interannual variability of the intensity of the Indian
summer monsoon measured by zonal wind shear between 850 and 200
hPa. Red and green lines represent the observation and model
results, respectively.
[Figure 2]: Simulated snow anomalies in the winter preceding the
monsoon season. Upper and lower panels correspond to the weak and
strong monsoon years, respectively.
[Figure 3]: Simulated soil moisture anomalies in the preceding
spring season. Upper and lower panels correspond to the weak and
strong monsoon years, respectively.
El Nino
As is seen on the previous page, El Nino is one of the most dominant
modes of short-term climate variability. The El Nino fluctuation of
equatorial sea surface temperature and associated atmospheric Southern
Oscillation (collectively called ENSO) has been well reproduced by a
coupled atmosphere-ocean climate model that explicitly computes motion
in the upper oceans. The simulated amplitude is comparable to that
observed. The dominant role of El Nino in regulating the monsoon
system has also been reconfirmed.
Within the coupled model, errors in individual atmosphere and ocean
models tend to get amplified through air-sea interactions. Therefore,
simulating a realistic climate by a coupled model is not an easy task.
Coupled simulations need to be validated carefully against
observations, and efforts to improve the model should be
continued. Better simulations should go hand-in-hand with a better
understanding of the dynamics of the climate system.
[Figure 4]: Sea surface temperature changes in the equatorial
eastern Pacific. A comparison between observation and the coupled
model simulation.
[Figure 5]: Spatial distributions of sea surface temperature
changes associated with El Nino. Upper and lower figures show the
observation and model results, respectively.
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Last revised: Tuesday, 30-Jul-2002 15:02:12 JST