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Comparing Fig. 5(a) with Fig. 5(b), it can be found that the difference
between the influence of the pure El Nino and that of the positive phase
of the PIM is quite large. The influence of the positive phase of the PIM
can increase the drought in India. Although the mean summer rainfalls
in 1983 (figure omitted) and 1997 are near average in India and slightly
more than average in some regions of India,
29
other 5 years among the
seven cases receive less rainfalls. It indicates that less rainfall in India is
a main feature during the positive phase year of the PIM. Therefore, it is
worth discussing that the occurring of the Indian Ocean temperature dipole
causes the Indian rainfall in El Nino year being not negative anomaly. The
relationship between ENSO and Indian rainfall is weakened recently much
because of other factors including the role of Atlantic circulations.
30
In
spite of the so-called “Weakening ENSO-Monsoon relationship” India faced
severe droughts during the moderate El Nino episodes of 2002 and 2004.
Studies have shown that the impact of ENSO events on Indian Monsoon
rainfall is modulated by the decadal variability in monsoon rainfall and
depends on the prevailing epoch; i.e. the impact of El Nino (La Nina) in
more severe during the below (above) normal rainfall epochs.
32
,
33
,
35
The
Indian Monsoon rainfall has been under the below normal epoch since last
four decades, hence the impact of 2002 and 2004 El Nino episodes may have
resulted in droughts over India.
The above observation analysis suggests that the PIM and its role are
significantly different from the Pacific ENSO. For isolating the influence
of SSTA on climate change, ensemble simulation is used for further
investigating the climate effect of the PIM. The R42L9 numerical model
in this paper is a global generation spectral model developed by State
Key Laboratory of Numerical Modeling for Atmospheric Sciences and
Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics,
Chinese Academy of Sciences. The horizontal resolution is roughly
2.8125(lon)
1.66(lat) with a rhomboidal truncation at wavenumber 42.
The vertical adopts the terrain-following Sigma coordinate with nine levels.
The model uses a reference atmosphere, and semi-implicit time-integration
scheme with 15-min time step. This model was successful in modeling
climate characteristics.
31
Control experiments and sensitivity experiments have been performed
with the time integration from 1 January to 30 September. The monthly
mean SST is used in the control experiment. In the sensitivity experiment,
the forcing SST uses the monthly mean SST plus the SSTA in the forcing
region 15
◦
S-15
◦
N, 40
◦
E-75
◦
W which is twice that (Fig. 2(d)) of the positive
∗
