Rainfall in India

Although the relationship between the Indian summer precipitation and ENSO in recent years is some what weakened, on the whole, the summer anomalous rainfall in India is still a typical example of the ENSO impacts. One of the causes of the weakened relationship between the Indian summer rainfall and ENSO is considered the influence of the Indian Ocean SSTA. If the composite effect of the SSTA in both the Pacific and the Indian Ocean is taken into account, it may make the relationship between our presented composite mode and the Indian summer rainfall become relatively good. Figure 5 shows the Indian summer rainfall corresponding to the pure El Niño year and the positive phase of the PIM, respectively. More rainfall than average occurs in the middle part of India, less rainfall happens in the northern part and the southwestern part of India corresponding to the pure El Niño year (Fig. 5). The whole India receives substantially less rainfall than the average corresponding to the positive phase of the PIM (Fig. 5(b)).

over land (PREC/L) in summer for positive phase of (El Niño) of the ENSO mode (a), (b) positive phase of the PIM. Unit: mm/JJA. The dark shading and light shading show the i-test significance at the 0.05 and 0.1 levels, respectively.

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

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Fig. 6. Precipitation anomalies in summer (JJA) by the SSTA forcing of positive phase of the PIM. Unit: mm/d.

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Fig. 6. Precipitation anomalies in summer (JJA) by the SSTA forcing of positive phase of the PIM. Unit: mm/d.

phase of the PIM after 16 April. In order to decrease influences of initial error and other errors, the control experiment and sensitivity experiment apply seven ensemble members starting from some different initial states. The seven ensemble members mean is used. From the difference between the control experiment and the sensitivity experiment, it can be the anomalous response to the SSTA forcing.

Figure 6 gives the precipitation anomaly forced by the SSTA of positive phase of the PIM. We can find that substantial negative anomaly occurs in Indian Peninsula, Indochina, and Indonesia, which is quite consistent with Fig. 5(b). But it differs from Fig. 5(a) of ENSO. The numerical results can be compared well with the observation evidence. The positive rainfall anomaly occurs from Southwest China to Northeast China with center in Sichuan Province and Guizhou Province of China and Korea in numerical simulation. The Changjiang-Huaihe River Basin and large part of South China also get more rainfall. This indicates that the numerical results in most area of China are the same as those observed. But in few areas of China the numerical results do not agree with those observed. The mechanism of summer precipitation in China is complex and middle-high latitudes are not all influenced by SSTA. Collectively, the numerical results are comparable well with those observed. The numerical simulation further indicates that the PIM has important impact on Asian climate which differs from that of ENSO mode.

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