Correlation between positive ONI and rainfall index
The relationship between positive summer ONI (April, May, June) and rainfall index of monsoon (July, August, September) and post monsoon (October, November, December) season is shown in fig 2. During positive summer ONI, when there was positive El Niño (positive ONI) influence, negative correlation with rainfall (deficit rainfall than normal) was observed in northern Indian plains (Uttar Pradesh, Himachal Pradesh), southern India (Kerala, Tamil Nadu), western part of India (Haryana, Delhi, Chandigarh), and western coastal states (Goa, Maharashtra, Telangana). A negligible relationship was found in the states of Arunachal Pradesh, Maharashtra, eastern Rajasthan, east and west Madhya Pradesh, whereas, the western states (i.e., Rajasthan, Gujarat), northeastern (i.e., Arunachal Pradesh, Assam, Meghalaya) northern (i.e., Jammu and Kashmir, Punjab), and eastern states (i.e., Bihar, Jharkhand, West Bengal) showed positive correlation indicating excess rainfall than normal.
The association between positive monsoon ONI (July, August, September) and rainfall index (October, November, December), was found negatively correlated in the north eastern states (Assam, Nagaland, Meghalaya, Tripura, Manipur), eastern part of India (West Bengal, Orissa), western part (Gujarat), and western coastal states (Maharashtra, Kerala, Goa) indicating deficit rainfall than normal. On the other hand, the northern states (Haryana, Delhi, Chandigarh, Himachal Pradesh, Jammu and Kashmir, Punjab), Madhya Pradesh, Rajasthan and Telangana showed positive correlation indicating excess rainfall than normal.
Correlation between rainfall index and dengue case index
The correlation between rainfall index (July, August, September) and dengue case index (July, August, September) showed negative correlation in most of the northeastern states (Nagaland, Manipur, Mizoram, Tripura), northern states (Haryana, Himachal Pradesh, Chandigarh and Delhi), Chhattisgarh, western part (Gujarat, eastern Rajasthan), and western coastal states (Mahasrhtra, Goa) (fig. 3) indicating a increase in dengue with rainfall deficit in the monsoon season. However, when the analyses in respect of Delhi was done with lag periods, a lag period of 1-3 month was found between ‘rainfall index’ and ‘dengue case index’. On the other hand in the eastern states (Bihar, Jharkhand), eastern coastal states (Andhra Pradesh, Telangana, Orissa), and western part of Rajasthan, positive correlation was found indicating dengue outbreaks during excess rainfall.
Further, fig. 3 shows negative correlation between post-monsoon rainfall (October, November, December) and dengue case index (October, November, December), in the northeastern states (Nagaland, Manipur, Mizoram, Tripura), eastern coastal states (Tamil Nadu, Pondicherry, Andhra Pradesh, Talangana), northern India (Haryana, Chandigarh and Delhi), and southern part of western coast (Kerala, Karnataka) indicating increase in dengue cases due to post monsoon rainfall deficit. The eastern states (Bihar, Jharkhand, West Bengal), Madhya Pradesh, northern Chhattisgarh, Orissa), western part of India (i.e., Gujarat, Rajasthan), and western coastal states (western Maharashtra, Goa) register positive correlation between post-monsoon rainfall and dengue case index, representing an intensification in dengue during excess post-monsoon rainfall. However, when the analyses were undertaken with lag period, it was found that with 1-3-month lag period higher correlation was found between rainfall index and dengue case index.
Correlation between positive ONI and dengue case index
The relationship between positive Summer ONI (April, May, June) and dengue case index (July, August, September, lag period of three months) has been shown in fig. 4a, indicating that most of the states like Arunachal Pradesh, Chhattisgarh, Haryana, Uttarakhand, Andaman and Nicobar Islands, Delhi, Daman and Diu show positive correlation (0.69 to 0.84). These states are likely to register dengue outbreaks in monsoon season after high positive summer ONI. On the other hand, negative corrleation was found in the states of Assam, Himachal Pradesh, Meghalaya, Manipur, Mizoram, Uttar Pradesh, Orissa, Jammu and Kashmir and Andhra Pradesh indicating decrease in dengue cases after positive summer ONI (strong El Niño). The states like Gujarat, Karnataka, Rajasthan, Dadra, and Nagar Haveli and Puducherry show ‘poor correlation’ indicating negligible impact of summer El Niño on dengue cases (fig. 4a).
The mapping of correlation coefficient between positive monsoon ONI (July, August, and September) and dengue case index (October, November, and December) is shown in fig. 4b. A positive correlation (0.40-0.68) was found in the states of Arunachal Pradesh, Chhattisgarh, Haryana, A & N Islands, Daman and Diu, Delhi and Haryana indicating that with positive monsoon ONI increase in dengue cases is likely to be witnessed in post monsoon months i.e. October, November and December. On the other hand, negative correlation was found in the states of Assam, Himachal Pradesh, Andhra Pradesh, Jharkhand, Meghalaya, Manipur, Nagaland, Sikkim and Mizoram reflecting decrease in dengue cases after positive monsoon ONI (monsoon El Niño). Rest of the states show ‘very low or no correlation’.
Predicted Dengue outbreak states based on ENSO outlook of 2020
As per probabilistic ENSO outlook (CPC/IRI) (fig. 5) extracted in mid-June 2020, there are moderate to high chances (40-58%) of normal condition (neutral) between July 2020 to December 2020 indicating the possibility of normal rainfall during monsoon and post-monsoon season in India. Therefore, the north-eastern state i.e. Arunachal Pradesh, western part (Gujarat, western Madhya Pradesh), eastern coast (Andhra Pradesh, Tamil Nadu), and northern part of western coast (Maharashtra, Daman and Diu, Goa) are likely to have dengue outbreaks during July to September (monsoon season) of 2020. The eastern coastal states (Andhra Pradesh, Telangana, Orissa), central India (Madhya Pradesh, Chhattisgarh), western coast (Goa, Maharashtra, Daman and Diu, Dadra Nagar Haveli), north-eastern states (Mizoram, Tripura), and the state of Uttarakhand are likely to have dengue outbreaks during post-monsoon months from October to December 2020, provided surveillance and interventions remain unchanged. (fig. 6a and 6b). Moreover, in the early 2021 i.e. February 2021 to March 2021, very high probability of neutral scenario of ENSO is projected.
Local-scale analysis results
As per results, some states (Arunachal Pradesh, Chhattisgarh, Haryana, Uttarakhand, Andaman and Nicobar Islands, Delhi, Daman and Diu) are positively correlated with positive ONI while some are negatively correlated (Assam, Himachal Pradesh, Meghalaya, Manipur, Mizoram, Uttar Pradesh, Orissa, Jammu and Kashmir and Andhra Pradesh). Further, local scale analysis, by identifying one district each from positively and negatively correlated states, was done to find out the association between positive ONI and dengue cases. It was found that in Delhi and Visakhapatnam, 1-3 months lag between positive ONI and reported dengue cases was observed. The results also showed that 2-month lag was found between monthly rainfall index and dengue cases in both the places (Delhi and Visakhapatnam cities). Lag month-wise correlation values between dengue cases and positive ONI, rainfall are shown in table 1. The largest El Niño events (ONI) resulted in very limited number of dengue cases in Visakhapatnam in 2015 (fig. 7) while Delhi recorded a very high number of dengue cases in 2006, 2009, 2015 (fig. 8). Thus, the results show that the association between positive ONI and occurrence of dengue cases in identified areas is statistically significant based on hypothesis test.
Table 1. Correlation Coefficients between monthly positive ONI and Dengue Cases; and rainfall and Dengue Cases (2014-2018) with different lag periods in Delhi and Visakhapatnam (*indicates p value at 5% significance level)
Lag-month
|
Delhi (Correlation coefficient, ‘r’)
|
Visakhapatnam (Correlation coefficient, ‘r’)
|
Positive ONI & dengue cases
|
Rainfall & dengue cases
|
Positive ONI & dengue cases
|
Rainfall & dengue cases
|
0
|
0.368
|
-0.053
|
-0.541
|
0.316
|
1
|
0.429*
|
0.189
|
-0.545*
|
0.661
|
2
|
0.401*
|
0.412*
|
-0.552*
|
0.742*
|
3
|
0.217
|
0.285
|
-0.537*
|
0.501*
|
4
|
-0.152
|
0.093
|
-0.498
|
0.141
|
5
|
-0.178
|
0.079
|
-0.44
|
-0.111
|
6
|
-0.131
|
0.088
|
-0.431
|
-0.293
|
S Error
|
0.105
|
0.058
|
0.019
|
0.147
|