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The details on the scaling factors (s in the following equation) used at ECMWF to produce these relative indices are shown below:.
Niño3.4 Niño3 Niño4 Niño1+2
month
1 1.233 1.239 1.300 1.208
2 1.262 1.291 1.264 1.154
3 1.303 1.297 1.246 1.067
4 1.306 1.213 1.243 1.049
5 1.219 1.138 1.275 1.070
6 1.152 1.112 1.294 1.073
7 1.079 1.080 1.150 1.055
8 1.092 1.110 1.107 1.074
9 1.155 1.150 1.184 1.114
10 1.185 1.189 1.233 1.158
11 1.194 1.199 1.256 1.197
12 1.212 1.213 1.296 1.222
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