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The difference between the forecasted traditional and relative Niño-3.4 index for all winters (Dec – Feb) from 1991-2024 from all NMME forecasts issued between September and November. Because the global tropics have been warming continuously during this period, an increase in temperature in the Niño-3.4 region is more commonly accompanied by an increase in temperature in the surrounding oceans, so the relative temperature difference between the Niño-3.4 region and surrounding tropics is decreasing. Therefore, the traditional Niño-3.4 index has been consistently higher than the relative index.