Fig. 4: Drought is more likely to develop during El Niño or positive IOD than La Niña or negative IOD.
a Pearson correlation coefficient between the 12-month Standardised Precipitation Index (SPI-12) and the Oceanic Niño Index (ONI) leading by 6 months over the period January 1901 - December 2022. Colours show correlation coefficients statistically significant at the 99% level based on a Student’s t-test. SPI-12 was calculated using monthly rainfall totals from AGCD141. The ONI was obtained from ref. 142 based on ERSSTv5. b Percentage of times that an El Niño event preceded a drought peak. Droughts were defined when SPI-12 < −1 and only droughts that persisted for at least 6 months were included. A drought peak is defined as the middle month of each drought event. El Niño events were identified when the ONI exceeded 0.5o for at least 3 consecutive months. The El Niño occurrence is counted between 6 months before SPI-12 < −1 and the drought peak. Over the period 1901–2022 the count is summed for all identified droughts and divided by the total number of droughts. Colours show significant frequencies of El Niño based on a binomial distribution at the 95% level. c Same as (b) but for La Niña. La Niña events were identified when the ONI remained below −0.5o for at least 3 consecutive months. d–f Same as (a–c) except for the Dipole Mode Index (DMI). The DMI143 was obtained from https://psl.noaa.gov/gcos_wgsp/Timeseries/DMI/ for the period 1901-2022. Positive (negative) IOD events were identified when the DMI was greater (less) than 0.5o for at least 3 consecutive months.
