Multidecadal Variability and Secular Change in Summer Rainfall over the Indo-Gangetic Plain and Eastern China: Observations, Simulations, and Origin

The Indo-Gangetic Plain and the lowlands/plains eastward of the Tibetan Plateau exhibit substantial – at times, precarious – trends in summer precipitation since the mid-20th century. These include the declining rainfall over the Gangetic Plain, and a dipole trend pattern over eastern China that is referred as the South-Flood North-Drought (SFND) pattern. The trends have been attributed to increased aerosol and dust loadings, significant land-use land-cover change, and increased greenhouse gas emissions, among others. Interestingly, multidecadal natural variability is not a commonly cited cause. The presence of oppositely-signed trends in the first-half of the 20th century in some of the same regions prompted this reassessment of the role of multidecadal SST variability in generation of rainfall trends over monsoon Asia.

Analysis of the century-long precipitation and SST records suggests that the post-1950 decline in rainfall over the northwestern-central Gangetic Plain and northeastern China is linked with North Pacific decadal SST variability. The rainfall decline over the Himalayan foothills and northeastern Gangetic Plain, and increasing rainfall over the lower Yangtze River basin (the southern SFND center), on the other hand, are found linked with SST Secular Trend (or secular variability) in this analysis.

The SST anomalies associated with North Pacific decadal variability extend well beyond the midlatitude Pacific – into the tropical Indian and Pacific basins where oppositely signed SST anomalies are present. It is hypothesized that the tropical Indian Ocean SST anomalies modulate the meridional ocean-continent thermal contrast, and thus the large-scale distribution of monsoon rainfall over the Asian continent. The findings need corroboration from additional analyses of the observed circulation and thermodynamics fields, and the hypothesis needs support from controlled climate model experiments.