Fast enhancement of the stratification in the Indian Ocean over the past 20 years

Abstract Indian Ocean (IO) stratification has important effects on the air-sea interaction, ocean dynamics and ecology. It is, therefore, of significance to investigate the changes in IO stratification. In this study, we use Ensemble Empirical Mode Decomposition (EEMD) to extract the nonlinear long-term trend in the upper IO stratification quantified by potential energy anomaly. The results show that the strengthening of the stratification is spatially and temporally non-uniform. Specifically, the trend of stratification intensified gradually before 1996, but accelerated rapidly after 1996. Temperature and salinity changes play a crucial role in the fast enhancement of stratification and its regional differences. Temperature variations dominate the stratification trend in ∼90% of the IO area, while the contributions of salinity changes are mainly in the Southeast Indian Ocean (SEIO). Vertically, the rapid enhancement of stratification is caused by the trend of temperature and salt in the upper 400 m. We further perform temperature budget analysis and find that the warming trend in the upper 400 m South of IO is mainly modulated by vertical advection and meridional advection, while the warming in the North of IO is mainly induced by air-sea heat fluxes. Salinity budget analysis shows that ocean advection has played a primary role in modulating SEIO salinity over the past 20 years.