Revisiting null hypotheses for Indian Ocean interannual variability

While the Indian Ocean Dipole (IOD) plays a crucial role in the interannual variability of Indian Ocean climate, its co-variability with El Niño-Southern Oscillation (ENSO) makes it difficult to accurately quantify its independent contribution, with the scientific community struggling to find a consensus on the relationship between these two major tropical modes. 

Recent modelling experiments that separate the independent roles of ENSO and IOD in tropical climate variability suggest a causal relation in which the Indian Ocean variability is energized by ENSO, while playing a damping role for Tropical Pacific variability. Here, we use reanalysis datasets and climate simulations to revisit fundamental null hypotheses regarding the relationship between these two modes, with the aim of quantifying the variability of IOD independent of ENSO and identifying the type of relation between these modes, ultimately proposing a novel improved null hypothesis for Indian Ocean variability.  

To achieve this, we focus on a series of simple statistical models and assess their skill in representing Indian Ocean variability given the knowledge of the Pacific state. In particular, we divide our analysis between “one-way” models that assume that Indian Ocean predictability comes only from Pacific state variables, and “two-way” models where the two basins can influence each other. Lastly, we compare the results of these statistical models with Pacific Pacemaker experiments, revealing an inconsistency between these two approaches.