Revisiting null hypotheses for ENSO-IOD relationship

The Indian Ocean Dipole (IOD) mode is a major component of the interannual variability of Indian Ocean Climate and plays a crucial role in determining temperature and precipitation patterns across the basin. However, IOD variability correlates significantly with El Niño-Southern Oscillation (ENSO) making it difficult to quantify the IOD independent contribution to the interannual variably of the Indian Ocean basin. Despite the numerous studies investigating the relationships between IOD and ENSO, the scientific community still lacks consensus on the interdependence of these two major tropical modes. Some authors even question the assumption that IOD is an actual independent mode of variability, proposing conceptual and statistical model that represent IOD as the expression of ENSO in the Indian Ocean. 

Inspired by some recent modelling experiments that separate ENSO and IOD independent role in Tropical climate variability, we use reanalysis datasets to revisit fundamental null hypotheses for the relationship between ENSO and IOD. We focus on two of the simple statistical models that have been proposed as null hypothesis in the last two decades, the second being an extended version of the first one that accounts for how these modes are modulated by the seasonal cycle, the so-called Combination mode.  

Both these models rely on the assumption that IOD is essentially a local response to ENSO forcing, which is in contrast with our assessment of IOD-ENSO relationship obtained by applying a novel methodology that uses a seasonally staggered cross-basins EOFs analysis. In addition, the seasonality of SST patterns revel that the IOD dominates Indian Ocean variability in autumn (i.e., leading EOF mode), which occurs only if one takes advantage to the longer records available today. Moreover, the addition of recent observations yields to results that are significantly different with respect to older studies, leading to different conclusions on IOD strength and independence.