On the role of the Indian Ocean as a precursor of ENSO

In recent years, research has suggested that the Indian Ocean dynamics and specifically, the Indian Ocean Dipole (IOD) might influence the state of ENSO with lead times up to 15 months, potentially exceeding past the spring predictability barrier, thus providing information key for the skillful forecasting of El Niño events. This study analyzes the ocean subsurface temperature in the tropical band to assess the role of the Indian Ocean as a precursor of the ENSO. Composite analyses of El Niño events suggest that subsurface temperature anomalies in the easternmost region of the Indian Ocean are the inverse to those present in the eastern Pacific Ocean. The East Indian Ocean region shows a significant lagged correlation with the West Pacific Ocean, leading the generation of Kelvin waves in the thermocline. The most significant lagged correlation corresponds to 6-8 months. These results suggest the existence of an ENSO precursor in the Indian Ocean. In order to investigate this relationship, extended empirical orthogonal functions (EEOF) were used to identify propagating structures between the basins. The first two leading extended modes portray the full ENSO cycle. The first mode is consistent with the recharge (discharge) of the Pacific Ocean heat content, which is a precondition for El Niño (La Niña) development. The second mode shows the propagation of a Kelvin wave, causing the onset of El Niño and La Niña events. At the same time, there is a response in the Indian Ocean: while the Kelvin wave is moving eastward in the Pacific Ocean, there is an inverse activity that develops in the eastern Indian Ocean and expands to the western Pacific Ocean. This study seeks to analyze the connection between these phenomena, the atmospheric counterparts, and points towards the simultaneous development of an atmospheric and oceanic bridge, which could, in turn, provide information beyond the spring predictability barrier. Furthermore, this study suggests that tropical Indo-Pacific climate variability should be studied as a whole rather than as separated basin modes.