A seasonal footprinting mode in the southern Indian Ocean leading to the Indian Ocean Dipole event

This study performs statistical analyses and a case study to show that the subtropical Indian Ocean (IO) can cause an Indian Ocean Dipole (IOD) in the form of a seasonal footprinting mode (SFM). This SFM is independent from the El Niño/Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The combined effects of SFM and ENSO can explain the different lifetimes and strengths observed for the IOD. During the positive phase of the SFM, an anomalously high pressure induces anti-cyclonic winds in the southern IO and forces a dipole sea surface temperature (SST) anomaly where the positive/negative pole locates at the southwest/northeast southern IO during January-February. With the seasonal evolution of the IO monsoon and the eastward shift of the high pressure anomaly, the anomalously cool SST gradually moves northeastward to the southeastern tropical IO and strengthens in the following months. Due to the stronger southeasterly wind and heat loss, the SST off Sumatra-Java coast cools after May-June, leading to an early positive IOD onset. The pressure anomaly in the subtropical area may be related to Antarctic variability, suggesting a teleconnection between high-latitude and mid-latitude climate that can further affect the tropics.