Two Characteristic Patterns of the Summer Indian Ocean Dipole.

This study identifies two distinct patterns of the summer Indian Ocean dipole (IOD)—the coastal IOD and the offshore IOD—named based on the proximity of their eastern pole to Sumatra. Their spatial characteristics, evolutionary mechanisms, relationships with ENSO, impacts on precipitation, and the factors controlling the simulation performances of climate models are discussed. The coastal IOD has the same eastern pole as the conventional IOD, which is located off the coast of Sumatra, but its western pole is situated in the central region of the tropical Indian Ocean (TIO). In contrast, the offshore IOD shares the same western pole as the conventional IOD, which is located off the coast of Somalia, whereas its eastern pole is positioned in the central TIO. Regarding their evolutions, while they initially develop similarly, their later evolutions differ due to their distinct pole locations: The offshore IOD peaks in summer, while the coastal IOD can be sustained into autumn. The coastal IOD correlates to preceding and late ENSO states, but the offshore IOD does not, making it an independent internal mode of TIO. The two IODs affect climate differently, with only the coastal IOD affecting Australian rainfall. Climate models exhibit varied levels of performance in simulating the two IODs. Specifically, a stronger link between spring TIO rainfall and ENSO, as well as stronger southeasterly monsoon winds in the southern TIO, can enhance the coastal IOD modeling, while a stronger summer Somali jet benefits the simulation of the offshore IOD. Distinguishing these two IODs has implications for accurate diagnosis and prediction of the summer climate surrounding the TIO. Significance Statement: The Indian Ocean dipole (IOD) significantly influences the monsoon climate. Previous studies use a unified IOD index based on all-season sea surface temperature (SST) data of tropical Indian Ocean (TIO), resulting in a representation biased toward the autumn peak season. However, understanding SST patterns during summer is crucial for Asian summer monsoon precipitation, and what are the characteristic patterns of summer IOD remains unclear. This study fundamentally advances the understanding of the IOD's seasonal diversity by identifying two distinct dipole patterns in the summer TIO. These patterns differ from the conventional IOD in terms of spatial characteristics, evolutions, ENSO relationships, and precipitation impacts. Distinguishing between these two IODs is anticipated to improve the diagnosis and prediction of summer monsoon rainfall. [ABSTRACT FROM AUTHOR]