Abstract:
Currently, tropical inter-basin interactions are a cutting-edge research topic worldwide, and the impacts of the Atlantic and Indian Oceans on Pacific Ocean climate variability have garnered extensive attention. Atlantic Niño, the dominant interannual mode in the tropical Atlantic, plays an indispensable role in the development and evolution of El Niño–Southern Oscillation (ENSO). Observation results show that the relationship between Atlantic Niño in boreal summer and ENSO in subsequent winter (hereinafter referred to as the Atlantic Niño–ENSO relationship) is nonstationary and exhibits an interdecadal variation feature. However, the key physical factors affecting the interdecadal variations in the Atlantic Niño–ENSO relationship remain unclear. Through observations and the long-term Community Earth System Model 1 (CESM1) simulation archive, this study analyzes the key factors affecting this relationship and draws the following conclusions. (1) The Indian Ocean dipole (IOD) signals in summer and autumn are an important factor regulating the Atlantic Niño–ENSO relationship. Specifically, when Atlantic Niño stimulates abnormal Walker circulation in the tropical Atlantic–Pacific Ocean, a negative IOD can stimulate abnormal Walker circulation in the tropical Indo-Pacific Ocean, whose sinking branch strengthens the original sinking branch of the abnormal Walker circulation in the Atlantic–Pacific Ocean. Such a strengthened sinking branch can induce anomalous, low-level easterly winds in the equatorial Pacific Ocean and thus promote the development of the La Niña event in subsequent seasons and vice versa. (2) The initial heat content state of the Pacific Ocean is another key factor that modulates the influence of Atlantic Niño on ENSO. Atlantic Niño occurring while the equatorial Pacific Ocean is in a “discharged” state (meaning that the zonal mean equatorial thermocline is shallower than normal) facilitates the subsequent formation of La Niña and vice versa. Noteworthily, these two key factors, namely the IOD signals and initial heat content state of the Pacific Ocean, are independent of each other, and their modulation effects on the Atlantic Niño–ENSO relationship are comparable. In conclusion, when focusing on the influence of the tropical Atlantic Ocean on the Pacific climate variability, the influences of the Indian Ocean, specifically IOD signals, and the initial heat content state of the Pacific Ocean should be considered. In light of the limited amount of the observational data, the conclusions of this study based on the long-term CESM1 simulation archive can supplement the past observation results and contribute to the understanding of the interaction between tropical ocean basins.