24th Pacific Science Congress (PSC-24)

Geological archives and climate model simulations indicate that the variability of the El Niño-Southern Oscillation (ENSO) changes significantly over orbital timescales. While precessional forcing has been proposed as a key driver, the underlying mechanisms remain unresolved due to complex interactions between internal variability and external forcing. Here, a high-resolution configuration of the AWI Earth System Model, optimized for tropical dynamics, was applied to simulate ENSO activity across a full precessional cycle. Our results reveal that ENSO is more frequent and intense during periods of austral summer perihelion (and boreal summer aphelion), as in the present day. This strong ENSO variability arises from asymmetric continental heating: austral summer perihelion introduces relatively strong warming on Australia, east of the Indo-Pacific Intertropical Convergence Zone (ITCZ). Because deep convection favors the warmest areas, this causes the ITCZ and the Indo-Pacific Warm Pool (IPWP) to shift eastwards. As a result, the Pacific’s east-west thermal contrast is reduced, lowering the threshold for east-west oscillations of convection and thereby amplifying ENSO activity. In contrast, boreal summer perihelion enhances Afro-Eurasian heating, to the westward of IPWP, displacing the ITCZ and IPWP westward and limiting the eastward expansion of deep convection, resulting in weaker ENSO activity. Proxy records of precipitation and sea surface temperature across the Indo-Pacific support this changes in climate state. By understanding the astronomical modulation of ENSO's long-term behavior, our findings reveal that asymmetric continental heating—also a key signal of global warming—can serve as an indispensable framework for predicting tropical climate variability.
 

Precession,El Nino-Southern Oscillation,Indo-Pacific Warm Pool,Zonal Continental Distribution