Quantitatively Isolating Extratropical Atmospheric Impact on the Tropical Pacific Interannual Variability in Coupled Climate Model

Recent studies have demonstrated a significant control of extratropical atmospheric forcing on the tropical Pacific interannual variability. However, it remains unclear how the extratropical atmospheric signal is transferred equatorward via potential pathways. Here aided by regional coupled data assimilation and partial restoring techniques, the extratropical atmospheric impact on the tropical Pacific interannual variability is quantitatively isolated into atmospheric, oceanic and coupled ocean-atmosphere teleconnections at different latitudes in a coupled general circulation model. Results show that poleward of 20 degrees, the atmospheric pathway dominates the extratropical impact on the tropical Pacific, accounting for 90% of the total contribution. Towards the equator, less contribution is attributed to the atmospheric pathway, while more to the coupled ocean-atmosphere teleconnections and the oceanic pathway. Poleward of 10 degrees, contribution from the atmosphere rapidly decreases to 46%, while impact from coupled interactions significantly increases to 46% and that from the ocean slightly increases to 8%. Composite analyses show that coupled interactions between 10 degrees and 20 degrees significantly impact the ENSO (El Nino-Southern Oscillation) onset by modulating the Pacific meridional mode and the preconditioning of equatorial Pacific heat content.