A Critical Role for North Pacific Meridional Mode in the ENSO Response to Orbital Precession

Abstract Orbital precession modifies the intensity of the annual cycle at millennial time scales and is a major external driver of El Niño–Southern Oscillation (ENSO) variability in both proxy records and climate model simulations. We examine precession’s influence on ENSO through a subtropical pathway, the Pacific meridional mode (PMM), using a suite of NCAR Community Earth System Model, version 1.2 (CESM1.2), experiments that simulate five precessional extremes: perihelion at autumnal equinox (AE), winter solstice (WS), vernal equinox (VE), summer solstice (SS), and zero eccentricity (E0). We investigate mechanisms that may moderate the PMM’s influence on ENSO such as the strength of midlatitude stochastic forcing via the North Pacific Oscillation, changes in the climatological mean state, and the wind–evaporation–sea surface temperature (SST) (WES) feedback. We find that orbital precession strongly influences PMM variability, the PMM’s ability to trigger El Niño events, and ENSO diversity. Precessional extremes characterized by a more southerly intertropical convergence zone (ITCZ) and stronger trade winds (WS and AE) have more variable PMM behavior and a PMM that is more effective at triggering El Niño events, particularly central Pacific events. Precessional extremes characterized by a more northerly ITCZ and weaker trade winds (SS and VE) have reduced PMM variability and a PMM that is a less-reliable precursor to El Niño events. The PMM response to precession is driven by variations in surface wind fields that moderate the strength of WES feedback, the mechanism by which PMM anomalies grow and propagate. Understanding the sensitivity of ENSO to subtle shifts in the mean state contextualizes past variability and aids in anticipating future change. Significance Statement Orbital precession alters the seasonal distribution of solar insolation around Earth and has vast impacts on the climate system including El Niño–Southern Oscillation (ENSO). Here, we examine how precession influences ENSO through a subtropical ENSO precursor: the Pacific meridional mode (PMM). We use climate models to test how the influence of warm PMM events on El Niño events may vary under extreme states of orbital precession. We find precession moderates the variability of the PMM, the ability of the PMM to trigger El Niño events, and the spatial diversity of El Niño events. These results help us to interpret past climatic changes and understand the sensitivity of the tropical Pacific to small variations in external forcing.