Anthropogenic Disruption of Orbital‐Scale Seasonal Asynchrony in the Tropical Pacific Walker Circulation During the Holocene

Abstract

The zonal sea surface temperature gradient (SSTG) in the tropical Pacific governs the Pacific Walker Circulation (PWC) and global hydroclimate. Although the annual‐mean PWC is thought to have declined since the Mid‐Holocene, the seasonal architecture of its response to external forcing remains poorly constrained, obscuring the distinction between natural variability and anthropogenic trends. Here, using a pair of transient Holocene simulations that isolate orbital forcing from orbital‐plus‐GHG forcing, we demonstrate a fundamental seasonal asymmetry inherent to the tropical Pacific. Pure orbital forcing drives a winter weakening but summer intensification of the SSTG, amplified by orbital modulation of the mixed‐layer depth and the resultant differential thermal sensitivity of the eastern Pacific cold tongue. Crucially, anthropogenic GHG emissions since the Mid‐Holocene disrupt this natural rhythm: GHG forcing disproportionately warms the eastern Pacific across all seasons, counteracting orbital‐driven summer SSTG intensification while accelerating its winter decline. This forcing synergism eliminates the seasonally anti‐phased behavior inherent to orbital cycles, resulting in a uniform PWC weakening that is unresolvable in annual‐mean paleo‐proxies. Our findings establish a pre‐anthropogenic seasonal baseline and highlight the sensitivity of the tropical Pacific to anthropogenic forcing, providing a long‐term context for interpreting recent and future trends.