Seasonally dependent changes in the North Pacific subtropical anticyclone during the mid-Holocene: Dominant role of mid-latitude baroclinicity

Abstract

Understanding mechanisms behind North Pacific subtropical anticyclone (NPSA) changes is pivotal for interpreting mid-Holocene climate variations. This study investigates NPSA changes between mid-Holocene and preindustrial periods using simulations from 15 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The results reveal pronounced seasonal disparities: the NPSA weakens and contracts in spring but strengthens and expands in summer. These variations stem from orbitally induced changes in meridional and zonal thermal structures, which modulate the NPSA by redistributing precipitation-released latent heat and by altering the North Pacific westerly jet. In spring, reduced insolation shifts North Pacific baroclinic zones southward, as indicated by meridional temperature gradient changes, and attenuates land–sea contrasts in surface equivalent potential temperature (θ_e). The southward-shifted baroclinic zone displaces the westerly jet southward, accounting for NPSA weakening over its northern–central sector, while the attenuated land–sea θ_e contrast enhances oceanic convective precipitation, weakening the NPSA's eastern flank. In summer, the NPSA intensification primarily results from reduced large-scale precipitation over the North Pacific due to diminished mid-latitude baroclinicity caused by increased insolation. This summertime NPSA intensification is additionally fueled by enhanced convective precipitation over Asian land and diminished convective precipitation over the North Pacific, associated with orbitally amplified land–sea θ_e contrasts. This study highlights the primary role of altered meridional temperature gradients, which largely determine atmospheric baroclinicity, in shaping the NPSA, rather than the previously emphasized land–sea thermal contrasts. The simulation plausibly explains discrepancies in NPSA changes inferred from proxies in the western and eastern North Pacific.