Mechanisms of South Pacific hydroclimate variability on decadal to multi-decadal time scales

The South Pacific Convergence Zone (SPCZ) is the dominant perennial rainfall feature of the Southern Hemisphere, yet the physical mechanisms driving its variability on decadal to multi-decadal timescales remain poorly constrained. Using prescribed sea-surface temperature (SST) perturbations in the atmosphere-only IGCM4 model, we investigate how three major modes of low-frequency climate variability – the Inter-decadal Pacific Oscillation (IPO), Atlantic Multi-decadal Variability (AMV), and Southern Ocean SST-driven mid-latitude jet shifts – modulate South Pacific hydroclimate. IPO forcing produces the most substantial and spatially coherent SPCZ response: a positive (negative) IPO anomaly drives a north-eastward (south-westward) shift in the SPCZ. This behaviour arises from coupled dynamic and thermodynamic dynamic changes, with anomalous moisture convergence – rather than altered Rossby wave refraction – emerging as the dominant control on SPCZ position. By contrast, AMV-forced atmospheric tele-connections exert only weak and statistically insignificant impacts on South Pacific precipitation; any apparent signal is best interpreted as an alias of IPO-like SST anomalies in the Pacific. Southern Ocean SST anomalies induce significant shifts in the Southern Hemisphere mid-latitude jet and associated Hadley–Ferrel cell structure, but these changes do not generate a coherent SPCZ displacement. Instead, precipitation anomalies reflect large-scale regions of anomalous ascent and descent, driven by Hadley and Ferrel cell shifts, rather than modifications to SPCZ dynamics.