Decadal‐scale drivers of South Pacific hydroclimate variability

Abstract 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 multidecadal time‐scales are poorly understood. Using prescribed sea‐surface temperature (SST) perturbations in the atmosphere‐only Intermediate Global Circulation Model version 4 (IGCM4), we investigate the direct atmospheric mechanisms by which three major modes of low‐frequency climate variability—the Interdecadal Pacific Oscillation (IPO), Atlantic Multidecadal Variability (AMV), and Southern Ocean SST‐driven midlatitude jet shifts—modulate the South Pacific hydroclimate. IPO forcing produces the most substantial and spatially coherent SPCZ response: a positive IPO anomaly drives a northeastward shift in the SPCZ, while a negative IPO shifts the SPCZ southwestward. This behaviour arises from coupled dynamic and thermodynamic changes, with anomalous moisture convergence—rather than altered Rossby‐wave refraction—emerging as the dominant control on SPCZ position. By contrast, AMV‐forced atmospheric teleconnections 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 midlatitude jet and associated Hadley–Ferrel cell structure, but these changes do not generate a coherent SPCZ displacement. Instead, precipitation changes reflect large‐scale regions of anomalous ascent and descent, driven by Hadley and Ferrel cell shifts, rather than modifications to SPCZ dynamics.