Seasonal and inter-annual variability of western subtropical mode water in the South Pacific Ocean

The seasonal and inter-annual variability of the western subtropical mode water (hereafter STMW) in the South Pacific Ocean was examined using the Bluelink ReANalysis 2.1 (BRAN2.1) in terms of heat budget. The analysis of heat content change suggested that the seasonal cycle of surface heat flux played a dominant role in the formation of the STMW in the South Pacific Ocean. However, the surface heat flux and the East Australian Current (EAC) heat transport tended to compensate one another during STMW production. Out of phase or different amplitude of the components led to warming or cooling of the mixed layer, and the heat transport by the EAC in the formation of the STMW cannot be ignored. The correlation between volume anomalies of the STMW and net surface heat flux was insignificant, indicating that the inter-annual variability of the STMW was equally influenced by surface thermal forcing and ocean dynamic processes, such as horizontal advection. This study revealed the important role played by the EAC in the inter-annual variability of the STMW, i.e., a weakened heat transport by the EAC led to an increased volume anomaly of the STMW in the South Pacific Ocean. The STMW production can be further enhanced by La Nina, which drives positive anomaly in sea surface salinity in the western South Pacific and creates a favourable preconditioning for surface cooling in austral winter.