Effect of Subsurface High Spiciness Variability on the Weakened Upper Ocean Stratification in the Western Tropical Pacific Since 2000

Recent studies have reported that global upper ocean stratification has intensified under the influence of global warming. Nevertheless, using both observations and long-term reanalysis data, we find that concurrent with the significant ocean warming, the increasing trend of upper ocean stratification attenuated in the western tropical Pacific after 2000. This study highlights the effects of subsurface high spiciness (warm and salty) anomalies on the attenuation of the increasing stratification trend, which mirrors distinct vertical structures in the long-term changes in temperature and salinity. Subsurface warming, rather than sea surface warming, is the primary driver of the increase in the ocean heat content and the weakening of ocean stratification as it substantially reduces the magnitude of the trend in the vertical thermal density gradient after 2000. These phenomena are closely associated with processes on isopycnals and reflect ocean dynamics driven by large-scale atmospheric circulation. Intensified tropical trade winds and accelerated undercurrents facilitate the convergence of heat and salt in the western tropical Pacific. In addition, changes in the depth of the isopycnal layers induced by anomalous wind stress curl contribute to the subsurface spiciness variability through heaving. Notably, the consistent increase in the salinity from the surface to the subsurface also contributes to the post-2000 weakening trend of the vertical haline density gradient. Analysis within the isopycnal framework demonstrates that the emergence of subsurface salty water effectively offsets the negative contribution of the thermal component to the weakening of density stratification in relatively deeper layers.