Salinity Variability Modes in the Pacific Ocean From the Perspectives of the Interdecadal Pacific Oscillation and Global Warming

Ocean salinity can be used as an important indicator of climate change by measuring the global hydrological cycle. In our research, observations and reanalysis data sets are applied to determine that, from 1950 to 2018, long-term salinity change in the upper tropical Pacific Ocean was dominated by two empirical orthogonal function (EOF) leading modes on decadal to longer time scales. The first leading mode is defined as the decadal mode, which is related to the Interdecadal Pacific Oscillation (IPO). Additionally, the second leading mode is a global warming-related mode. The decadal mode is highly correlated with the IPO index and its decadal fluctuation significantly enhanced after the 1990s. Salinity budget analyses demonstrate that the freshwater flux term is the major contribution to decadal salinity change. We also proposed that the importance of ocean circulation modulation tends to be increasingly significant after the 1990s. The relative importance of the two salinity modes reversed over the upper 400 m. The global warming-related mode strengthened and became the dominant mode in the subsurface ocean, while the decadal mode weakened. The subsurface freshening is mainly induced by diapycnal mixing and the poleward migration of the ventilated region. Both of these processes were seen in subsurface isopycnal layers, which reveals a connection between ocean interior dynamic modulation and climate change.