By analyzing the upper-ocean properties of observation-based hydrographic data and validated oceanic reanalysis products, this study presents multidecadal changes of oceanic surface and subsurface thermal conditions in the tropical northwest Pacific Ocean (TNWP) and South China Sea (SCS) during 1960-2015. The analysis reveals that a transition of a 30-yr trend took place in 1980s during the analyzed period for both the surface and subsurface environment. Generally, the warming trend of sea surface temperature (SST) in the TNWP has a similar multidecadal change to that in the SCS. However, a huge accumulating rate of upper-ocean heat content above the 26°C isotherm (UOHC26) showed up in the TNWP (about 3 times compared to that in the SCS) in the last 30 years. In the TNWP, the southward shift of the North Equatorial Current on the multidecadal time scale induces the vertical displacement of isotherms, leading to a strong subsurface warming around the top of the thermocline. Secondarily, the Pacific decadal oscillation (PDO)-related SST regulates the thermal structure in the mixed layer. The multidecadal UOHC26 in the SCS is mainly attributed to the PDO-related SST and further modulated by the isothermal variability caused by the change of basin-scale SCS circulation.
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