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<p>Ocean salinity evidently exhibits the change in its climatological mean and long-term trend due to the acceleration of hydrological cycle under global warming. However, at presents, systematic research is still lacking on decadal and long-term changes of salinity and their contributions to the upper ocean stratification, an important factor affecting vertical mixing and sea surface temperature (SST). In this study, the ensemble empirical mode decomposition (EEMD) method is used first to extract the nonlinear trend in salinity during 1940–2019, and then the corresponding long-term trends and their impacts on the vertical stratification in the tropical Pacific are analyzed. The results indicate that the salinity changes are regionally- and time-dependent, with the salinity trends in different regions of the tropical Pacific showing differences during different periods relative to SST trend. For example, sea surface water becomes fresher in the western tropical Pacific and southern Pacific convergence zone under global warming, while it becomes saltier in the southeastern Pacific. As a result, under the combined contributions of temperature and salinity with their compensated effects, the sea surface density decreases significantly in the tropical Pacific with the largest decrease centered in the warm pool, while the subsurface density in the western tropical Pacific increases. These contrasting changes act to enhance the vertical differentiation in the density between the surface and subsurface waters, leading to more stable ocean stratification. The mixed layer becomes shallower near the equatorial dateline but deeper in the warm pool, mainly due to salinity contributions. In addition, salinity and temperature make different contributions to variations of barrier layer thickness in different regions, with the changes of salinity (temperature) leading to the thickening of barrier layer located at 160°E east (west). It is illustrated that salinity variations in the Pacific affect the ocean thermodynamic processes under global warming, which then modulate the climate variability.</p>

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Upper ocean salinity and temperature changes and their contributions to the stratification in the tropical Pacific under global warming

  • 期刊论文

  • 发表日期：

    2025年1月19日

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  • Hai Zhi,
  • Rong-Hua Zhang,
  • Meng Dong,
  • Minmin Wu

摘要

Ocean salinity evidently exhibits the change in its climatological mean and long-term trend due to the acceleration of hydrological cycle under global warming. However, at presents, systematic research is still lacking on decadal and long-term changes of salinity and their contributions to the upper ocean stratification, an important factor affecting vertical mixing and sea surface temperature (SST). In this study, the ensemble empirical mode decomposition (EEMD) method is used first to extract the nonlinear trend in salinity during 1940–2019, and then the corresponding long-term trends and their impacts on the vertical stratification in the tropical Pacific are analyzed. The results indicate that the salinity changes are regionally- and time-dependent, with the salinity trends in different regions of the tropical Pacific showing differences during different periods relative to SST trend. For example, sea surface water becomes fresher in the western tropical Pacific and southern Pacific convergence zone under global warming, while it becomes saltier in the southeastern Pacific. As a result, under the combined contributions of temperature and salinity with their compensated effects, the sea surface density decreases significantly in the tropical Pacific with the largest decrease centered in the warm pool, while the subsurface density in the western tropical Pacific increases. These contrasting changes act to enhance the vertical differentiation in the density between the surface and subsurface waters, leading to more stable ocean stratification. The mixed layer becomes shallower near the equatorial dateline but deeper in the warm pool, mainly due to salinity contributions. In addition, salinity and temperature make different contributions to variations of barrier layer thickness in different regions, with the changes of salinity (temperature) leading to the thickening of barrier layer located at 160°E east (west). It is illustrated that salinity variations in the Pacific affect the ocean thermodynamic processes under global warming, which then modulate the climate variability.