Salinity plays a vital role in regulating ocean density,stratification and circulation,and serves is an indicator of the coupling between the ocean,atmosphere and land through the water cycle.Thus,salinity is always regarded as a natural“water gauge”.However,a holistic analysis of subsurface salinity down to 2000m is lacking,and there has been no quantification of the uncertainty in the salinity seasonal variation.Therefore,a rigorous analysis on ocean subsurface salinity with uncertainty estimate is needed and performed here.First,the thesis describes a complete view of the seasonal cycle and spatial distribution of upper 2000 m ocean salinity from regional to global scales based on the latest IAP salinity data and other multi-source salinity data.The new data show a robust seasonal variation in global ocean salinity from the surface down to 350 m depth that exceeds the observational uncertainty,and in some regions the variation can be detected at 2000m.Pronounced sea surface salinity(SSS)seasonal signalincluding the Northwest Pacific,Northwest Atlantic,Northeast Indian Oceans;and tropical area in Pacific,Atlantic,and Indian Oceans.In north tropical Pacific(5°N to 30°N),the sea surface becomes fresher(saltier)in the first half of the year and gets saltier(fresher)in the second half of the year.However,when it comes to south tropical Pacific(20°N to 5°N),it shows opposite seasonal signal.This is because the surface precipitation and evaporation changes associated with monsoons and the seasonal changes in atmospheric circulation.In the middle and high latitudes,the evaporation-minus-precipitation is mismatched with local salinity changes,suggesting that river runoff,sea ice change and ocean dynamics have a controlling role.The zonal mean salinity variation shows that SSS anomalies extend down to 40~80m and are generally consistent with E-P,indicating that the role of salinity as“rain gauge”extends to a depth of 40~80m.Below~100m,ocean dynamics plays a vital role in salinity changes,which should be associated with the seasonal variation of winds driven circulation variations.Spatially,the concept of the“salinity as rain gauge”does not work well in the coastal regions and polar regions.It is likely that other processes are dominant in these regions.Besides,a comprehensive assessment of 40 models from the Coupled Model Intercomparison Project phase 5(CMIP5)and 33 models from the CMIP phase 6(CMIP6)have been carried out to determine the climatological and seasonal variation of ocean salinity from the surface to 2000 m.The general pattern of the ocean salinity climatology can be simulated by both the CMIP5 and CMIP6 models from the surface to 2000 m depth.However,this study shows an increased fresh bias in the surface and subsurface salinity in the CMIP6 multi-model mean,with a global average of-0.44 g kg-1 for the sea surface salinity(SSS)and-0.26 g kg-1 for the 0–1000 m averaged salinity(S1000)compared with the CMIP5 multi-model mean(-0.25 g kg-1 for the SSS and-0.07 g kg-1 for the S1000).In terms of the seasonal variation,both CMIP6 and CMIP5 models showed positive(negative)anomalies in the first(second)half of the year in the global average SSS and S1000.The model simulated variation in SSS was consistent with the observations,but not the S1000,suggesting a substantial uncertainty in simulating and understanding the seasonal variation in subsurface salinity.The CMIP5 and CMIP6 models overestimated the magnitude of the seasonal variation of the SSS in the tropics in the region 20°S–20°N but underestimated the magnitude of the seasonal change in S1000 in the Atlantic and Indian oceans.Our assessments show new features of the model errors in simulating ocean salinity and support further studies of the global hydrological cycle,thus,recommend using an observation constraint model(i.e.reanalysis)for the dynamical analysis.Finally,a salinity balance analysis of the upper ocean was conducted in the Pacific Ocean using ECCO reanalysis data to clarify the facts that cause the seasonal variation of salinity in the Pacific Ocean at middle and low latitudes.The results show that the distribution and seasonal variation of freshwater fluxes in the middle and low latitudes of the Pacific Ocean are at same phase with the surface salinity seasonal cycle.Besides,they are basically consistent in spatial distribution,indicating that freshwater fluxes play a vital role in the seasonal variation.The distribution of freshwater fluxes are mainly dominated by changes in precipitation anomalies,which in turn affect the seasonal cycle of regional salinity.The advectiving term does not have a pronounced effect on the seasonal variation of salinity,except in equatorial Pacific where it plays a moderating role in the seasonal variation of salinity.The residual term is weaker than the contribution of other processes in most of the Pacific Ocean thus can be neglected.Based on salinity budget,moisture budget is also used to further find the origin of the seasonally varying freshwater term:the anomalous convergence(dispersion)of the lower wind field corresponds to the rising(sinking)branch of the middle atmosphere,leading to an increase(decrease)of precipitation in the sea,which in turn contributes negatively(positively)to the increase of salinity in the sea. |