THE GLOBAL HYDROLOGIC CYCLE LINKS THE OCEAN ATMOSPHERE CRYOSPHERE AND CONTINENTS AND IS GREATLY IMPACTED BY THE EARTH'S CLIMATE. AS AN ESSENTIAL PROXY OF THE CHANGING WATER CYCLE THE VARIATIONS IN OCEAN SALINITY INDICATES SUBTLE CHANGES IN THE EARTH'S CLIMATE SYSTEM THAT ARE DIFFICULT TO MEASURE THROUGH OTHER APPROACHES. WHILE NUMEROUS STUDIES HAVE FOCUSED ON THE SURFACE SALINITY CHANGE THE SUBSURFACE WATER ESPECIALLY THE DEEP AND ABYSSAL OCEANS ARE LESS DISCUSSED DUE TO LIMITED OBSERVATIONAL DATA. AS SUCH A SIGNIFICANT PORTION OF THE GLOBAL OCEAN IS LESS STUDIED IN THE ASSESSMENTS OF GLOBAL SALINITY CHANGE AND THE EXCHANGES BETWEEN THE UPPER AND DEEP LAYERS OF THE GLOBAL OCEAN ARE USUALLY IGNORED THE INFERENCE ABOUT THE CHANGES IN GLOBAL WATER CYCLE FROM MERELY UPPER OCEAN SALINITY MEASUREMENTS ARE PROBABLY NOT ACCURATE. IN THE PAST FEW YEARS THE OCEAN REANALYSES PRODUCTS HAVE BEEN PROVED AS A POWERFUL TOOL TO TIE TOGETHER REAL-WORLD MEASUREMENTS WITH CLIMATE MODELS AND BEEN UTILIZED FOR ANALYZING THE CHANGES ON BOTH THE SURFACE AND SUBSURFACE OCEAN. ONE OF THE STATE-OF-THE-ART OCEAN STATE ESTIMATES IS ECCO (ESTIMATING THE CIRCULATION&CLIMATE OF THE OCEAN) VERSION 4 WHICH ARE DYNAMICALLY CONSISTENT AND AT THE SAME TIME CONSTRAINED BY GLOBAL-SCALE OBSERVATIONS PROVIDING OPPORTUNITIES FOR DESCRIBING AND INTERPRETING CHANGES IN THE GLOBAL OCEAN. HERE WE PROPOSE A THOROUGH AND DETAILED EXAMINATION OF THE MULTIDECADAL CHANGES IN THE GLOBAL OCEAN SALINITY AND THE CONTROLLING DYNAMICAL MECHANISM WITH THE ECCO ESTIMATE. PRELIMINARY RESULTS WITH ECCO HAVE ALREADY INDICATED THAT A SET OF PHYSICAL PROCESSES IN THE OCEAN INTERIOR COULD CONTRIBUTE TO THE OCEAN VERTICAL SALT TRANSPORT INCLUDING OCEAN CIRCULATION MESOSCALE EDDIES CONVECTION ETC. A DEEPER LOOK INTO THOSE PROCESSES AS WELL AS THE EXTERNAL FORCINGS WILL IMPROVE OUR UNDERSTANDING OF THE RESPONSE OF THE OCEAN INTERIOR TO THE CHANGING CLIMATE. ONE OF THE OVERARCHING SCIENCE GOALS IN THE EARTH SCIENCE RESEARCH PROGRAM IS "CLIMATE VARIABILITY AND CHANGE." AS CLIMATE CHANGE ACCELERATES THE EVOLUTION OF THE GLOBAL WATER CYCLE WHICH IS ANOTHER OVERARCHING SCIENCE GOAL IS INEVITABLE. IN THIS PROJECT WE AIM TO DESCRIBE THE CHANGES OF THE SALINITY FROM THE ECCO ESTIMATE UNDERSTAND THE CONTROLLING PROCESSES BY EVALUATING AND COMPARE THE CONTRIBUTIONS OF MAJOR PARTITIONS OF THE VERTICAL SALT TRANSPORT EXAMINE THE CHANGES IN THE EXTERNAL FORCINGS AND FURTHER INVESTIGATE THE RESPONSE OF OCEAN INTERIOR TO THE FORCING CHANGES. RESULTS OBTAINED FROM THESE WORKS WILL PROVIDE INSIGHTS INTO INTERPRETING THE OBSERVED CHANGING WATER CYCLE AS WELL AS THE UNSETTLED DEEP OCEAN CHANGE.