By participating in the 2017-2020 OSTST, we have in the past 4 years explored and clarified the nature and causes of the multi-scale upper ocean circulation variability in the following 3 research areas: (1) Kuroshio Extension variability and its role in controlling the dedacal variability of the North Pacific climate system, (2) Characterization of interannually-modulating mesoscale eddies in the global oceans, and (3) Exploration of fine-scale SSH signals. This presentation will summarize the major results and provide references.