We thank three reviewers for their insightful suggestions and comments. This work was supported by the Key State Basic Research Program of China under Grant 2012CB417401, the National Natural Science Foundation of China (Grants 40906007, 40890152), and the Open Foundation of Physical Oceanography Laboratory, OUC, under Grant 200902.
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The Southern Ocean is the region between 37.5° and 65°S in the South Pacific and between 35° and 65°S in the other Southern Hemisphere oceans; the zonal boundaries between the Atlantic, Indian, and Pacific Oceans are 30°E, 145°E, and 70°W, respectively; same as the ACC referred to in this paper.