Response of the North Pacific subtropical countercurrent and its variability to global warming
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Response of the North Pacific subtropical countercurrent (STCC) and its variability to global warming is examined in a state-of-the-art coupled model that is forced by increasing greenhouse gas concentrations. Compared with the present climate, the upper ocean is more stratified, and the mixed layer depth (MLD) shoals in warmer climate. The maximum change of winter MLD appears in the Kuroshio–Oyashio extension (KOE) region, where the mean MLD is the deepest in the North Pacific. This weakens the MLD front and reduces lateral induction. As a result of the reduced subduction rate and a decrease in sea surface density in KOE, mode waters form on lighter isopycnals with reduced thickness. Advected southward, the weakened mode waters decelerate the STCC. On decadal timescales, the dominant mode of sea surface height in the central subtropical gyre represents STCC variability. This STCC mode decays as CO2 concentrations double in the twenty-first century, owing both to weakened mode waters in the mean state and to reduced variability in mode waters. The reduced mode-water variability can be traced upstream to reduced variations in winter MLD front and subduction in the KOE region where mode water forms.
KeywordsNorth Pacific subtropical countercurrent Global warming Decadal variability Mode waters Subduction rate Mixed layer depth
We thank the anonymous reviewers for constructive and helpful comments. This work is supported by the Qianren project, Changjiang Scholar Program, Natural Science Foundation of China (40830106, 40921004), National Key Program for Developing Basic Science of China 2007CB411803 and 2010CB428904, the US National Science Foundation, and the Japan Agency for Marine Earth Science and Technology.
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