Response of the North Pacific subtropical countercurrent and its variability to global warming |
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| Authors: | Lixiao Xu Shang-Ping Xie Qinyu Liu Fumiaki Kobashi |
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| Institution: | (1) Physical Oceanography Laboratory, and Ocean–Atmosphere Interaction and Climate Laboratory, Qingdao, China;(2) Department of Meteorology, International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii;(3) Faculty of Marine Technology, Tokyo University of Marine Science and Technology, Tokyo, Japan;(4) Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan; |
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| Abstract: | 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. |
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