Vertical and horizontal structures of the North Pacific subtropical gyre axis
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Mean structures of the North Pacific subtropical gyre are investigated using Argo float observations and several wind stress products, with a particular focus on the gyre axis, which is defined as a large-scale boundary between eastward motions on the northern part of the subtropical gyre and westward motions on the southern part. Structures of the gyre axis are different between the regions east and west of about 180°. In the west, the gyre axis is affected by the strong Kuroshio Extension jet, showing discrepancy with features expected from the Sverdrup balance and ventilated thermocline theory. In the east, depth-integrated geostrophic circulation displays the two gyre axes at about 25°N and 30°N in association with a local cyclonic wind stress curl anomaly. It is expected from the Sverdrup balance that the two gyre axes would extend to the west across the subtropical gyre, but the actual geostrophic circulation exhibits only one gyre axis around 30°N to the west. The vertical structure of the subtropical gyre exhibits a remarkable northward shift of the gyre axis with depth in the east, consistent with the southward decrease in the thickness of density layers around the gyre axis, while the northward shift is less obvious in the west, where the gyre axis lies along the southern flank of the Kuroshio Extension. The gyre shift is relatively smaller in the central mode water (CMW) layer due to the horizontal uniformity of the CMW, suggesting a dynamic effect of the CMW on large-scale circulations of the subtropical gyre.
KeywordsNorth Pacific subtropical gyre Wind-driven circulation Gyre axis Sverdrup balance Ventilated thermocline theory
Comments and suggestions from the editor and anonymous reviewers helped to greatly improve the manuscript. This study is partially supported by Grants-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology, Japan (22106007, 23340139). The MOAA GPV was obtained from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) website. The JRA25 wind stress data was obtained from the Japan Meteorological Agency website, the J-OFURO from the J-OFURO website of Tokai University, the NCEP/NCAR and NCEP/DOE data from the NOAA website, and the MERRA from the NASA website. The authors would like to acknowledge these data providers.
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