Abstract
Interannual-to-decadal variations in the subtropical countercurrent (STCC) and low potential vorticity (PV) water and their relations in the North Pacific Ocean are investigated on the basis of a 60-year-long hindcast integration of an eddy-resolving ocean general circulation model. Although vertically coherent variations are dominant for STCC interannual variability, a correlation analysis shows that an intensified STCC vertical shear accompanies lower PV than usual to the north on 25.5- to 26.1-σθ isopycnal surfaces, and intensified meridional density gradient in subsurface layers, consistent with Kubokawa’s theory (J Phys Oceanogr 29:1314–1333, 1999). The low-PV signals appear at least 2 years before peaks of STCC, propagating southwestward from the subduction region.
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Notes
Around 160°–140°W, positive PV correlations are found to the north of positive zonal velocity anomalies. In contrast to that found in Fig. 10a, these are not accompanied by negative PV anomalies below. The positive PV anomalies are probably due to advection by westward current anomalies found there (not shown) on a background of strong zonal PV gradients.
Indeed, simulated STF in early spring also show weak correlation (r ~ 0.4 at the maximum) between the Ekman convergence and meridional temperature gradient to about 100-m depth. This is consistent with the result of Qiu and Chen (2010) but the correlation is weaker and spatially incoherent (not shown).
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Acknowledgments
The OFES simulations were conducted on the Earth Simulator under the support of JAMSTEC. We thank the members of the OFES group, including Drs. H. Sakuma, Y. Masumoto, and T. Yamagata, for their efforts and support in the model development. Our thanks are extended to Drs. A. Kubokawa, F. Kobashi, and E. Oka for useful discussions. This study is partially supported by Grand-In-Aid for Scientific Research defrayed by the Ministry of Education, Culture, Sports, Science and Technology of Japan (22106006, 23340139). IPRC/SOEST publication #790/8196.
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Nonaka, M., Xie, SP. & Sasaki, H. Interannual variations in low potential vorticity water and the subtropical countercurrent in an eddy-resolving OGCM. J Oceanogr 68, 139–150 (2012). https://doi.org/10.1007/s10872-011-0042-3
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DOI: https://doi.org/10.1007/s10872-011-0042-3