Abstract
By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m deep continental slope seabed off the southeastern coast of Hokkaido, Japan. The results indicate that the current intensifies toward the bottom and is directed nearly toward the shore, reaching an average speed of ~2.5 cm s−1 just above the bottom. The thickness of the along-slope northward component of the bottom-intensified current varied within the range of 50–350 m. We found that the current thickness change was caused by oceanic barotropic disturbances, produced by the intensification of the Aleutian Low, largely related to the El Niño–Southern Oscillation and modified through the excitation of bottom-trapped modes of coastal-trapped waves. This finding improves the prediction accuracy of the the bottom-intensified current change, being beneficial for suspended sediment studies, construction and maintenance of marine structures, planning of deep drilling, and so on.
Highlights
The northward component of the bottom-intensified current was found to thicken on an interannual timescale; using an ocean general circulation model (OGCM) dataset, we found that the current travels along the onshore continental slope region of the Japan and Kuril–Kamchatka trenches
By using a rotary empirical orthogonal function (EOF) analysis [21–23], we examined the characteristics of the interannual variation of the bottom-intensified current
The time coefficient of the first rotary EOF mode exhibits interannual variation, with significant positive peaks in the winters of 2002/2003, 2007/2008, 2009/2010, 2011/2012, and 2015/2016 (Figure 6a). Most of these positive peaks lag behind the positive peaks of the inverted North Pacific Index (NPI) anomaly, which represent the strengthening of the Aleutian Low, principally due to the occurrence of El Niño events, e.g., [34]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Knowledge of mid-depth and deep currents is quite limited, in comparison to seaand near-surface currents It is because current velocity data are derived from “snapshot”. Variations in the current velocity structures just above the seafloor surface, even on timescales longer than the inertial period, have not been sufficiently observed. To elucidate the vertical structure of such long-term variations, we must deploy ADCPs near the bottom pressure gauge to obtain current velocity profiles in long term. Near the bottom pressure gauge at PG1, a portable observation system (cable end station) has been installed (Figure 1b). This system comprises of an ADCP, hydrophone, video camera, and other components and is connected to the battery pack [16].
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