The variability of the East Sakhalin Current induced by winds over the continental shelf and slope
Abstract:Long-term current measurements of the East Sakhalin Current (ESC) in the Sea of Okhotsk are analyzed using the technique of empirical orthogonal functions (EOFs) in the frequency domain. The first and second EOFs at subtidal frequencies represent motions over the continental shelf and slope, respectively, corresponding to the variability of the two cores of the intense ESC. The first EOF can be explained by the first-mode coastal trapped wave (CTW). The structure of the second EOF is similar to that of the second-mode CTW to the first approximation. According to the distribution of the cross-spectra between EOFs and the wind stress over the whole area of the Sea of Okhotsk, the first EOF is correlated with the alongshore component of the wind stress over the northern and western shelves. The distribution of the phase of the wind stress, which is correlated with the first EOF, indicates that a resonance between the CTW and wind stress drives the motion represented by the first EOF at lower frequencies. At higher frequencies the phase of the wind stress correlated with the first EOF is almost uniform in space, being consistent with the greater speed of phase propagation of the EOF compared with that for the free CTW at these frequencies. The second EOF is correlated with the wind stress curl in the central part of the Sea of Okhotsk. The motion by the second EOF is confined over the slope at lower frequencies and becomes large over the shelf at higher frequencies. This change in the structure of the second EOF is consistent with the results of the numerical experiment of the flow induced by the offshore forcing by Chapman and Brink (1987). The phase of the wind stress curl which is correlated with the second EOF changes clearly in space at some frequencies, suggesting that the motion represented by the second EOF propagates along the isobath with the coast to the right. The wind stress curl contains the wavenumber resonant with the lowest two or three modes of CTWs.
Document Type: Research Article
Publication date: November 1, 2005
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