Variations in Water Properties and Current Velocity at Station K2 in the Western Subarctic North Pacific in Relation to Wind Changes

The western subarctic gyre (WSAG) of the North Pacific shrank due to the attenuation of the westerly wind and the Aleutian Low from the late 1990s to the mid-2000s. Applying the altimetry-based gravest empirical mode (AGEM) method to hydrographic and altimetric sea surface height data, we estimated the vertical distributions of potential temperature and salinity at station K2 (47˚N, 160˚E). Associated with the WSAG shrinkage, the AGEM-derived halocline depth at K2 was found to be deepened at an equivalent rate to the deepening of the temperature minimum layer. By using a Rossby wave model forced by wind stress, we calculated the potential density variation at K2 due to the first to fourth baroclinic modes. The halocline and main pycnocline deepening is caused mainly by the weakening of the westerly wind. While the first baroclinic mode variation in potential density attenuates during propagation, the higher, particularly second and third, modes are locally excited through a quasi-resonant amplification mechanism and have greater impacts on the main pycnocline depth deepening. From July 2016 to July 2018, we deployed a mooring system, on which an upward-looking 75 kHz acoustic Doppler current profiler was installed at a depth of approximately 400 m. The rotary empirical orthogonal function (EOF) modes of current velocity vector variation on timescales longer than 10 days are similar to those of the baroclinic Rossby modes. In particular, the second EOF mode has a 180˚ phase reversal at a depth of ~150 m under the winter mixed layer base. Furthermore, significant clockwise rotary spectral peaks were observed down to ~150 m depth in frequency ranges of around inertial frequency (1.47 cpd). The near-inertial current variation is excited at the sea surface by wind. Thus, in addition to the pycnocline depth change due to the higher baroclinic Rossby waves, the wind-forced near-inertial variation yields vertical velocity shear at a depth of ~150 m.