Abstract
The northward shrinkage of the North Pacific western subarctic gyre (WSAG) in the early 2000s is associated with a sea surface height (SSH) elevation and is correlated to sea surface wind stress change. By using a Rossby wave model forced by wind stress, which computes the component variations due to the barotropic and first to fourth baroclinic modes, we estimated decadal changes in SSH and main pycnocline depth in the subpolar region. Realistic decadal SSH elevation and deepening of the main pycnocline depth associated with the northward shrinkage of the western subarctic gyre from the late 1990s to the mid-2000s were reproduced by the model. The sea surface elevation was caused primarily by the barotropic Rossby wave response to the relaxation of the Ekman suction due to the attenuation of the Aleutian Low by frequent La Niña occurrences after the late 1990s in addition to the long-term weakening of the westerly wind. The northward shrinkage of the WSAG was found to be associated with the intensification of an anticyclonic circulation centered around 43–44 ∘ N, 170–175 ∘ E. The westerly wind weakening deepened the main pycnocline in the western subarctic region through the baroclinic Rossby wave mode response to the wind stress change, which mostly accounts the equivalent halocline deepening at station K2 (47 ∘ N, 160 ∘ E). While the first baroclinic mode variation of the water density significantly attenuates during propagation, the higher mode variations, particularly the second and third mode variations, are locally excited through a quasi-resonant amplification mechanism and have profound impacts on the depth of the upper main pycnocline.
Highlights
In the subpolar North Pacific, a basin-scale cyclonic circulation, called the subpolar gyre, is driven by the sea surface wind stress over the entire subpolar ocean (Dodimead et al 1963; Ohtani 1973; Favorite et al 1976; Nagata et al 1992)
Focusing on the wind stress changes in the subpolar North Pacific, we examined the interannual to decadal variations in sea surface height (SSH) related to the northward shrinkage of the western subarctic gyre (WSAG) using a dynamical model of the Rossby waves forced by changes in the wind stress and damped by horizontal and vertical eddy dissipation
SSH depressions related to the intensification of the Aleutian Low in winter of El Niño years, which were not comparably observed by the satellite altimetry, were produced probably because topographic effects such as the joint effect of baroclinicity and bottom relief (JEBAR) were not taken into account in the model
Summary
In the subpolar North Pacific, a basin-scale cyclonic circulation, called the subpolar gyre, is driven by the sea surface wind stress over the entire subpolar ocean (Dodimead et al 1963; Ohtani 1973; Favorite et al 1976; Nagata et al 1992). Decadal fluctuations in the subpolar region are well explained by long Rossby wave responses to wind stress changes These low-vertical-resolution models are insufficient to examine the observed potential density change associated with the northward shrinkage of the WSAG reported by Wakita et al (2010, 2013, 2017) and Nagano et al (2016). Kawabe (2000, 2001) solved the vorticity gradient equation (e.g., LeBlond and Mysak 1978) with wind stress forcing to take into account the propagations of disturbances by Rossby waves and calculated interannual sea level variations at tide gauge stations in the North Pacific subtropical region Adopting this method to the SSH changes in the North Pacific subpolar region, we can compute the changes in SSH and water potential density due to the individual barotropic and baroclinic mode changes excited by the wind stress changes. The φn function is the nth eigenfunction, satisfying d dz 1 dφn N2 dz
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