Quasi-geostrophic ocean response to real wind forcing: the effects of temporal smoothing

A one-degree, flat bottom, eight-layer quasi-geostrophic model of the North Pacific Ocean is forced by six different wind stress curl datasets, all derived from seven years of twice daily analyses at the European Centre for Medium Range Weather Forecasts, 1980-86. The six datasets, with nominal averaging times of 1, 2, 3, 7, 14, and 30 days, are obtained by carefully filtering in the frequency domain which greatly reduces the variance, it also smoothes spatially, reducing high wavenumber variance to a greater degree than variance near wavenumber zero. The wind stress curl is neither temporally nor spatially stationary with the high frequency variance being much larger during the winter season and over the northern half of the North Pacific. Its spectrum does not appear to be isotropic in wavenumber. High frequency forcing produces higher kinetic energies at all depths with an annual cycle that is related to that of the high frequency variance in the wind stress curl. The deep kinetic energy and the intra-annual streamfunction variance are used to quantify the relative importance of the high frequency barotropic Rossby waves. A 3-day average wind forcing gives a considerable advantage over 7- and 14-days, whilst the variance is typically only 30% of the 3-day value when forced with 30-day averaged wind curls. (from authors' abstract)