Generation of Annual Rossby Waves in the North Pacific

Abstract

A new mechanism is proposed for the generation of the annual-periodbaroclinic Rossby waves which have been observed in the central NorthPacific by Kang and Magaard. It is shown that annual north-southfluctuations in the eastern boundary current off Vancouver Islandcan efficiently generate first baroclinic mode Rossby waves throughoutthe central North Pacific. In particular, to the southwest of VancouverIsland the direction, wavelength and speed of phase propagation associatedwith the far field (asymptotic) wave solution agree favorably withobservations. Also, along and fixed latitude contained within theobserved latitude band (30?40�N), the amplitudes of the verticaldisplacement at 300 m and surface horizontal current speed both monotonicallyincrease to the west, in agreement with the observed trends for thesequantities. However, the existence of this monotonicity in the solutionappears to be quite sensitive to the north-south spatial structureof the fluctuating coastal current. For example, the oscillationsof a point source (delta function) current excite a wave field witha fairly uniform amplitude across most of the observation region.Within the context of a reduced gravity, quasi-geostrophic model,the theory presented here is quite general. For example, baroclinicwaves of other than annual period could be investigated, and theradiation pattern due to a number of coastal sources could be determined.