Geostrophic Scatter Diagrams and the Application of Quasigeostrophic Free-Mode Theory to a Northeast Pacific Blocking Episode

Abstract

Throughout the latter part of January and most of February 1989 a large-scale intense blocking event occurred over the northeast Pacific Ocean. During its lifetime the block exhibited two distinct spatial configurations corresponding to Omega and dipole shapes, respectively. A time series of scatter diagrams of 5-day-averaged 500-mb geostrophic streamfunction versus potential vorticity (? vs q) is computed. It is suggested that both the Omega and dipole forms may correspond to free modes. It is shown that immediately prior to the block formation there is a rapid steepening of the q(?) scatter diagrams associated with a strong increase in net vorticity being advected out of the block region. A suggestion is made that this analysis may be useful in identifying flow configurations that are capable of initiating blocks. It is possible to separate the geostrophic streamfunction into a product of horizontal and vertical components. The vertical structure problem is solved analytically using a constant lapse rate approximation for the background atmosphere, and a constant value for the gradient of the mean flow potential vorticity with respect to the geostrophic streamfunction. It is shown that as the block develops, the contribution of the gravest, quasi-barotropic mode dramatically increases and forms the dominant mode. Immediately prior to the formation of the dipole configuration, a small increase in baroclinicity was observed near the block interior. It is suggested that this event could be evidence of eddy forcing similar to that proposed for the maintenance of North Atlantic dipole blocks.