The Role of Instability during the Onset of Blocking and Cyclogenesis in Northern Hemisphere Synoptic Flows

Abstract

A study is made Of the three-dimensional instability properties of a sequence of daily instantaneous Northern Hemisphere flow fields during the period 1 November? 16 November 1979. The growth rates phase frequencies, and structures of the ten fastest growing modes have been analyzed on each day. We have concentrated on instability modes associated with the most dramatic developmental in particular, with the formation of a block in the Gulf of Alaska between 5 and 12 November. Associated with the later stages of this block formation are large-scale equivalent barotropic dipole modes which are stationary or slowly propagating and which have quite large growth rates. In particular, an 6 November the fastest growing mode is a mature blocking mode in the Gulf of Alaska with an e-folding time of only 1.6 days. The early stages of block formation are characterized by the formation of large westward tilting dipole onset-or-blocking modes with largest amplitudes upstream of the blocking region. Substantial changes in the locations of the cyclogenesis modes take place with the regions of preferential development occurring in the winter climatological storm tracks over the Pacific, Atlantic and Siberian regions during early November. With the formation of a block in the Gulf of Alaska, there is a splitting and northward deflection of some cyclogenesis modes near the block between 5 and 12 November. During the later stages of the period, a block in the North Atlantic-Arctic region causes some Atlantic cyclogenesis modes to he deflected southward. The roles of barotropic and topographic instability in the formation of mature blocking modes are examined. The implications of the results for different theories of blocking are discussed.