North atlantic blocking during January 1979: Linear theory

The second half of January 1979 was a time of persistent and large‐scale blocking in the North Atlantic region. In this study we examine the instability properties of the flow during this period and relate the growing modes to the development that occurs. We have analysed the growth rates, phase frequencies and structures of the ten fastest‐growing modes, within a five‐level quasi‐geostrophic model incorporating spherical geometry, on each day for the period between 10 and 30 January 1979. Large‐scale equivalent barotropic dipole or multipole instability modes are prevalent in the North Atlantic during the second half of January. These modes are stationary or slowly propagating and grow rapidly with efolding times as short as 1.7 days in some cases. We find that there is a close connection between these modes and the dynamical developments observed subsequently a few days later. Associated with the early stages of block formation, there are large‐scale eastward propagating westward tilting onset‐of‐blocking wave‐train modes. the onset‐of‐blocking modes also appear to be associated with rapid cyclogenesis off the east coast of North America. The formation of the North Atlantic blocking system and diffluent flow also produces corresponding changes in the storm tracks. During the early parts of this period the fastest‐growing monopole cyciogenesis instability modes have regions of preferential development in the climatological storm tracks over the Pacific, Atlantic and Siberian regions. Subsequently, however, the Atlantic cyclogenesis modes split into wave trains poleward and equatorward of the block. In the developments during this period, we also examine the roles of the adjoint eigenmodes and of optimal perturbations which produce the largest initial tendency of the L 2 norm squared of the streamfunction.