The development of perturbations or errors is considered during periods of observed Northern Hemisphere blocking. The evolution of initial onset-of-blocking mode perturbations is studied using a two-level tangent linear model with time-dependent basic states taken from observations in the first half of November 1979. During this period a blocking high formed in the Gulf of Alaska between 5 and 12 November followed by decay and the subsequent development of a block in the North Atlantic. Initial baroclinic wave train perturbations, with large amplitudes stretching from East Asia into the central Pacific, propagate eastward as they grow and become equivalent barotropic. Large-amplitude large-scale perturbations form over the Gulf of Alaska as the observed block amplifies. Subsequently, downstream development occurs with rapid amplification of the perturbation near the east coast of North America followed by the formation of large-scale tripole anomalies in the blocking region over the North Atlantic. As the Gulf of Alaska and North Atlantic blocks amplify, there are close similarities between the simulated anomalies and dominant normal mode instabilities of the instantaneous observed flows. The inverse problem of what are the precursors to a given observed or specified forecast error is examined. Anomalies are specified as equivalent barotropic normal modes or similar structures with large amplitudes in the Gulf of Alaska. The precursors are smaller-scale wave train disturbances emanating from the East Asia-west Pacific sector. The sensitivity of the precursors to the structure of the prescribed error in the blocking region is examined and condition numbers for the propagator are calculated.
|Number of pages||18|
|Journal||Journal of the Atmospheric Sciences|
|Publication status||Published - 15 Jul 1998|