The Role of Finite-Time Barotropic Instability during Transition to Blocking

Abstract

Linear instability analysis is applied to study the role of barotropic dynamics in the evolution of blocking events during winter 1990/91. Finite-time interval instabilities (i.e., nonnormal-mode structures defined as the singular vectors of the tangent propagator) growing over periods of 4 days have been computed using adjoint methods. Correlation between large values of the singular vector amplification rate and the occurrence of blocking onset in the real atmosphere is studied. A correspondence is found between periods with the largest singular vector amplification rates and periods either leading to blocking formation or covering the mature phase of blocks. It is shown that at final time the singular vectors tend to have largest amplitude in the same regions of planetary wave ridging where blocks develop. On average, singular vectors developing on the Pacific have larger growth rates than those in the Euro?Atlantic region. The analysis of some case studies indicates a qualitative similarity between observed tendencies and their projections onto the five leading singular vectors, although correlation coefficients between actual and projected fields are small. The cases with largest tendency correlation are associated with the formation of blocking dipoles from preexisting planetary-scale ridges of larger meridional scale. Overall, our results indicate that barotropic instability is mostly driven by planetary wave amplification rather than being the cause of it, and mainly contributes to a rather mature stage of blocking development. Energetics of barotropic perturbations indicate that dipole structures similar to blocking patterns can efficiently gain energy from the planetary-scale flow provided that the longitudinal gradient of the basic-state zonal wind ub in the jet exit has a comparable magnitude to the meridional gradient of ub near the jet core. It is shown that an anomaly reinforcing the basic-state ridge on the eastern side of the Pacific and/or Atlantic Ocean (therefore increasing the magnitude of the longitudinal wind gradient) is necessary for a dipole structure to emerge as the fastest growing perturbation.