A study on precursors to blocking anomalies in climatological flows by using conditional nonlinear optimal perturbations

This work explores the inverse problem of what are the precursors to a given blocking anomaly in climatological flow over the Atlantic and Pacific Oceans, respectively. Blocking anomaly in geopotential height field is specified as a dipole structure which is dominated by a strong positive anomaly centred at about 60 degrees N and a weak negative anomaly south of it. The method of conditional nonlinear optimal perturbation (CNOP) is applied to investigate the above problem by using a T21L3 quasi-geostrophic model and its tangent linear and adjoint versions. Results show that both for the Atlantic and Pacific blockings, the precursors are baroclinic synoptic-scale wave train disturbances, whose maximum amplitudes are located upstream of the corresponding blocking regions. The disturbances, which mostly focus on the northward flanks of the corresponding Atlantic and Pacific upper-level jet, take on a northeast-southwest trend. However, the leftover parts located in the southward flanks of the corresponding upper-level jet take on a northwest-southeast trend. This structure is favourable for the precursors to gain more kinetic energy from the horizontal shear of the basic flow. Further energy analysis reveals that the available potential energy contributes more to the initial precursors, and with time, the kinetic energy dominates the structures. With the help of potential vorticity analysis, it is shown that the planetary-scale eddy feedback seems to advect anticyclonic (cyclonic) vorticity to the high pressure (low trough) region of blocking, and thus it promotes the onset of blocking. In addition, more specific differences for the precursors to blocking anomalies over the two oceans are compared. Copyright (C) 2010 Royal Meteorological Society