A quantitative analysis of the physical mechanisms governing the life cycles of persistent flow anomalies

A new diagnostic approach, piecewise tendency diagnosis (PTD), is employed to study the dynamics of weather regime transitions. Originally developed in a quasigeostrophic (QG) framework by Nielsen-Gammon and Lefevre (1996) for adiabatic quasigeostrophic flow on a β-plane, the PTD partitions local geopotential height tendencies into a linear combination of dynamical and physical forcing terms that can be related to specific source mechanisms within a potential vorticity (PV) framework. Here the PTD is extended to account for spherical geometry, diabatic heating, and ageostrophic dynamical processes. The extended PTD is then used to identify the mechanisms responsible for the development and decay of persistent large-scale cyclonic and anti-cyclonic flow anomalies over the North Pacific and Atlantic Ocean during wintertime. Unlike potential enstrophy analyses, which are also based on PV principles, PTD allows a quantitative assessment of the contribution of the vertical superposition and horizontal deformation of PV anomalies toward development, the latter linked to barotropic growth mechanisms in low frequency variability.; It is demonstrated that PTD is a useful diagnostic paradigm for studying large-scale dynamical processes in the midlatitude atmosphere. Height tendency patterns obtained by summing the PTD forcing terms correspond very well in most cases to the observed height tendencies. The composited PTD analyses reveals that linear dynamic advections, specifically baroclinic growth and horizontal deformation of PV anomalies, provide the largest forcing of large-scale development over the North Pacific. Linear Rossby-wave propagation significantly opposes large-scale development by transporting wave activity away from the development region. Barotropic deformation, and secondarily baroclinic growth and nonlinear dynamic advections are the most important mechanisms for growth during Atlantic anti-cyclonic development whereas the Atlantic cyclonic event develops from a more complex combination of the linear and nonlinear advections. All four PFA types decay as a result of wave energy propagating away from the anomaly. Nonlinear and non-quasigeostrophic forcings are second order but assist in some cases during early time periods. Height tendencies associated with diabatic heating are also second order and generally positive over most of the key region during anti-cyclonic growth and decay and negative during cyclonic anomaly growth and decay.