Potential Vorticity on Isentropic Surfaces: Climatology and Diagnostics

Abstract

A 25-year dataset of potential vorticity on the 315-K isentropic surface is built from the National Meteorological Center (NMC) final analysis archive. Potential vorticity is calculated from the nonlinear gradient wind balance using temperature and geopotential fields, since the wind field is not available in the early part of the archive. The validity of this calculation is assessed by comparing the results with potential vorticity obtained directly from European Centre for Medium-Range Weather Forecasts (ECMWF) analyzed winds. The error due to the nonlinear balance approximation turns out to be smaller than the difference between ECMWF and NMC analysis. The possibility of studying diabatic forcing as a residual of the equation of potential vorticity conservation is examined. The average potential vorticity forcing found in this way is consistent with the authors knowledge of the mean diabatic heating. The amplitude of the residual decreases through the analysis period, reflecting the improvement in the observational network and in the analysis schemes. Next the authors demonstrate that this dataset can be used for studies of transient-mean flow interactions. The authors present diagnostics of the transient feedback by separating the contribution of vortical and thermal terms on the isentropic surface. Also, the contribution of high-frequency (periods less than 10 days) and low-frequency (periods greater than 10 days) transients is examined. On the 315-K surface, transients act mostly in reducing the potential vorticity gradient through thermal terms and accelerate the zonal flow through vortical forcing. Finally, these diagnostics are also applied to a long ensemble of blocking events, and the authors study the anomaly of transient feedback during these events. It is found that transients have primarily an advective effect, forcing the dipole structure to retrograde westward. Thermal and vortical terms have a very distinct action on the blocking anomaly. Vortical forcing is constructive and advective, whereas thermal forcing is dissipative and makes the dipole rotate clockwise.