Differences Between Low-Frequency Jet and Pressure Variability in the Extratropical Troposphere

Canonical patterns and indices of low-frequency atmospheric variability are ubiquitous, yet a firm dynamical interpretation has remained elusive in many cases. This challenge remains despite a long history of exploring both covariance on a horizontal surface and the three-dimensional structure of the atmospheric general circulation's climatology.

This study compares jet (zonal wind) variability both near the tropopause and in the lower troposphere with lower tropospheric pressure variability, using primarily the entire period of record from the Era-Interim reanalysis. A rich three-dimensional structure exists in the variance of the zonal wind field that cannot be captured by pressure variability in the lower troposphere or, for that matter, on any horizontal surface. Correspondingly, many of the pressure-based canonical indices of low-frequency atmospheric variability exhibit inconsistent relationships to physically-intuitive reorganizations of the sub-tropical and polar front jets. Pressure-based canonical indices are often also associated with zonal wind variability that is substantially offset from the peaks of variance within the three-dimensional zonal wind field. Neither result is encouraging in terms of relating canonical patterns and indices to the dynamics of the general circulation. These conclusions are robust in a variety of seasons and also in intra-seasonal and inter-annual explorations. Similar results and conclusions are also derived using detrended data, other reanalyses, and state-of-the-art coupled climate model output.

A case study examines the practical importance of the aforementioned mismatch between horizontal pressure variability and the fully three-dimensional variability of the atmospheric circulation. Sea level pressure (SLP) variability in the central Arctic is found to have strong associations with stratospheric circulation variability, but only weak associations to SLP and tropospheric circulation variability outside the central Arctic.

Collectively, this study demonstrates that the implicit assumption of strictly deep barotropic structures in atmospheric low-frequency variability obfuscates not only a clear definition of the leading tropospheric patterns, but also efforts to ascribe a clear dynamical interpretation to each of them. A fully three-dimensional exploration of tropospheric circulation variability is warranted.