The Maintenance of Extratropical Intraseasonal Transient Eddy Activity in the GEOS-1 Assimilated Dataset

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Abstract
Diagnostic potential enstrophy (PE) analyses are formulated for studying the dynamics of transient eddies within discrete frequency bands. The PE analyses are derived for nonconservative quasigeostrophic motion on a sphere and relate local PE tendencies to 1) conversions from the climatological-mean flow, 2) nonlinear dynamical sources, and (3) nonconservative physical sources. Local conversions are associated with cross-gradient residual eddy fluxes of pseudopotential vorticity (PV) while the nonlinear and nonconservative sources of PE result from correlations between PV anomalies and anomalous PV source terms. The PE analyses provide a useful local diagnostic approach for studying fundamental aspects of transient wave dynamics within a PV-based framework. Wintertime-average PE analyses are constructed for both bandpass (BP) and low-pass (LP) intraseasonal timescales. These analyses are applied to the multiyear GEOS-1 assimilated dataset to ascertain the primary sources and sinks of BP and LP transient eddy activity in the Northern Hemisphere extratropics. In the interior troposphere, eddy PE is found to maximize near 400 hPa. The PE analyses applied at this level indicate that the primary source of both BP and LP eddy activity is a local conversion from the wintertime-mean flow. These conversions are associated with residual eddy PV fluxes that are directed down the wintertime-mean PV gradient. Thus, both BP and LP transients act to reduce the wintertime-mean PV gradients in the upper troposphere. For BP eddies, there is a significant additional PE source in the storm tracks associated with latent heat release. Over the North Pacific this additional source is locally of the same order as the mean-flow conversion. The sources of BP and LP PE are largely offset by enstrophy sinks associated with nonlinear eddy processes. At second order, latent heating anomalies provides weak dissipation of LP PE in regions of frequent blocking activity. These signatures are reproduced in parallel diagnoses of the NCEP–NCAR Reanalyses. Radiative processes provide relatively weak and uncertain contributions to the BP and LP PE budgets while turbulent mixing of heat and momentum furnish negligible contributions.

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