The European Centre for Medium-range Weather Forecasting (ECMWF) Reanalysis v5 (ERA5) data set (1979 – 2020) is used in many climate studies. The present monthly mean data set of Eliassen-Palm fluxes (EP fluxes) and EP flux divergences, was created from twice-daily gridded values of ERA5 winds and temperature on pressure surfaces and is intended to fill a gap in availability. It is compatible for use with ERA5 monthly mean data.
The EP flux is a diagnostic tool for assessing wave propagation and wave-mean flow interaction. It is a vector representation for the propagation of synoptic and planetary Rossby wave activity in the meridional plane. An upward component indicates a poleward heat flux while an equatorward component indicates a poleward momentum flux. EP flux divergence implies a source of Rossby wave activity, and EP flux convergence implies absorption of Rossby wave activity. EP flux divergence represents the body force, or net effect of waves on the zonal mean zonal wind, with EP flux convergence causing deceleration of zonal mean westerlies and EP flux divergence causing acceleration. The primary effect of a region of EP flux convergence, however, is to induce poleward motion, with an associated mean meridional circulation and quadrupole of temperature anomalies in the meridional plane. EP fluxes are useful in investigating many planetary and synoptic scale weather phenomena such as the Quasi-Biennial Oscillation (QBO) and Sudden Stratospheric Warmings (SSWs).
The meridional and vertical components of the EP flux are calculated in pressure coordinates using the following initial conditions:
Ground density (rho_0 ):P_0/R_0/T_0 Ground pressure (P_0): 1013hPa
Ground temperature (T_0): Temperature at 1000hPa
Earth radius (a) = 6378km
Dimensions in the data:
Latitude: 90°N-90°S (grid spacing is 0.25°, as in the ERA5 reanalysis)
Pressure: 1 hPa-1000 hPa (levels are the same as in the ERA5 reanalysis)
Time: January 1979 – December 2020; December, January, February (DJF) only; 00z and 12z
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Why Are Stratospheric Sudden Warmings Sudden (and Intermittent)?
This paper examines the role of wave–mean flow interaction in the onset and suddenness of stratospheric sudden warmings (SSWs). Evidence is presented that SSWs are, on average, a threshold behavior of finite-amplitude Rossby waves arising from the competition between an increasing wave activity A and a decreasing zonal-mean zonal wind [Formula: see text]. The competition puts a limit to the wave activity flux that a stationary Rossby wave can transmit upward. A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once [Formula: see text] drops below a certain fraction of u REF , a wave-free, reference-state wind inverted from the zonalized quasigeostrophic potential vorticity. This fraction is 0.5 in theory and about 0.3 in reanalyses. We propose [Formula: see text] as a local, instantaneous measure of the proximity to vortex breakdown (i.e., preconditioning). The ratio r generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, similar to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective time scales. The model’s response to a variety of transient wave forcing and damping is discussed.
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- Award ID(s):
- 1909522
- NSF-PAR ID:
- 10163332
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 77
- Issue:
- 3
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 943 to 964
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JAS-D-19-0249.1
