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| Titel |
The influence of solar wind on extratropical cyclones – Part 1: Wilcox effect revisited |
| VerfasserIn |
P. Prikryl, V. Rušin, M. Rybanský |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 1 ; Nr. 27, no. 1 (2009-01-06), S.1-30 |
| Datensatznummer |
250016353
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| Publikation (Nr.) |
 copernicus.org/angeo-27-1-2009.pdf |
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| Zusammenfassung |
A sun-weather correlation, namely the link between solar
magnetic sector boundary passage (SBP) by the Earth and upper-level
tropospheric vorticity area index (VAI), that was found by Wilcox et al.
(1974) and shown to be statistically significant by Hines and Halevy (1977)
is revisited. A minimum in the VAI one day after SBP followed by an increase
a few days later was observed. Using the ECMWF ERA-40 re-analysis dataset
for the original period from 1963 to 1973 and extending it to 2002, we have
verified what has become known as the "Wilcox effect" for the Northern as
well as the Southern Hemisphere winters. The effect persists through years
of high and low volcanic aerosol loading except for the Northern Hemisphere
at 500 mb, when the VAI minimum is weak during the low aerosol years after
1973, particularly for sector boundaries associated with south-to-north
reversals of the interplanetary magnetic field (IMF) BZ component. The
"disappearance" of the Wilcox effect was found previously by Tinsley et
al. (1994) who suggested that enhanced stratospheric volcanic aerosols and
changes in air-earth current density are necessary conditions for the
effect. The present results indicate that the Wilcox effect does not require
high aerosol loading to be detected. The results are corroborated by a
correlation with coronal holes where the fast solar wind originates.
Ground-based measurements of the green coronal emission line (Fe XIV, 530.3 nm)
are used in the superposed epoch analysis keyed by the times of sector
boundary passage to show a one-to-one correspondence between the mean VAI
variations and coronal holes. The VAI is modulated by high-speed solar wind
streams with a delay of 1–2 days. The Fourier spectra of VAI time series show
peaks at periods similar to those found in the solar corona and solar wind
time series. In the modulation of VAI by solar wind the IMF BZ seems to
control the phase of the Wilcox effect and the depth of the VAI minimum. The
mean VAI response to SBP associated with the north-to-south reversal of
BZ is leading by up to 2 days the mean VAI response to SBP associated
with the south-to-north reversal of BZ. For the latter, less
geoeffective events, the VAI minimum deepens (with the above exception of
the Northern Hemisphere low-aerosol 500-mb VAI) and the VAI maximum is
delayed. The phase shift between the mean VAI responses obtained for these
two subsets of SBP events may explain the reduced amplitude of the overall
Wilcox effect.
In a companion paper, Prikryl et al. (2009) propose a new mechanism to
explain the Wilcox effect, namely that solar-wind-generated auroral
atmospheric gravity waves (AGWs) influence the growth of extratropical
cyclones. It is also observed that severe extratropical storms, explosive
cyclogenesis and significant sea level pressure deepenings of extratropical
storms tend to occur within a few days of the arrival of high-speed solar
wind. These observations are discussed in the context of the proposed AGW
mechanism as well as the previously suggested atmospheric electrical
current (AEC) model (Tinsley et al., 1994), which requires the presence of
stratospheric aerosols for a significant (Wilcox) effect. |
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