The influence of solar wind on extratropical cyclones – Part 1: Wilcox effect revisited

Abstract

Abstract.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) B Z 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-toone 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 B Z seems to conCorrespondence to: P. Prikryl (paul.prikryl@crc.ca) trol 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 B Z is leading by up to 2 days the mean VAI response to SBP associated with the south-tonorth reversal of B Z . 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 solarwind-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 me...

The influence of solar wind on extratropical cyclones – Part 2: A link mediated by auroral atmospheric gravity waves?

“…The mean high-level cloud area index decreases to a minimum near the arrival of an interaction region at the leading edge of a high-speed plasma stream and then maximizes a few days later. In the extratropics, this response is consistent with the effect on tropospheric vorticity found by Wilcox et al (1973Wilcox et al ( , 1974 and verified by Prikryl et al (2009). In the tropics, the observed highlevel cloud response is similar to that in mid-to-high latitudes, namely a decrease in high-level cloud cover at CIR arrival followed by an increase a few days later, which may be due to ducted auroral gravity waves stimulating convection in tropical storms and cyclones.…”

“…A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-tohigh latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al (1974) and verified by Prikryl et al (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes.…”

“…The latter conditions may prevent the ducted waves from reaching the equator, although they may still reach mid latitudes. The results presented in this paper, the verified and updated results on solar wind modulation of vorticity area index (Wilcox et al, 1974;Prikryl et al, 2009), and the observed tendency of significant extratropical storms to occur within a few days of arrival of high-speed plasma streams ) all point to a link between the solar wind and the troposphere. The results indicate that interaction between solar wind generated auroral AGWs and weather systems is possible.…”

“…Because of lower quality and sparsity of cloud estimates at very high latitudes, we limit the analysis to the latitude range from −70 • to +70 • . Prikryl et al (2009) verified the relationship between solar magnetic sector boundary passage and tropospheric vorticity (Wilcox et al, 1974) that most likely relates to synoptic scales. It is noted that a corotating interaction region (CIR) at the leading edge of a solar wind stream is a more accurate marker of auroral activity onset than the sector boundary, which may differ by 1-2 days from the stream interface.…”

“…In the companion paper, Prikryl et al (2009) use the ECMWF ERA-40 re-analysis dataset for a period from 1963 to 2002 to verify the Wilcox effect for the northern as well as the Southern Hemisphere winters. It is shown that the vorticity area index (VAI) at 300 and 500 mb is modulated by highspeed solar wind streams flowing from coronal holes.…”