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

Abstract

Abstract. Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (<440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. 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 amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field B Z component shifting southward. When the IMF B Z after the stream interface shifts northward, the autumn-winter Correspondence to: P. Prikryl (paul.prikryl@crc.ca) effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in springsummer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF B Z component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.

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

“…Auroral AGWs may release instabilities that lead to tropospheric convection, convective clouds and storminess (Prikryl et al, 2009). Using the International Satellite Cloud Climatology Project (ISCCP) D1 dataset, a statistically significant response of high-level cloudiness to fast solar wind from coronal holes is found (Prikryl et al, 2003(Prikryl et al, , 2006(Prikryl et al, , 2009. These results are consistent with the previous finding of solar wind influence on mid-latitude tropospheric circulation (Wilcox et al, 1973;Lundstedt, 1984).…”

“…Prikryl et al (2001Prikryl et al ( , 2003 have suggested that auroral atmospheric gravity waves (AGWs) are another candidate for the "missing link" between the solar wind and tropospheric weather. Auroral AGWs may release instabilities that lead to tropospheric convection, convective clouds and storminess (Prikryl et al, 2009). Using the International Satellite Cloud Climatology Project (ISCCP) D1 dataset, a statistically significant response of high-level cloudiness to fast solar wind from coronal holes is found (Prikryl et al, 2003(Prikryl et al, , 2006(Prikryl et al, , 2009.…”

“…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.…”

“…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.…”

Temperature changes over storms from measurements of spacecraft TIMED

“…Note that nowadays there are researches in which the existence of the inverse connection is discussed, i. e. when the AGWs starting their spread from auroral ionosphere regions have an inuence onto extratropical cyclones [28]. Rees et al [30] identied sources at high latitudes of strong vertical winds of more than 100 m/s, resulting from local geomagnetic energy input and subsequent generation of thermospheric gravity waves.…”