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Cases of mesoscale cloud bands in extratropical cyclonesare observed a few hours after atmospheric gravity waves (AGWs) are launchedfrom the auroral ionosphere. It is suggested that the solar-wind-generatedauroral AGWs contribute to processes that release instabilities and initiateslantwise convection thus leading to cloud bands and growth of extratropicalcyclones. Also, if the AGWs are ducted to low latitudes, they couldinfluence the development of tropical cyclones. The gravity-wave-inducedvertical lift may modulate the slantwise convection by releasing the moistsymmetric instability at near-threshold conditions in the warm frontal zoneof extratropical cyclones. Latent heat release associated with the mesoscaleslantwise convection has been linked to explosive cyclogenesis and severeweather. The circumstantial and statistical evidence of the solar windinfluence on extratropical cyclones is further supported by a statisticalanalysis of high-level clouds (<440 mb) extracted from the InternationalSatellite Cloud Climatology Project (ISCCP) D1 dataset. A statisticallysignificant response of the high-level cloud area index (HCAI) to fast solarwind from coronal holes is found in mid-to-high latitudes duringautumn-winter and in low latitudes during spring-summer. In theextratropics, this response of the HCAI to solar wind forcing is consistentwith the effect on tropospheric vorticity found by Wilcox et al. (1974) andverified by Prikryl et al. (2009). In the tropics, the observed HCAIresponse, namely a decrease in HCAI at the arrival of solar wind streamfollowed by an increase a few days later, is similar to that in the northernand southern mid-to-high latitudes. The amplitude of the response nearlydoubles for stream interfaces associated with the interplanetary magneticfield BZ component shifting southward. When the IMF BZ after thestream interface shifts northward, the autumn-winter effect weakens orshifts to lower (mid) latitudes and no statistically significant response isfound at low latitudes in spring-summer. The observed effect persiststhrough years of low and high volcanic aerosol loading. The similarity ofthe response in mid-to-high and low latitudes, the lack of dependence uponaerosol loading, and the enhanced amplitude of the effect when IMF BZcomponent shifts southward, favor the proposed AGW link over the atmosphericelectric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requiresthe presence of stratospheric aerosols for a significant effect and shouldproduce negative and positive cloud anomalies in mid-to-high and lowlatitudes, respectively. However, if the requirement of aerosols for the AECmechanism can be relaxed, the AGW and AEC mechanisms should work in synergyat least in mid-to-high latitudes

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