The Impact Of Solar Variability On The Northern Hemisphere Winter Climate And Its Possible Mechanism

The relationship between solar activity and the Northern Hemisphere winter climate(NHWC) was investigated using solar 10.7 cm radio flux data from NOAA and reanalysis data provided by NCEP/NCAR. The results show that solar activity correlates well with the NHWC, furthermore, the relationship between them are affected by decadal and interannual climate backgrounds drastically. On decadal scales, interestingly, we find that solar-climate relationship differs at strong and weak solar activity periods, in addition, the influence of solar variability on NHWC changed owing to the decadal climate transition in the late 1970 s. Besides, on the interannual scale, in terms of solar-climate linkages, there exists significant differences between west and east quasi-biennial oscillation(QBO) phase. The main conclusions are as follows:(1) Solar activity correlates well with the winter atmospheric circulation over East Asia(EA). However, interestingly, this relationship exhibits large differences between strong and weak solar activity periods, suggesting an asymmetric solar influence on the East Asian winter climate(EAWC). Further investigation indicates that the linkage between solar activity and the EAWC is robust during active solar periods, while during inactive phases, the connection is fairly weak in comparison. During the active solar period, with strengthening of solar activity, the geopotential height in the mid-troposphere increases significantly over the mid-latitudes of the EA, resulting in a weakened East Asian trough(EAT). Meanwhile, southerly anomalies exist at 850 h Pa over EA, and the Siberian high is weakened in addition to the East Asian winter monsoon(EAWM) at the surface, giving rise to significantly positive temperature anomalies in most parts of the EA. Nevertheless, during inactive solar periods, there is almost no obvious link between the interannual variability of solar activity and the East Asian winter atmospheric circulation. Comparison of the strong solar period with the weak period shows that the abnormal planetary wave activity and sea surface temperature(SST) anomalies over the northwestern Pacific could be crucial for such an asymmetric solar influence. During the active solar period, when solar activity becomes stronger, planetary waves in the stratosphere propagate toward the equator abnormally, resulting in the divergence of the Eliassen-Palm flux in high-latitude areas and enhancement of the Arctic Oscillation and high-latitude westerlies. Meanwhile, the SST in the tropical northwestern Pacific decreases with the increase in solar activity, which reduces the thermal contrast between the Eurasian continent and the Pacific, and weakens the meridional flow patterns, resulting in an inactive EAWM.(2)As a result of climate change, the relationship between solar activity and boreal winter climate had undergo an interdecadal change in late 1970 s. During 1952-1978, solar influences on tropical pacific are mediated by the El Ni?o–Southern Oscillation(ENSO). In high solar(HS) years, there is a positive feedback between central pacific SST and atmospheric circulation linked with convection activity, giving rise to enhanced local warming there. In addition, ENSO-associated SST anomaly by solar forcing would affect extra-tropical planetary wave activity, which would increase geopotential height and weaken the polar vortex in the Arctic, thus warm the polar regions. In 1979-2011, solar activity affects climate in Eurasia, Africa and Greenland through Stratosphere Troposphere coupling. High solar activity corresponds to positive AO and enhanced zonal wind in mid latitudes, the Eurasia is warmed by warm advection from the North Atlantic, while the North Africa is cooled by strengthened northeasterly coupling with enhanced Azores high. In addition, positive AO favors a stable polar vortex, which traps the cold air in the Arctic areas, thus the temperature in Greenland is lower than normal.(3)Under westerly QBO, significant solar signals in NH mid-lower stratosphere are robust during late winter, and are largely restricted in polar regions, butsolar influences on low latitudes are rather weak. During February, the polar night jet and polar vortex are weakened by planetary wave activities under higher solar activity, corresponding to the negative phase of the Northern Annular Mode(NAM); besides, strong westerly anomalies appears in mid-latitude regions, the influence of solar activity on the the Arctic polar regions has achieved its strongest state. While in east QBO phase, the sun changes temperature and atmospheric circulation in low-latitude Northern Hemisphere(NH) stratosphere mainly. With seasonal march, solar influences on the tropic fades during late winter, but its impact on northern high latitudes becomes evident through modulating planetary waves. This signal has already been isolated from the one in the tropics during March.