Low Solar Activity
According to Science Daily, a report published on April 15, 2010 in IOP Publishing's, Environmental Research Letters, explains how low solar activity may be to blame for some weather anomalies. Despite an increase in global temperatures, the UK experienced one of its coldest winters in the last 160 years, in 2009. Lack of normal solar activity results in a phenomenon called blocking, where jet streams over the Atlantic get lost in an anticyclone before reaching Europe. If lack of solar activity affects the jet stream, it stands to reason that higher than normal sun activity would also have an effect.
High Solar Activity
Depending on where the solar flux is and far it extends, it can reach Earth's upper atmosphere; the flux can cause the Earth's magnetic field to shift or warp. This shift in the magnetic field causes a subsequent shift in the jet stream. Solar flares and coronal mass ejections (CMEs) have a direct effect on solar flux.
Sun Spots
Sunspots may be the cause of solar flares and CMEs. When solar flares are large -- enough to induce a domino effect -- its effect on the jet stream directly influences ocean currents, resulting in such phenomenon as El Nino or La Nina. A simple explanation developed by Mitch Battros, the host of Earth Changes Media, is: sunspots = solar flares = magnetic field shift = shifting ocean and jet stream currents = extreme weather and human disruption.
Solar Flares
Solar flares are sudden, violent eruptions on the sun's surface. Energy released from solar flares can be equivalent to millions of hydrogen bombs. During a flare, heated protons and electrons eject into space, creating weather changes on Earth. Due to the distance between the sun and Earth, effects felt on Earth do not occur until several days after the flare.