The climate is a huge and complicated system and it is affected by all sorts of things. The factors that you hear most about at the moment are greenhouse gases such as carbon-dioxide or methane.
In the 1920s scientists noticed that the level of Lake Victoria in East Africa seemed to vary on an 11-year cycle. This seemed to correlate with the 11-year sunspot cycle. Sun spots are less bright regions of the sun which look like dark blotches on its surface. Their number increases and decreases on various cycles, the strongest one being an 11-year period.
Interestingly, at the period of most sunspots the solar output is about 0.1% higher than 5-and-a-half years later at their minimum. It has been suggested that changes in the sun's output could be causing the present warming of our planet.
Argentinian physicists have accrued a lovely set of data for the flow rate of the Rio Paran dating back to 1904. The Rio Parana is the 4th-largest river in the world so it gives you a good measure of the average rainfall over a huge area and not just local variations.
They have found 3 main effects on the rainfall that can accommodate for an El Nino year. Although reversals in the circulation of water in the Pacific had a big effect they also have found another strong correlation between the number of sunspots and the river level. The more sunspots, the more energy is released by the sun and the more energy there is available to evaporate water which will then fall as rain. But, unfortunately for the greenhouse effect sceptics, they have also found a slow 20% increase in flow rate. This increase in flow rate has sped up over the last 30 years which demonstrates it was unrelated to the sunspots: casting doubt on the theory held by global warming sceptics.
At they very least they have found some useful tools for predicting which years are likely to be bad for flooding and their data may feed into better climate models in the future.