It tends to come in autumn. The Venetians call it acqua alta--the seemingly seasonal flooding of their historic city center. But a paper published in the Journal of Geophysical Research Atmospheres suggests that Italians eager to predict the next flood shouldn't study the Earth's seasons, but should instead look at the sun. A project led by David Barriopedro at the Universidade de Lisboa in Lisbon, Portugal analyzed hourly recordings of water levels in the city from 1948 to 2008. His team noted a correlation of these "high-surge events" and the eleven-year solar cycle: Periods of maximum solar activity, when sun spots usually appear, seemed to herald the acqua alta. Historically, finding a causal link even between longer term climate changes and sunspots has proven difficult. As a National Oceanic and Atmospheric Administration (NOAA) site on sunspots says, a period of very low sunspot activity from the 1600s to the 1700s, dubbed the Maunder Minimum, corresponded to cold temperatures in Western Europe, a period known as the Little Ice Age. This could be a coincidence, the site says, given the complexity of the Earth's climate. The study of Venice's floods also says that any link between these short-term weather events and the sun is "indirect." The researchers note that the solar activity corresponds to changes in pressure systems over the Atlantic Ocean, and these pressure systems in turn correspond to how storms move over Europe. As New Scientistreports:
Records of air pressure over Europe over the same period revealed "acqua alta years" saw a lot of low-pressure systems over the north Adriatic Sea, while in quiet years these systems were further south. This makes sense, because flooding events in Venice are known to be triggered by low-pressure systems from the Atlantic.
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