Jan 16, 2012
African thunderstorms show up in Scotland
Lerwick Observatory in Shetland, roughly 60 miles north-east of the Scottish mainland, has proved a valuable – and rare – source of measurements of atmospheric electricity since the 1920s. Investigations by scientists from the University of Reading, UK, revealed that air electricity in Shetland intensifies when ocean temperatures rise; they believe that this is likely to be because of increased lightning in east and central Africa.
"Visiting the Met Office site in Lerwick turned up amazing new information about these atmospheric electricity measurements: data tabulations and notebooks, hardly any of which had been analysed," Giles Harrison of the University of Reading told environmentalresearchweb. "As the fine weather air electricity arises from currents flowing from distant thunderstorms, it seemed worth comparing these measurements with a factor influencing global meteorology: this led to the comparison with El Niño."
Although measurements at Lerwick began in the 1920s, the "fair weather" conditions needed to remove local weather effects were only noted from 1957, and nuclear weapons tests contaminated the data for a period from the late 1950s. So the researchers used records of potential gradient from 1968 until 1984, when measurements ceased. They compared them to the Niño3.4 anomaly index, which shows sea-surface temperature anomalies in the Pacific Ocean and so indicates the state of the El Niño Southern Oscillation.
"Our analysis demonstrates that the natural network of electric currents flowing from thunderstorms globally is influenced by the El Niño Pacific temperature fluctuations," said Harrison. "This is a further example of the remarkable inter-connectiveness of the atmosphere–ocean system: water temperature changes in the Pacific ultimately couple to air electricity in Shetland."
The potential gradient data from Lerwick exhibited a daily cycle that closely mirrored the classic "Carnegie curve", with a minimum at around three Universal time (UT) and a maximum at 19 UT. These changes correspond to total thunderstorm area worldwide.
The atmospheric electricity readings were most sensitive to the El Niño index between 10 and 16 UT. African thunderstorms tend to peak at these times, indicating that the African electrical storm response to El Niño dominated. According to the researchers this implies that El Niño boosts thunderstorm activity in east and central Africa, which is consistent with the increased precipitation observed in Kenya and east Africa during an El Niño.
Harrison believes that by taking advantage of atmospheric electricity measurements made anywhere, this technique provides a new approach to understanding how El Niño is linked to weather changes elsewhere on the planet. He says the results also quantify how the fine weather atmospheric electricity is influenced by natural variations.
"Previous work has studied the responses in rainfall and lightning distributions to El Niño," said Harrison. "Our analysis adds new information from examining the time of day the changes are observed – the contributions to atmospheric electricity from different parts of the world occur at different times of the day in Shetland, which pointed to involvement of African lightning."
Now the team plans to investigate whether the relationship between atmospheric electricity and El Niño is present in other sources of atmospheric electricity information, and how strong it is.
The team reported their work in Environmental Research Letters (ERL).