Until now, little was known about these winter-melt events, but using 50 years' worth of weather-balloon data, Ben Brock from Northumbria University, UK, and colleagues from the Centre for Scientific Studies and University of Concepcion, both in Chile, have gained a clearer understanding of how, when and why glaciers melt in winter.

In order to study glacier melting behaviour, the scientists needed a high-altitude temperature record. Such data are few and far between, so instead Brock and his colleagues searched for a good proxy. What they found was the Puerto Montt radiosonde, which has been operating in southern Chile since 1960. Using recent meteorological records over five winters, from two Chilean glaciers situated between 38°–42°S, the researchers showed that the atmospheric temperature readings from the Puerto Montt radiosonde were representative of near-surface air temperature at the glaciers. This enabled them to build up a record of near-surface glacier temperatures from 1960 to 2010.

From this record Brock and colleagues estimated when temperatures rose above freezing, causing a winter-melt event. On average they found that each winter has 21 melt days and approximately 0.28 m of glacier melt. However, there is a lot of variability around that average. "The number of melt days in each winter ranged from just five in 1967 to 38 in 1979 – that's well over one day in three," Brock told environmentalresearchweb. "The highest amount of melt was 58 cm in 1979 and the least 4 cm in 1967."

By analysing the meteorological data associated with melt events, the scientists gained an understanding of what causes winter melts. They identified two types of climate event that brought warm pulses of air over the glaciers during winter. The first was simply north-westerly winds, bringing warm air from subtropical parts of the Pacific Ocean. The second was adiabatic warming, when a large ridge of high pressure causes warm air to descend. "Interestingly the warm air often overrides colder air underneath," said Brock. "During the warmest events, the glaciers at around 2000 metres altitude are actually warmer than places at sea level."

Over the 50-year study period the researchers saw no trend associated with global warming. Instead they did discover a relationship with the El Niño Southern Oscillation Index, whereby most melt events tended to occur in-between the El Niño and La Niña states. "More work is needed to understand this relationship, but it is likely to be associated with changes in large-scale atmospheric circulation patterns across the south Pacific," explained Brock.

So is it just Chilean glaciers that sometimes melt in winter, or is this a global phenomenon? Brock and his colleagues think that significant winter melts are unlikely to occur in the cold, dry climate associated with continental glaciers, but that they could be common on "maritime" glaciers situated in the warmer and moister climates closer to coastal regions. Along with South America, maritime glaciers are found in Norway and New Zealand.

The findings are published in Environmental Research Letters (ERL) , as part of the ERL Focus on Cryospheric Changes in a Changing Climate.