Highlights
A paper published yesterday in the journa
l Geophysical Research Letters claims the harsh winter weather is consistent with a warming world.
The paper blames the severe weather on warmer temperatures in the Northern Hemisphere caused by climate change. Warmer air means the atmosphere can hold more water. When continental Europe and northern America cool in autumn, the atmosphere can’t hold as much moisture and dumps it as snow.
During the first three weeks of October 2009, snow cover increased rapidly over Europe and persisted throughout winter. The cold and reflective snow cooled the atmosphere above it: the air in the mid-latitudes – around 45 degrees north – became colder and denser and began to sink towards the ground.
Scientists believe sinking air and lower temperatures at mid-latitudes help knock the (AO) – a large-scale atmospheric pressure pattern that switches between two modes – into its “negative” state.
During a “negative” AO, air sinks at mid-latitudes and rises over the poles. The “positive” phase is the other way around – air is sinking over the poles and rising at mid-latitudes. The “negative” phase of the AO tends to bring colder weather to the US and northern Europe.
Widespread snow at mid-latitudes – the paper says – leads to a “negative” AO through a six-step process. During step one and two, air cools and sinks over the snow-covered mid-latitudes. As the air cools and descends, it causes the atmosphere above it – around five to 31 miles above the surface – to warm. This warming causes upper air turbulence, which – in step five and six – propagate to the surface as a negative AO.
Last year’s negative AO was unusually severe because the snow hung around throughout the winter and kept temperatures low. This triggered the six-step cycle twice in quick succession – once in late October/November and again in late January. Usually during a negative AO, the Northern Hemisphere only cycles through these steps once per winter.
Cohen, J., Foster, J., Barlow, M., Saito, K., & Jones, J. (2010). Winter 2009–2010: A case study of an extreme Arctic Oscillation event Geophysical Research Letters, 37 (17) DOI: 10.1029/2010GL044256
The source article Why was last winter so cold? And is this a problem for climate change? | Outdoor Science .
