On the early onset of the NLC season 2013 as observed at ALOMAR

On 21 May the ALOMAR RMR-lidar in Northern Norway detected the first noctilucent clouds (NLC) in 2013. This unusual early NLC onset was accompanied by ∼6 K lower temperatures and higher water vapor mixing ratios at NLC altitudes from the end of April until the beginning of June. The zonal mean temperature and dynamic conditions in the Arctic middle atmosphere deviated in spring 2013 significantly from the mean conditions of the last 20 years. Furthermore the planetary wave activity in the high latitude stratosphere was enhanced from 20 April to beginning of May. The colder and wetter upper mesosphere in May 2013 is attributed to this unusual late planetary wave activity in the stratosphere, introducing a strong upwelling in the mesosphere, lower temperatures and an upward transport of water vapor, which finally resulted in earlier existence conditions for mesospheric ice particles. We regard this as a first evidence for intra-hemispheric coupling in the northern hemisphere extending from the stratosphere into the mesopause region. Yet it is unclear whether this is an unusual extreme event or an indicator for a change in the circulation due to the observed long-term cooling of the middle atmosphere.