Research Highlights for Stratosphere and Climate

High above the equator winds slowly change from blowing eastward to westward and back again roughly every 28 months in a natural climate rhythm known as the quasibiennial oscillation (QBO). These regular winds have been recorded since the 1950s and emerge from natural processes within the tropics e.g. clouds, convection, rainfall and the wave disturbances arising from these. The latter break down high up in the stratosphere, analogous to waves on a beach.

A strong link exists between stratospheric variability and anomalous weather patterns at the Earth’s surface. Specifically, during extreme variability of the Arctic polar vortex termed a “weak vortex event”, anomalies can descend from the upper stratosphere to the surface on timescales of weeks. Subsequently the outbreak of cold-air events have been noted in high Northern Latitudes, as well as a quadrapole pattern in surface temperature over the Atlantic and western European sectors and it is currently not understood why certain events descend to the surface while others do not.

The figure shows blocking frequency, for blocking events that persist at least five days, during Northern Hemisphere winter (December-January-February) in two subsets of CMIP5 coupled ocean-atmosphere general circulation models. The model-mean blocking frequency shown in panel (a) is for models with high vertical resolution in the 5-15 km atmospheric layer, while that in panel (b) is for models with coarse vertical resolution in this layer. The 5-15 km layer encompasses the upper troposphere and lowermost extratropical stratosphere.

The ability of the climate models submitted to the Coupled Model Intercomparison Project 5 (CMIP5) database to simulate the Northern Hemisphere winter climate following a large tropical volcanic eruption is assessed. When sulfate aerosols are produced by volcanic injections into the tropical stratosphere and spread by the stratospheric circulation, it not only causes globally averaged tropospheric cooling but also a localized heating in the lower stratosphere, which can cause major dynamical feedbacks.