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Tropical triggers for polar stratospheric warmings

A diagram showing the layers of Earth's atmoshere.

This diagram shows the layers of Earth's atmosphere. (View a larger version in UCAR's Digital Image Library.) Sudden stratospheric warmings occur in the the stratosphere, located between the troposphere and mesosphere. The WACCM model is one of the few comprehensive climate models that extends from Earth’s surface through the stratosphere and beyond. 

A new study from NCAR uses an innovative computer model to investigate events called sudden stratospheric warmings (SSWs) in the Arctic atmosphere. The study focuses on how two atmospheric patterns based in the tropics, the El Niño–Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO), affect SSWs.

SSWs occur during winter when the polar vortex of westerly winds in the Northern Hemisphere slows or even reverses direction, usually over the course of a few days. They are accompanied by a dramatic rise in stratospheric temperatures. The Arctic’s stratosphere is highly variable during the winter due to this phenomenon. A major SSW occurs about every two years, and several minor events usually occur each winter.

Scientists know that the frequency of SSWs is influenced by factors that originate outside of the polar stratosphere, including ENSO, QBO, the Sun’s 11-year cycle, and volcanic eruptions. But because observations of the stratosphere are only available for the past 50 years, it is difficult for scientists to sort out how these different forcings influence the stratosphere separately and together.

In a study led by Jadwiga Richter, researchers used NCAR’s Whole Atmosphere Community Climate Model (WACCM) to run simulations zeroing in on how ENSO and QBO affect the frequency of SSWs. WACCM is one of the few comprehensive climate models that extends from Earth’s surface through the stratosphere and beyond.

The study found that removing only one forcing (either ENSO or QBO) results in an SSW frequency slightly reduced but similar to the control simulation; removing both forcings, however, results in a significantly decreased SSW frequency, from six to one warming per decade. This shows that either ENSO or QBO is needed (in addition to solar forcing) to produce a realistic frequency of SSWs in climate models. The study also suggests a non-linear interaction between ENSO and QBO in the polar stratosphere.

In addition, the study found that ENSO and QBO lead to SSWs with different characteristics. QBO causes SSWs that are very intense and that impact stratospheric temperatures between December and June. ENSO causes SSWs that are less intense and that occur primarily January through March.