The 2018–2019 Arctic stratospheric polar vortex Weather

The stratospheric polar vortex (SPV) is a planetary-scale cyclonic circulation that forms over the winter pole each year in the stratosphere (the layer of the atmosphere 10–50km above the surface) and is encircled by the westerly polar night jet stream. The vortex develops due to seasonal radiative cooling owing to the Earth's axial tilt; air within the vortex becomes isolated and can cool to below −80°C as a result of the lack of solar heating. In the Northern Hemisphere (NH), the SPV is highly variable on both intra- and inter-annual timescales. The distribution of the oceans, continents and mountain ranges produces large-scale planetary waves in the mid-latitude tropospheric polar jet stream. Planetary-scale waves can also be formed by anomalous heating associated with tropical convection, such as the Madden–Julian Oscillation (MJO) or the El Niño–Southern Oscillation (ENSO). These waves can propagate vertically into the westerly winds of the SPV and break in the stratosphere (akin to waves breaking on a beach), depositing their momentum there and decelerating the westerly flow. Such waves can only propagate into regions of westerly flow; this communication of wave activity from the troposphere to the stratosphere is absent in the summertime when stratospheric easterlies are present. The stratospheric circulation typically only supports large-scale waves of wavenumber 1 or 2 (whereas many higher wavenumbers are present in the troposphere). Contrastingly, the Southern Hemisphere (SH) SPV is relatively strong and stable with less inter-annual variability due to the symmetric Southern Ocean encircling Antarctica.