Absorbing and reflecting sudden stratospheric warming events and how they impact the weather

Sudden stratospheric warming (SSW) events have received increased attention in the media since their impacts on the troposphere, where our weather happens, have led to a few notable events in recent years. Such as the SSW in early February 2018, which caused the polar vortex to weaken and split, leading to a significant breakdown and the redirection of frigid Arctic air southwards and eventually triggered the "Beast from the East" that arrived in the UK later that month.

2018 SSW sequence of polar stratospheric zonal wind reversal and propagation towards the troposphere. Source: Copernicus

However, each SSW is different and to understand the global impact of these events requires analysing the characteristics of the stratosphere-troposphere coupling or lack of. SSW events are classified into two types based on how effectively the stratospheric polar vortex absorbs or reflects upward-traveling planetary waves from the troposphere. The resulting type dictates how the event impacts subsequent surface weather patterns

Absorbing SSW

The first is the absorbing type of SSW, which has a longer timescale as well as a larger meridional extent due to the persistent incoming planetary waves from the troposphere. The absorbing type of SSW is related to the annular mode on the surface through poleward and downward migration of the deceleration region of the polar night jet.

This can result in the following impacts in the stratosphere and troposphere:

• Longer Duration: The polar vortex remains in a highly disrupted state for a longer period, sometimes a month or more.
• Wider Impact: It has a larger meridional (north-south) extent
• Surface Effects: there is a higher probability of significant, long-lasting cold weather outbreaks in mid-latitude regions, such as North America and Europe, often associated with a negative Arctic Oscillation (AO) and negative North Atlantic Oscillation.

The February 2018 SSW is an example of an absorbing SSW, which split the polar vortex into 2 ‘daughter’ vortices.

Reflecting SSW

The other is the reflecting type. This is characterized by a quick termination of the warming episode due to the reflection of planetary waves in the stratosphere, which leads to an amplification of tropospheric planetary waves inducing strong westerlies over the North Atlantic and blocking over the North Pacific sector.

This can result in the following impacts in the stratosphere and troposphere:

• Quick Termination: The stratospheric warming episode terminates quickly, and the polar vortex can recover more rapidly.
• Localized Impact: The reflected energy leads to an amplification of tropospheric planetary waves, which induces strong westerlies (winds from the west) over the North Atlantic and a high-pressure blocking pattern over the North Pacific sector.
• Surface Effects: The impact on surface weather is typically short-lived and tends to focus cold weather events more specifically over North America, while Europe might experience milder, westerlies-driven, wet, and windy conditions.

Source: Stratospheric Polar Vortex wave absorption/reflection and effect on surface climate

The minor SSW in late November just gone is an example of a reflective SSW. Reflection of planetary waves upwards into the stratosphere have been reflected back down into the troposphere, leading to a quick termination of the warming episode with a restrengthening of the SPV. While the reflection of planetary waves back down into troposphere leads to an amplification of tropospheric planetary waves inducing strong westerlies over the North Atlantic and blocking over the North Pacific sector.