January 18, 2021

Is stratospheric warming under way?

For several days, so-called “sudden stratospheric warming” has been anticipated over the north pole. The phenomenon was discovered in Berlin in 1952. For that reason it is also known as the “Berlin Phenomenon”, although it is actually the entire polar region that is affected. It involves a spontaneous, substantial warming of the stratosphere in winter. The stratosphere extends some 50 kilometres from the end of our “weather layer”, the troposphere, at about 12 kilometres altitude, up to the transition to the mesosphere. Within a few days winter temperatures increase here from the usual -70°C up to -20°C. This warming is often preceded by a cooling of the mesosphere. Stratospheric warming is accompanied by a weakening or even reversal of the west wind that dominates in the stratosphere during winter (see. EOC News of March 2, 2018). These west winds flow around the polar region counter-clockwise in a more or less circular path and are responsible for the formation of a large low pressure area. Stratospheric warming can lead to the disruption of the polar vortex, which then separates into two low pressure areas that drift southward, bringing with them cold arctic temperatures. This can lead to the paradoxical situation that ground temperatures in arctic regions are warmer than those in temperate latitudes.

Wind speed on January 17, 2021

Newspaper articles have recently appeared about the current situation in the stratosphere. There are speculations that the sudden onset of winter in Spain may be caused by processes in the stratosphere. For example, at the end of December a division of the polar vortex was predicted already for the beginning of the year. So far, there has only been a strong shift of the vortex southward over the European continent (see figure 1, left). The vortex is indeed also definitely weaker, but it has not yet decisively separated into two parts.

Wind speeds on January 4, 2021
at a pressure level of 10 hPa (ca. 30 km altitude). The flow lines clearly show a quasi-bi-modal structure of the large-scale circulation in the northern hemisphere



So-called planetary waves are the reason for such disruptions to the polar vortex. They are responsible for a regular succession of high and low pressure areas at mid-latitudes. Planetary wave activity in the stratosphere (10 hPa, ca. 30 km altitude) was intense at the end of December and only noticeably decreased in January (see figure 2).

Planetary wave activity (PWA)
from July 2020 to Jan. 7, 2021 in the northern hemisphere compared with the climatologic mean value (white) and twice the standard deviation (red) for each day of the year between January 1,1979 and December 31, 2020. The numbers 1 to 3 represent wave numbers (the number of wave maxima along a latitude). Each individual year is shown as a light grey line in the background. Data basis: temperatures from global ERA5 reanalyses.

Increased planetary wave activity can significantly weaken the polar vortex. It begins to meander and shift its location until, should the weakening persist, it collapses and splits, with stratospheric warming as the result. That was almost the case on January 4 this year. A quasi-bi-modal structure could be detected (see figure 1, right), but the vortex has not yet finally divided.

At the same time we are now observing a strong La-Niña event, a global weather phenomenon that last took place in 2015/2016. That normally means that the planetary waves are especially weak, so that the vortex remains poly-symmetric and undisturbed (see EOC News “Ozone hole 2020” of Dec. 10, 2020) while the mesopause region is characterized by above-average high temperatures. Until mid-December this could also be seen quite clearly in our data (see figure 3).

The mesopause temperature measured at four stations in Europe
[grey: Zugspitze (47°N, 11°O), Haute-Provence (44°N, 6°O), Wuppertal (51°N, 7°O) and Abastumani, Georgia (42°N, 43°E)] and the mean value (black/red). At all stations, December was noticeably warmer than in 2019 (red). In addition, all stations show the same behaviour for January of this year: as a whole, the mesopause has cooled by about 20 K since mid-December, and since then the temperatures vary in synchrony at all stations.

In the past three weeks they quickly dropped by about 20°C. The same temperature decline was recorded at four European measuring stations, a clear indication that planetary waves are associated with this drop. So there must be a large, supra-regional shifting of air masses in the mesosphere, which could in fact be a sign of imminent stratospheric warming.

As part of the international global Network for the Detection of Mesospheric Change (NDMC), DLR operates infrared spectrometers at several locations to monitor temperatures in the mesopause, such as at the Schneefernerhaus Environmental Research Station on the Zugspitze mountain.