Recall how man-made CO2 emissions are supposed to cause a greenhouse effect in the troposphere, meaning the stratosphere above it is supposed to cool. Lately, that has not been the case at all – at least over the northern hemisphere, 65°N and above, over the better part of the last year.
Schneefan here writes on an interesting phenomenon now taking place in the Arctic stratosphere. Much of what follows is a translation, along with some points I’ve added myself.
The upper stratosphere over the Arctic, her writes, has never been so warm in March since satellite records began back in 1979: a sudden stratospheric warming (SSW) with an expected splitting of the polar vortex is underway (polar vortex split).
The plot above shows the sudden warming (red line) on March 7, 2016 (day 67 of the year) occurring in the upper stratosphere at 10 hPa (approx. 30 km altitude), between 65°N and 90°N. The red line significantly surpassed the high mark (upper black line) for March and is far above average (line with green dots). Source: www.cpc.ncep.noaa.gov/.
Through the strong warming and related pressure increase, a warm high has formed over the Arctic in place of the cold polar vortex. This means the polar vortex will split over the coming days and get displaced southwards – (polar vortex split):
ECMWF projection of March 8, 2016 for temperatures in the upper atmosphere at 10 hPa (approx. 30 km altitude) for March 16, 2016. The polar vortex has split due to the high pressure caused by the sudden stratospheric warming (SSW) over the pole. Two completely separated cold cores form over Siberia and Western Europe/North Atlantic. At this latitude the zonal wind has reversed from west to east. Source: http://www.geo.fu-berlin.html.
When there is a reversal in the temperature gradient and circulation (west wind shifts to east wind) from 60°N latitiude to the North Pole at in 10 hPa (approx. 30,000 m altitude), this is known as a major warming. Whether or not this ushers in a “final warming” and thus a switch from the Arctic winter to a summertime circulation remains to be seen…
The SSW and expected polar vortex splitting along with the reversal of zonal winds from west to east will also have major impacts on the circulation of air masses at the lower atmospheric levels of the northern hemisphere. The polar jet will meander and lead to enhanced exchange between polar and subtropical air masses: That means more cold air blasts into the middle latitudes are expected for the northern hemisphere.
The ECMWF prognosis from 9 March 20016 consequently sees an intensified March winter 2016 over large parts of Europe. Wintry temperatures at 850 hPa (approx. 1500 m) of -10°C on March 19,2016 over eastern Europe:
ECMWF forecast of March 9, 2016, for temperatures on March 19, 2016, in Europe. Cold Siberian air is expected to flow across Europe between high pressure positioned over the North Atlantic and a low over Russia and southern Europe. Source: www.wetterzentrale.de/html
These unusually cold calculations have been more or less confirmed by other weather models over the past few days. But as everyone knows, forecasts 10 days out are fraught with considerable uncertainty, and so we should not lose hope for milder spring weather.
Yet, this year it looks like we’ll have to wait a little longer than normal. But that shouldn’t surprise us, though, because statistics show that spring has been arriving later and later in Europe over the past 2 decades – and not earlier.