Stratospheric Warming

The past couple of days has featured a warming event in the stratosphere centered between the 2mb and 10mb height. This event has proven to be larger in magnitude than even the past few years. As the warming occurs, thicknesses increase, and the thermal wind balance then implies the mean zonal wind switches to the east. This has already occurred down to the 30mb level, which constitutes a major warming.

Since few people know much about what goes on 20 kilometers above their heads, here's a quick lesson on the stratosphere:

The stratosphere is the portion of the atmosphere above the troposphere between about 12km and 48km, and 200mb and 1mb. In the lower stratosphere the temperature is at a minimum at the equator and has maxima at the summer pole and at about 45 latitude in the winter hemisphere. From thermal wind considerations the decrease of temperature poleward of 45 in winter requires a zonal vortex with strong westerly shear with height.

Every other year or so in the Northern Hemisphere this normal pattern of a cold polar stratosphere with a westerly vortex is interrupted. The polar vortex becomes highly distorted and breaks down, accompanied by a large-scale warming of the polar stratosphere, all within a few days. This can quickly reverse the meridional temperature gradient and (through thermal wind balance) create a circumpolar easterly flow.

A "major warming" in which the mean zonal flow reverses at least as low as the 30mb level occurs only about once every couple of years. If the magnitude of a stratospheric warming event is large enough, the warming can once in a while propogate down to the tropopause. The reason for the unsuredness of propopation is that the warming is occuring at a level where the density of the air is just 2% of the density of the air at the tropopause, and about .2% of the density of the air at the surface of the earth, and since temperature is directly proportional to density, significant changes in temperature can be lost as propogation downward occurs.

When it does happen however, the descending increased thicknesses in effect "squashes" the polar vortex, with warming at the pole the results in a displacement of arctic air to the south, and has a direct correlation to a negative AO and sometimes -NAO.

The current stratospheric temperature picture:


Propogation looks likely with this year's warming event. Already warming has taken place as far down as 50mb. The ECMWF shows the polar vortex at 100mb breaking down significantly by day 8.




The GFS ensemble forecast has been developing a -AO within the next two weeks, with a major -NAO developing thereafter. The ensemble AO forecasts have suffered so far this winter, so confidence associated with them is low. However the ECMWF 8-10 day mean H5 anomalies also support rising heights in the polar region. Whether or not this is necessarily due to the stratosphere is uncertain. The timing of the stratospheric warming induced -AO should be a little later into February.

So as a summary of the long term, expect the +PNA to break down through next week, with a large piece of energy entering the SW US. (Storm potential is being discussed in the winter weather outlook page). After a brief blast of arctic air Sunday through Tuesday, temperatures will return to normal. We will basically be looking at a repeat of our pattern through last December into early January, with the overrunning potential occuring next week, followed by the -NAO development. One thing we have seen this winter is the constant support for east coast cold from different sources every week or two: When the PNA wasnt favorable, the EPO was; when the EPO wasnt favorable, the NAO was, and when the NAO wasnt favorable, the PNA was. This has kept the east coast (or at least 40N) favorable for cold and snow for the past 30+ days. For the first week of February, we will see the -NAO being favorable for the east, while the Pacific pattern supports warmth. It has yet to be seen which will ultimately prevail. At this point, it looks like the southern half of the nation will likely see above normal temperatures for this period. By the second half of February, the affects of the stratosphere should force a moderate to strong -AO, possibly assisting in -NAO development and a return to an amplified pattern with east coast cold.