Tuesday, March 18, 2014

Stratospheric Polar Vortex Collapsing; Final Warming Occurring?

We're going through a collapse of the upper stratospheric polar vortex at this moment, and it's now possible that we are seeing the Final Warming event occurring.

The image above shows a six-panel graphic, each marking a day's observation of zonal wind anomalies by latitude, as the legend on the bottom shows, and by height in millibars, as the legend on the left shows. Going from the top row of panels to the bottom row, from least to most recent, we see a notable and rather sudden drop in positive zonal wind anomalies in the upper right-hand corner. Looking closer, we find this drop in anomalies over the 1 to 10 millibar level, in the highest reaches of the stratosphere (remember that we are based at roughly the 1000 millibar level, so 1 millibar is far higher than even what planes cruise at). Now, this transition from orange colors to blue colors in the top right hand corner of the most recent observation, marked under March 15 2014 means that the winds have reversed. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened. In this case, rather than the positive zonal winds just weakening a bit, it looks like they completely reversed in the far upper stratosphere, marking a collapse of the polar vortex at that level. Also note the lack of strong zonal winds in the mid and lower stratosphere, also indicative of a very fragile polar vortex.

An analysis of the 1 millibar level's temperature and height contours tells the story. In this view, covering the Northern Hemisphere (if you look closely you can see the continent outlines in white), we see high pressure duking it out with low pressure-- and winning. The high pressure has taken over the Arctic Circle, pushing the upper stratospheric polar vortex to the south. The swath of warmer than normal temperatures is also helping to weaken the polar vortex to the point that those zonal winds have reversed, meaning the polar vortex has fallen.

Let's take a look at another multi-panel graphic to get an even better handle on this situation. On the top panel, we see something we're already familiar with- the turquoise line denotes zonal wind values at the 1 millibar level, and the prominent dashed line indicates the separation line between positive zonal winds (upwards from the dashed line), and negative zonal winds (downwards from the dashed line). This graph tells us that as of late, zonal winds at the 1 millibar level have been sustained at negative levels, confirming this idea that the winds have reversed in the upper stratosphere, leading to a failure of the polar vortex. The panel just below that one shows us zonal winds for the 10 millibar level (blue) and the 30 millibar level (red). While these two levels are not seeing negative zonal winds, they're awfully close to zero on that graph, and that tells me something else is going on here. I'm led to believe that we are now under a final warming warning, if you will.

In the winter, there are sudden stratospheric warming (SSW) events that see massive quantities of warm air shoved up into the stratosphere, disrupting the polar vortex and sending frigid air down to lower latitudes, such as Eurasia and North America. However, in late winter or early spring, we see a massive warming event that reverses zonal winds to negative values for the summer. This means that we see the polar vortex disintegrate for the summer, replaced by dominant high pressure. This process is natural and happens every year. Now that we're in March, and we're seeing a stratospheric warming event that looks like it's dragging down positive zonal winds across the stratosphere to zero or negative values, it looks to me that this could very well be the Final Warming for the winter, heralding the end to the winter of 2013-2014 (at least in the stratosphere).

The 10 day ECMWF zonal wind forecast, the same format as we saw in the six-panel image at the top of this post (except valid 10 days from today), gives a prognosis that the Final Warming is either occurring or will occur soon. Even 10 days from now, we see slightly negative zonal winds now extending from the top of the stratosphere down to the 20 millibar level. Yes, we see a spot of positive zonal winds centered around the 30 millibar level on the right side of the graphic, but no big rebound to a strong polar vortex in 10 days' time tells me the stratosphere is shutting down for the summer.

This means, in layman terms, that winter could be over in just 2-4 weeks. It might seem like a long time, but it's better than not knowing if it will end at all, right?