Monday, March 24, 2014

Stratospheric Final Warming Occurring; Sustained Warmth Just 2-4 Weeks Away

In an update to an earlier post I wrote, where I evaluated the state of the stratosphere and possibilities for the final warming occurring, I believe we are indeed seeing the final warming event, meaning spring weather could be as little as two weeks away.

*Note: In previous posts, I addressed the potential of seasonal temperatures returning in just a handful of days. That forecast still stands- I am looking solely at the stratosphere in this post.

The image here shows a black line, which tells us observed temperatures in the 30 millibar level of the atmosphere, and also shows a superimposed gray line, which depicts the average temperature at the 30 millibar level at a given time. If we look at observed temperatures right now, where the black line stops, we see that they are well above the gray line, as much as 20 degrees above normal. The 30 millibar layer is located very high up in the atmosphere, especially when you consider we live on the surface level, located at the 1000 millibar, and considering planes usually fly at 35,000 feet, or about 250 millibars. Now, this massive warming event is not an odd occurrence in the stratosphere. Every winter, we usually see a few instances where the stratosphere suddenly warms up, which we refer to as a sudden stratospheric warming (SSW), or major stratospheric warming, which is a weaker SSW. However, at the end of winter, the stratosphere flips from having persistent low pressure dominate the upper millibars (we call this the polar vortex (yes, it's actually located very high up in the atmosphere, NOT on the surface)) to persistent high pressure, as the stratosphere basically shuts down for the winter. This flip from low to high pressure is natural, and happens every year, usually in the early spring months.

How do we know when this flip occurs? We look for a big SSW event that reverses wind direction in the stratosphere. 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.

Let's apply that rule on zonal winds to this observed chart of 10 millibar winds. Oranges show positive zonal winds, or westerly winds, while blues define negative zonal winds, or easterly winds. We observed strong westerly winds in the upper stratosphere in December and into January, indicative of a strong polar vortex. However, as we began to move into February, we began to see weakening of these westerly winds, and perturbations of easterly winds, the result of brief sudden or major stratospheric warmings. These have usually seen the polar vortex weaken but then recover. However, at the bottom of the image, we see that there has not been a recovery yet. Based on the previous SSW events, the polar vortex should have recovered by now. Despite this, we see no resurgence of westerly winds. Considering it's now late March, it's time to face the facts: we're probably looking at the Final Warming event, thanks to the strong 30mb warmings and lack of a resurgence of positive zonal winds.

We can also take a look at past and present zonal winds across the whole stratosphere and troposphere, not just the 10 millibar layer. The image above shows a cross-section of latitude-by-height zonal winds, where positives are in oranges and negatives are in blues. They go from least recent on the top left, to most recent on the bottom right. You can also identify the images by the dates just above each image. Note how in each image, since March 16 to March 21, we have seen the lack of positive zonal winds in the upper stratosphere, shown at the top of each panel. We have also seen a weakening of positive zonal winds that still remain across the stratosphere, and even into the troposphere. Once again, the lack of positive zonal winds coming back after the SSW event looks to have ended tells me not only has the polar vortex essentially collapsed in the upper stratosphere, but the final warming is occurring.

This can be further confirmed by the presence of dominating high pressure at the 1 millibar level of the stratosphere. Note the lack of a sustained polar vortex...

To summarize:
• We are looking at the Final Warming occurring in the stratosphere.
• This event signals the end of winter in the stratosphere.
• Since the effects of stratospheric events take 2-4 weeks to propagate to the surface, we should see a cold shot followed by potentially sustained warmth roughly 2-4 weeks from now.