|Observed zonal winds on left|
Hour 240 forecasted zonal winds on right
Shown above is a combined image of observed zonal winds as of yesterday morning throughout the stratosphere (millibar levels shown on the left-hand side of the image), while the right image depicts zonal winds 10 days from now. Positive zonal wind values indicate the presence of a storm system (in this case, the polar vortex), while negative zonal wind values tell of an anticyclone (high pressure system) in the vicinity. The observed image of zonal wind values tells me that the polar vortex is weakly present from the 3 millibar level through the rest of the stratosphere. We see how the positive zonal wind anomalies extend well beyond the 100 millibar level that defines the border between the stratosphere and the troposphere. The forecast image shows that the positive zonal wind anomalies have been eradicated from the stratosphere, and it is this eradication that indicates the polar vortex has collapsed and entered its summer stage. When the polar vortex collapses in spring, high pressure takes over the stratosphere, ending the risk for sudden stratospheric warming events. In the fall, this high pressure system breaks down again and the polar vortex returns to re-ignite the risk of collapse in the winter, like we very nearly saw this past winter.
When the polar vortex takes its summer leave, we can anticipate the unusually chilly spring to end. The stratosphere was bubbling with anomalously warm areas of air in the upper stratosphere, meaning that the stratosphere was still in its winter stage. Because the stratosphere was in its winter stage, the lower atmosphere, including the troposphere was most likely still in a rather wintry stage. Because the stratosphere is getting into its summery phase, we can expect the surface weather to calm down as well.