This blog entry provides a quick overview of the stratosphere and the polar vortex.
Both are of immense importance for understanding and forecasting Earth’s weather patterns.
Earth’s atmosphere consists of five main layers, each with distinctive characteristics.
From top to bottom, they are the exosphere, thermosphere, mesosphere, stratosphere and troposphere (Figure 1).
The troposphere is home; the air we’re most familiar with. It ranges from 7 to 20 km in height – tallest at the equator and shortest at the poles.
On top of this, fully above all but the very tallest clouds (i.e. severe thunderstorms), resides the stratosphere. This layer is comparatively vast, ranging from 30 to 43 km in height.
Meteorologists typically refer to height in the atmosphere by its associated air pressure (in millibars; mb). The stratosphere ranges from about 100 mb at its base to 1 mb at its top.
For reference, at sea-level, air pressure typically ranges from 970 to 1040 mb (extremes 880 to 1070 mb). Essentially, the more air is above a given point, the more pressure it exerts on that point.
Figure 1: Diagram illustrating the height spans of Earth’s atmospheric layers. Please note that the pressure values on the right are approximate (also, one bar is one thousandth of the more commonly used millibar).
Floor One: Stratosphere
Through interactions across the boundary (known as the tropopause), the stratosphere has strong effects on weather patterns in the troposphere.
It influences everything from winter storminess in North America and Eurasia, to summer sea ice melt in the Arctic, to the location and expansiveness of strong tropical thunderstorms.
For this blog piece, I will focus on the path of influence that involves the ‘polar vortex’.
The Powerful Polar Vortex
Broadly speaking, this is a vast (thousands of miles across!) rotation of air that extends from the lower troposphere to the top of the stratosphere. In the Northern Hemisphere (NH), it spins anticlockwise.
It spins up mid-late autumn (Oct-Nov in NH) above the Arctic region. It then tends to strengthen until between mid-Jan and early Feb, but may fluctuate greatly depending on other troposphere-stratosphere interactions (see: stratospheric warming events).
Late winter onward, an overall weakening trend takes place. It usually dissipates by May.
Figure 2: Northern hemisphere cylindrical equidistant view of reanalysed wind speed (shaded) and direction (arrows) at 10 mb height (about 30 km altitude) averaged across 1st-2nd Jan 2020. The enormous circulation of the polar vortex is clear to see! Notice the offset from the North Pole – the vortex is not a stationary feature.
The Southern Hemisphere also has a polar vortex that’s generally absent in austral summer (Dec-Feb) and strongest in austral winter (Jun-Aug). It spins the opposite way to its NH counterpart.
This seasonal behaviour results from the larger autumn-winter cooling in the Arctic and Antarctic regions relative to the remainder of the planet. It becomes much colder there than in the North Atlantic, for example.
The physical laws governing our universe discourage such uneven energy distribution. To attempt to restore balance, they drive cold air out of the Arctic and warm air into it.
But this is quite literally not a straightforward process. The Earth’s spin causes its surface to move to the west relative to air that’s moving north or south, which means that relative to it, the air curves eastward.
In the stratosphere, the resulting eastward (i.e. westerly) flow encircles the hemisphere and so creates the boundary, known as the polar night jet, which forms the periphery of the polar vortex.
The polar vortex’s enormous cyclonic rotation strongly affects weather patterns in the mid-latitudes*.
In both hemispheres, the polar vortex acts to drive mid-latitude weather systems from west to east. The stronger it becomes (i.e. faster it rotates), the more eastward momentum it imparts.
This also reduces how readily weather systems can move northward or southward (due to inertia). Weather patterns become ‘flat’ with little in the way of cold air moving equatorward or warm air heading poleward.
This tends to bring fast-changing weather to the poleward half of the mid-latitudes. Temperatures vary around the long-term average, but are more often above it than below. Figure 3 illustrates this using the historical top-ten strongest Jan-Feb mean polar vortex strengths as an example.
Figure 3: Composite geopotential height (left) and temperature (right) anomaly maps for the ten years with the strongest Jan-Feb mean zonal wind speed at 30 hPa; a good measure of the polar vortex strength (faster wind, stronger vortex).
We’re talking locations such as Northern Europe and Asia (i.e. Eurasia) most of the USA and southern Canada, or in the other hemisphere, the far-souths of America and Australia and most of New Zealand.
You can really see this effect in their climate patterns; a tendency for frequent bouts of wet and windy weather late autumn through mid-winter, with the rest of the year typically calmer and slower-changing.
Wonderous Weak Vortex Events
Sometimes, the atmosphere ‘fights back’ against the polar vortex, assaulting it with high-energy waves that raise temperatures above the Arctic.
This weakens the vortex, reducing the eastward inertia of weather systems. A strong weakening can lead to all sorts of wacky weather patterns across the mid-latitudes, from severely cold and snowy to positively spring-like!
For a healthy dose of additional insight, see my blog entry on stratospheric warming events.
James Peacock MSc
Head Meteorologist at MetSwift