The Web of Life (18-2) Stratosphere

 Stratosphere

The stratosphere is 10–50 km above Earth’s surface. Living up to its name, the stratosphere is stratified. Higher up, the stratosphere hots up.

As to the heat: it’s the ozone (O3); or not. In the lower strata none forms. The energy of sunlight is not strong enough to form ozone there.

In the mid-strata, the ozone layer forms as O and O2 combine when they meet. Higher up, O3 is whacked into diatomic (O2) and atomic (O) oxygen by sunlight. At different altitudes, depending upon its intensity, sunlight creates and destroys ozone.

Stratospheric ozone provides a protective layer to life below. O3 absorbs nearly all shorter wavelength ultraviolet sunlight (< 32 nm). The ozone layer is at a higher altitude in the tropics, and at its lowest in the polar regions.

The strata of the stratosphere are generally stable, with little convective turbulence. Variations in the jet stream, along with other wind shear, stir the lower stratosphere.

That stir can be quite significant at times. Gyral variations in stratospheric winds directly perturb the layer below: the troposphere. They also affect deep-ocean currents.

 Polar Vortex

The stratosphere is somewhat different at the north pole. The polar vortex comprises the stratospheric winds that encircle the Arctic. These winds can extend from the lowest stratospheric layer to beyond the top of the stratosphere.

Polar vortex patterns typically last 2 years. Sudden warming events can occur, with these winds weakening or even changing direction. Sporadic breakdown can last for up to 2 months, affecting air circulation all the way to the surface, and the ocean below.

The North Atlantic Ocean circulation pattern influences Earth’s oceans by moving water around the planet like a conveyor belt. Hence, changes in circulation pattern speed have a cascading effect on the rest of the world.

South of Greenland lies Earth’s most important seawater downwelling region: the sinking of cold, salty water that drives the oceanic conveyor belt. This downwelling area is quite susceptible to temperature changes in the troposphere there. Even modest warming or cooling in the local troposphere can trigger or delay downwelling. Hence the subtle gyral relationship between events in the polar stratosphere and ocean currents worldwide.

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Bacteria live in the lower stratosphere. Some birds reach the lower stratosphere, notably the Bar-headed goose, which routinely flies over Mount Everest’s summit.

Aerosols layer in the stratosphere, especially sulfuric and nitric acids, which are naturally deposited from volcanic exhaust. Other aerosols and gases make their way into the stratosphere. Some pose a threat to the ozone layer upon which life depends.