E12 Composition and Vertical Structure

Topic

Earth's atmosphere is divided into layers defined primarily by temperature variations with altitude. The troposphere, extending from the surface to an average height of about 12 km, exhibits a temperature decrease of approximately 6.5°C per kilometer. Above it lies the stratosphere—spanning roughly 12 to 50 km—where temperature increases with altitude, primarily due to the absorption of ultraviolet radiation by ozone. The mesosphere is located between approximately 50 and 80 km, where temperature once again decreases with altitude, followed by the thermosphere. This alternating temperature profile forms the basis of the atmosphere's vertical structure.

The boundary between the troposphere and the stratosphere is known as the tropopause. Its altitude varies significantly with latitude: in the tropics, it may be situated between 16 and 18 km, whereas in polar regions, it is typically found between 8 and 10 km. This difference reflects the thermal and dynamic conditions of each region. Intense tropical convection raises the tropopause, while the colder, more stable air of polar regions keeps it at a lower altitude. Its position is thus the result of a balance between radiative and dynamic processes and can shift when this balance changes.

The stratosphere also features a large-scale atmospheric circulation known as the Brewer-Dobson circulation. Air rises primarily in tropical regions, moves toward higher latitudes, and eventually descends in extratropical and polar regions. This circulation is linked to atmospheric waves generated in the troposphere that propagate into the stratosphere and transfer momentum there. Consequently, they help drive the meridional movement of stratospheric air. The Brewer-Dobson circulation also plays a fundamental role in the distribution of ozone and other trace gases, transporting them from their regions of origin and altering their global distribution.

The interaction between atmospheric waves and the stratosphere can produce particularly intense events known as sudden stratospheric warmings. During these events, the temperature of the polar stratosphere can rise by several tens of degrees within a few days. Simultaneously, the stratospheric polar vortex—formed by strong winds circling the pole during winter—can weaken significantly and, in the most intense cases, undergo a wind reversal.

The consequences of these phenomena can propagate to lower levels of the atmosphere. The coupling between the troposphere and the stratosphere thus operates in both directions. From below, waves generated in the troposphere propagate upward and influence stratospheric circulation, including the Brewer-Dobson circulation and the polar vortex. From above, disturbances occurring in the stratosphere can subsequently propagate down to the troposphere, altering atmospheric circulation and pressure patterns for weeks.

When a sudden warming event severely weakens the polar vortex, the resulting effects can alter circulation patterns associated with the Arctic and the North Atlantic. These changes can favor specific atmospheric configurations that allow cold air to intrude into certain mid-latitude regions. There is thus a dynamic connection whereby disturbances originating in the troposphere first affect the stratosphere, and the resulting changes can subsequently influence near-surface weather.

The vertical structure of the atmosphere is not, therefore, merely a succession of independent layers. The troposphere, stratosphere, and upper regions form a system dynamically connected through radiation, circulation, and wave propagation. A disturbance generated in one layer can be transmitted vertically, alter circulation in another, and subsequently produce effects over great distances and on timescales of days or weeks.

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