General Circulation in the Middle Atmosphere

The atmosphere above the troposphere at altitudes of approximately 10 to 100 km, namely stratosphere, mesosphere, and lower thermosphere, is called the middle atmosphere.

In the middle atmosphere, convective clouds, which you usually see in the sky, do not form.

Figure 1 shows the latitude-height cross section of the zonal mean temperature of the Earth's atmosphere in July by colors. In this figure, the northern hemisphere is in summer and the southern hemisphere is in winter. In January, the latitudinal distribution of the temperature is nearly the opposite of this.

**Figure 1** Latitude–height cross section of the zonal mean temperature in July. The red color indicates a higher temperature. The color interval is 20°C. The temperature at the boundary between blue and light blue is -120°C, while that between yellow and orange is -40°C, and that between red and pink is 0°C.

The stratopause (the temperature maximum in the vertical) is observed at ~50 km. This is because the ozone layer absorbs ultraviolet radiation from the sun and warms the air at that height. A stratopause does not exist in the atmospheres of Mars and Venus because the ozone layer is unique to the Earth, which is created by living organisms.

The temperature maximum at the stratopause appears in the summer polar region.

In July, although the solar radiation at the time when the sun crosses the meridian is strongest at around 20 degrees north latitude, the sun sets there. On the other hand, the sun does not set in the summer polar region. Thus, this region continuously receives solar radiation, whose daily average is greater than at any other latitude. Hence, the temperature maximum at the stratopause appears in the summer polar region.

Here, there are two notable, unusual features.

The first is the existence of the stratopause in the winter polar region (the Antarctic region in Fig. 1).
This region is during the polar night, when the sun does not rise.

The second is the low temperature at the mesopause in the summer polar region (the Arctic region in Fig. 1), which is the lowest in the Earth's atmosphere. The temperature there is so low that even a small amount of water vapor (which does not come from the ocean but from the oxidation of methane) freezes and forms clouds. These clouds are known as polar mesospheric clouds, also called noctilucent clouds because they appear to glow at night. The polar mesopause is warmer in winter than in summer, even though it is exposed to the midnight sun during summer.

It is the general circulation in the atmosphere that causes these strange temperature features.

The thick dashed curves with arrows in Fig. 1 show the general circulation in the middle atmosphere. In the stratosphere, there is a single-cell circulation from the low latitudes to the winter pole (called the Brewer-Dobson circulation), and in the mesosphere, there is a single-cell circulation from the summer pole to the winter pole (which has not been named and is simply known as the mesospheric general circulation).

In the winter polar region, there are stratospheric and mesospheric downwelling, whereas in the summer polar region, there are stratospheric downwelling and mesospheric upwelling. These upward and downward flows greatly change the temperature.

Have you ever heard of adiabatic expansion? It is a phenomenon wherein the temperature decreases when the pressure decreases without transferring mass or heat exchange between the air mass and surrounding environment.

For example, when you uncork a beer bottle, a mist may form. This is because, as the highly pressurized air inside the bottle is released, the pressure drops, causing thermal expansion. This, in turn, causes water vapor to condense and fog to form. The same process occurs in the atmosphere.

The higher the altitude, the lower the air pressure becomes.
At the ground, the pressure is approximately 1,000hPa. It decreases to ~100hPa at a height of 15km, ~10hPa at 30km, ~1hPa at 50km, and ~0.01hPa at 80km.

Therefore, when air is lifted by an upward flow, the temperature decreases due to adiabatic expansion, and when air is pushed down by a downward flow, the temperature increases due to adiabatic compression.

This mechanism maintains low temperatures in the summer polar mesosphere and high temperatures at the winter polar stratopause.

Aside to that, during polar night, the lower stratosphere is cold enough, despite the downward flow, to produce clouds known as polar stratospheric clouds.

So why does this general circulation exist?

It exists because atmospheric waves generated in the troposphere propagate upward and deposit momentum in the stratosphere and mesosphere.
This is also called wave drag, because it acts like friction. However, it also acts like "negative friction;" thus, wave forcing is a more appropriate name.
Wave forcing creates this meridional circulation because the earth is almost spherical and spins on its axis; although there are some very interesting physics involved, I will not go into the details here.

Instead, the latitude–height section of the zonal mean zonal wind (Fig. 2) provides an idea of where the wave forcing is effective. The figure shows that the wind is very weak at all latitudes at heights of 20km and 90km. "Weak" refers to the small speed as seen by observers on the ground.

**Figure 2** Latitude–height section of the zonal mean zonal winds in July. Positive values indicate eastward (westerly) winds; negative values indicate westward (easterly) winds (CIRA86). Contour interval is 15m/s.

In other words, the air at these two altitudes is informed of the speed of the ground (the rotation speed of the Earth; 0m/s relative to the ground; and 380m/s eastward near Japan observed from an inertial system).

Hence, the waves generated in the lower atmosphere transmit the speed of the ground to the much higher atmosphere at 20km and 90km. The speed is close to zero, which means that frictional forces are at work.

The wave forcing that maintains the general circulation is currently the subject of intensive studies.

**Figure 3** Stream function of the circulation in the stratosphere in December-January-February (a) calculated directly from the atmospheric reanalysis data; (b) driven by waves (mainly Rossby waves) that are resolved in the model, calculated by the downward control principle (Haynes et al., 1991); (c) driven by waves (mainly gravity waves) that are not resolved, estimated as the difference between (a) and (b) (Okamoto et al., 2011).

Figure 3 shows the results of our research on the flow of stratospheric circulation obtained from atmospheric reanalysis data.

Previously, the stratospheric circulation was considered to be primarily maintained by wave forcing caused by Rossby waves propagating from the troposphere.

However, our analysis shows that the upwelling in the summer hemisphere associated with the stratospheric circulation in the winter hemisphere, which spreads beyond the equator, is induced by gravity waves. It is also suggested that the gravity waves contribute notably to maintaining the circulation in the lowermost stratosphere (around 100hPa) at low latitudes in the winter hemisphere.

The stratospheric zonal wind in the summer hemisphere is westward (see the stratosphere in the Northern Hemisphere in Fig. 2), in which Rossby waves cannot propagate. Therefore, the fact that only gravity waves can induce this upwelling is theoretically plausible.

Moreover, we are studying the structure and mechanisms of the general circulation in the middle atmosphere, as well as polar mesospheric and polar stratospheric clouds, using the first large-scale atmospheric radar in the Antarctic, PANSY radar (Project of the Antarctic Syowa MST/IS radar).