“Vertical and Interhemispheric Coupling in the Middle Atmosphere”
Kaoru Sato
Department of Earth and Planetary Science, The University of Tokyo
The neutral atmosphere, which is characterized by a constant mixing ratio, extends toa height of about 100 km above the earth’s surface. The layer above the troposphere,
which is the lowest atmospheric region, is called the middle atmosphere. The bulk of
the middle atmosphere consists of two main layers, the stratosphere and themesosphere, which are distinguished on the basis of temperature stratification. Part ofthe thermal structure of the middle atmosphere is far different from the state ofradiative equilibrium. This peculiar structure is maintained through Lagrangian mean
circulation driven by momentum and heat transport by Rossby waves and gravity
waves. The recent development of satellite observation technology has allowed us toexamine the middle atmosphere, including the whole mesosphere, and several
interesting and spectacular phenomena have been discovered.
This lecture will focus on two striking phenomena initiated by a well-known event
called sudden stratospheric warming (SSW), in which the polar winter temperaturerises by tens of degrees in a few days. One of these resulting phenomena is asignificant warming of the upper mesosphere of the summer hemisphere, whichdevelops almost simultaneously with or slightly after the SSW of the winterhemisphere. The other one is the disappearance and subsequent re-formation of thestratopause at an unusually high level, which sometimes occurs after a strong SSW
event.
Because of a lack of solar radiation, the stratopause, which is defined as the region oftemperature maximum at the top of the stratosphere, does not arise through ozone
heating in the polar winter. Instead, the polar winter stratopause is maintained byadiabatic heating associated with a downward branch of the Lagrangian mean flow inthe mesosphere driven by gravity waves originating mainly from the troposphere. In
contrast, SSW is caused by an enhanced downward branch of the Lagrangian mean
flow generated via the penetration of strong Rossby waves from the troposphere. The

modified temperature field in the stratosphere changes the horizontal wind field in thethermal wind balance, which significantly affects the upward propagation of gravity
waves in the stratosphere. The mesospheric circulation and hence the polar winterstratopause are also modified by the modulated gravity waves. The modification
sometimes extends to the other (i.e., the summer) hemisphere.
The overall scenario has been discussed in consideration of the processes describedabove. However, the gravity waves have such a small spatial scale that observational
evidence of them is hard to obtain. The timings of the elevation of the stratopause andof the interhemispheric coupling are not constant, and the reason is not clear.
Moreover, recent studies indicate that gravity waves do not always behave passively
toward large-scale Rossby waves in the middle atmosphere. The interplay of Rossby
waves and gravity waves may be a clue for developing a quantitative understanding ofthese phenomena.
International observation campaigns have been performed in recent years using
mesosphere–stratosphere–troposphere (MST), meteor, and medium-frequency (MF)radars; lidars; and imagers that can capture the modulation of gravity waves thatoccurs when an Arctic SSW event is initiated. These observational results will beinterpreted using the results of simulations performed with high-resolution generalcirculation models. This is an international project called The Interhemispheric
Coupling Study by Observations and Modelling (ICSOM, PI: KS). The dynamics ofvertical and interhemispheric coupling through the middle atmosphere and the recent
progress on this issue will be reviewed.