Interhemispheric Coupling Study by Observations and Modeling (ICSOM)


Recent observational and modelling studies suggest that the Northern and Southern Hemispheres of the earth atmosphere are potentially coupled by the Lagrangian mean flow in the mesosphere modulated by waves interacting with the mean flow. However, observations of modulated wave and flow fields which are needed for quantitative understanding of the interhemispheric coupling are not sufficient. Simultaneous observations of gravity waves at various locations are most important because they are a main driver of the Lagrangian mean flow in the mesosphere.

With the start of full system observation by the PANSY radar in the Antarctic in March 2015, a global mesosphere-stratosphere-troposphere (MST) radar network extending from the Arctic to the Antarctic has been realized. The MST radars are able to observe wind vectors with fine temporal and vertical resolutions including vertical wind components in the troposphere, stratosphere and mesosphere, although an observational gap of the middle and upper stratosphere remains. Thus, the characteristics of small-scale or short-period wave motions including gravity waves and the momentum fluxes associated with these waves can be estimated with a good accuracy.

In addition, recent high-resolution general circulation models enable an explicit simulation of gravity waves under ideal and/or climatological boundary conditions and allow us to examine the momentum budget in the MST region including gravity waves, although their resolution is currently not sufficient to resolve the entire gravity wave spectrum. Real atmosphere simulations utilizing such high-resolution models are still a challenge for the MST region. However, if such real atmosphere simulations are successful, they will help quantitative interpretation of the dynamical fields observed by the MST radar network, and the observations will provide invaluable validation data for the model improvement.

Therefore it is proposed to examine the interhemispheric coupling of the earth atmosphere through a combination of simultaneous observations by networking the MST radars over the world, complementary radio and optical instruments, and high-resolution model simulations of the observed atmosphere.

Science Targets

  • How are the mean wind (in particular, the meridional component) and temperature at respective sites modulated by the SSW?
  • How are gravity wave characteristics at respective sites modulated by the SSW?
  • How do the QBO and/or SAO at the time of the SSW affect the interhemispheric coupling by modulating equatorial gravity waves?
  • Is the latitudinal variation of the modulated mean fields and wave fields consistent with the theoretical expectation?
  • Are there any longitudinal variations of the modulated mean and wave fields?
  • Are high-resolution models able to successfully simulate variations of mean and wave (perturbation) fields observed at the respective ground-based observing sites? If so, how are the three dimensional structures of mean flow and temperature fields, and wave characteristics represented in these models? What dynamical processes cause such structures?


A detailed paticipant list is available from here.

Future plans

Several more simultaneous observations will elucidate the robust nature of interhemispheric coupling through the mesosphere. This includes an extension of the observation network and performing coordinated observations during SSWs and during a stable Arctic vortex. Observations in austral winter will also be interesting to examine the difference between the two hemispheres.

Program Director

Kaoru Sato, Professor (The University of Tokyo, Japan.)