Interhemispheric Coupling Study by Observations and Modeling (ICSOM)
BackgroundRecent 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 and high-resolution model simulations of the observed atmosphere.
- 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 network observation using MST/ST radars
- PANSY radar (MST/IS, 69°00'22"S, 39°35'24"E), Syowa Station in the Antarctic, Japan.
- Summer Mesosphere ST radar (ST, 68°34'36"S, 77°58'03"E), Davis Station in the Antarctic, Australia.
- Jicamarca radar (MST/IS, 11.95°S, 76.78°W), Lima, Peru.
- Equatorial atmosphere radar (ST, 00°12'S, 100°19'E), West Smatra, Indonesia/Japan.
- MU radar (MST/IS, 34°51'18"N, 136°06'19"E), Shiga, Japan.
- MAARSY radar (MST/IS, 69°18'N, 16°02'E), Andoya, Norway/Germany.
- EISCAT radar (IS, 69°35'N, 19°02'E), Tromso, Norway.
- EISCAT Svarbard radar (IS, 78°09'N, 16°03'E), Longyearbyen, Svalbard, Norway.
Observations which are complementary to those made by the MST/IS radar network in terms of physical quantities, spectral range of gravity waves, and height coverages are needed:
- MF radars
- Meteor radars
- Imagers and interferometers
- Advanced Mesospheric Temperature Mapper (AMTM)
High-resolution model simulations
- High-top NICAM focusing on high-latitude SH
- Low-top but global NICAM
Plan of observations and simulationsThe first research target of ICSOM is the interhemispheric coupling initiated from a sudden stratospheric warming (SSW) in the Arctic. The observation window is 15-31 January, 2016. Statistics indicate that SSWs, extreme events in the stratosphere, occur in winter twice every three years in the Arctic on average. Roughly speaking, the prediction of an SSW is possible five days before the onset. Following possible SSW prediction made each day from 10-20 January, 2016, MST radar observations will start at all sites. Observations will be necessary over at least ten successive days starting three days before the SSW onset, which allows for the time lag of the Antarctic atmospheric response to the Arctic SSW. Even in the case of no SSW during the observation window, simultaneous observations will still be beneficial: If another extreme event of vortex intensification (VI) is predicted, we will perform coordinated observations in a similar way. If normal conditions of the polar vortex are expected, we will perform continuous observations over about seven days to obtain reference data which can be compared with future successful SSW observations. Simultaneous observations for the troposphere and lower stratosphere in addition to mesospheric observations will also be important so as to distinguish the modulation of gravity wave characteristics by the SSW from that originating from the gravity wave source variation. Measurements and analysis of the state of potential modulators of gravity wave activity, such as planetary waves and tides, Quasi-Biennial Oscillation (QBO) and Semi-Annual Oscillation (SAO) will also be made.
Complementary observations utilizing MF and meteor radars, lidars, imagers, interferometers and radiosondes will also be performed. High resolution satellite observations covering the whole stratosphere are included for the analysis as well.
Simulation of high-resolution general circulation models covering the MST region will be made using super computers. Initial conditions for the model simulation will be made based on global analysis data covering the mesosphere such as MERRA. Regional models or models having a stretched grid configuration will also be used for simulations with finer resolutions. Mechanistic model studies will also be made to deepen our understanding of inherent mechanisms suggested by these observation and high-resolution model studies.