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Coupling Processes in the Equatorial Atmosphere (CPEA): a Project Overview Journal of the Meteorological Society of Japan, Vol. 84A, pp. 1--18, 2006 1 Coupling Processes in the Equatorial Atmosphere (CPEA): A Project Overview Shoichiro FUKAO Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, Japan (Manuscript received 18 November 2005, in final form 28 February 2006) Abstract The global-scale atmospheric motions are originally generated by the strongest convective motions in the equatorial region caused by absorption of strong solar radiations. Specifically, the western Pacific re- gion called the Indonesian Archipelago is known for its convections, which are the most active and the highest all over the globe due to its warmest ocean water. Therefore, through these global-scale motions, the atmospheric dynamics over the Indonesian equator result in the most significant influences to global atmospheric changes. The mechanisms of these atmospheric changes and fluctuations, however, have not yet been made clear due to the sparseness of observational data in that region. The Coupling Pro- cesses in the Equatorial Atmosphere (CPEA) is a six-year research project of Japan to study dynamical and electrodynamical coupling processes in the equatorial atmosphere by conducting various observa- tions in the Indonesian equatorial region. In the present paper we describe the outline of this project and show preliminary results from its first campaign conducted from March to May 2004. 1. Introduction ward. The energy and momentum of these The equatorial region is the source of many waves are transported upward and deposited unique atmospheric processes that couple the at high altitudes. They interact with each other entire atmosphere vertically from bottom to or with the background flow at higher altitudes top and horizontally from equator to pole. De- and generate typical global-scale oscillations, spite its importance for global change, the such as quasi-biennial oscillation (QBO) (e.g., equatorial atmosphere is not well studied and Baldwin et al. 2001) and semi-annual oscilla- the coupling processes are poorly understood. tion (SAO) (e.g., Garcia et al. 1997). These Due to the equatorial region’s having the maxi- interactions are quite important because it is mum amount of solar radiation over the globe, the small-scale processes of atmospheric waves cumulus convection is most active there, gener- generated in the lower atmosphere that control ating a variety of atmospheric waves, such as the large-scale process of global circulation in the equatorial waves, atmospheric tides, and grav- middle atmosphere (Garcia and Solomon 1985). ity waves (e.g., Andrews et al. 1987). The mini- Electrodynamics shows unique features above mal Coriolis force effect present near the equa- 100 km altitude in the equatorial ionosphere, tor allows the widest range of periods for their where geomagnetic field lines are parallel to propagation over the globe. Their amplitudes the Earth’s surface (or perpendicular to the grow with altitude while they propagate up- Earth’s gravitational force) (e.g., Kelley 1989). Mesoscale atmospheric gravity waves exist at Corresponding author: Shoichiro Fukao, Research high altitudes and are believed to seed the Institute for Sustainable Humanosphere (RISH), plasma instability that generates equatorial Kyoto University, Gakasho, Uji, Kyoto 611-0011, plasma bubbles in the nighttime F region iono- Japan. E-mail: [email protected] sphere (e.g., Kelley et al. 1981; Hysell et al. ( 2006, Meteorological Society of Japan 1990). Thus, various dynamical and electrody- 2 Journal of the Meteorological Society of Japan Vol. 84A namical processes in the equatorial atmosphere observations were conducted with the deploy- are vertically coupled via convection-induced ment of Doppler radars (ship, land, air) in the atmospheric waves. A research project called Indonesian Archipelago (Webster and Lukas Coupling Processes in the Equatorial Atmo- 1992). Meanwhile endeavors have been made sphere (CPEA) has been initiated in Japan to for establishing core observatories and/or facili- understand the basic processes of the vertical ties in the equatorial region to integrate these coupling on various spatial and temporal scales investigations (e.g., Gage et al. 1991; Tsuda occurring in the equatorial low, middle, and et al. 1995). For instance, the Equatorial Atmo- upper atmosphere and ionosphere. CPEA is sphere Radar (EAR) was installed right at the funded as a Grant-in-Aid for Scientific Re- equator in Kototabang, West Sumatra, Indone- search on Priority Areas by the Ministry of sia (0.20S, 100.32E) in March 2001 (Fukao Education, Culture, Sports, Science and Tech- et al. 2003a). It started observations in July nology (MEXT) of Japan during the six-year pe- 2001 in collaboration between the Radio riod September 2001 to March 2007. Science Center for Space and Atmosphere (now In the equatorial region, cumulus convection Research Institute for Sustainable Humano- is a significant source of atmospheric gravity sphere), Kyoto University of Japan and the waves (e.g., Pfister et al. 1993; Alexander et al. National Institute of Aeronautics and Space 1995; Salby and Garcia 1987; Ricciardulli and (LAPAN) of Indonesia. CPEA is based on these Garcia 2000). It is well known from OLR (Out- developments, with the EAR site serving as a going Long Wave Radiation) satellite data that focus for its activities, and will encourage an the tallest clouds (deepest convection) appear extension of the network to incorporate as over the Indonesian Archipelago (or the Indo- many techniques as possible to ensure that nesian maritime continent) (e.g., Nishida et al. the equatorial atmosphere is characterized as 2000). The activity of mesoscale gravity waves broadly as possible. with vertical wavelengths shorter than 10 km 2. Scientific background of CPEA is estimated from potential energy per unit mass at 20–30 km altitude (Tsuda et al. 2000). 2.1 Dynamical coupling in the equatorial It is particularly enhanced over the Indonesian atmosphere Archipelago, where the cumulonimbus clouds The equatorial region features oscillations are tallest. The good correlation between these having temporal scales of about four years two types of data well demonstrates that atmo- (tropospheric El Nino-Southern Oscillation spheric waves are most significantly generated (ENSO)), about two years (stratospheric QBO), by convection there. Therefore, the Indonesian a half year (SAO in the middle atmosphere), Archipelago was selected as the main site for and shorter periods (tropospheric ISO and CPEA. equatorial waves) (e.g., Baldwin et al. 2001; The Scientific Committee on Solar- Garcia et al. 1997). Recently, with rapid prog- Terrestrial Physics (SCOSTEP) has sponsored ress in observational techniques, signatures of a series of previous international scientific pro- ENSO, QBO, etc. have been observed in the grams, among them the Solar Terrestrial En- mesosphere and lower thermosphere (MLT) ergy Program (STEP; 1990–97), and Energy too (e.g., Baldwin et al. 2001). Although theo- Processes Including Coupling (EPIC; 1998– retical principles for wave-wave and wave- 2002) was sponsored as one of four post-STEP mean flow interactions have been established, mini projects. The TOGA /COARE (Tropical identification of the actual mechanisms that Ocean Global Atmospheres/Coupled Ocean At- produce the oscillations seen is still controver- mosphere Response Experiment) was an inter- sial (Dunkerton 1997; Fritts and Alexander national research program promoted by the 2003). These mechanisms must be investigated World Climate Research Programme (WCRP) in detail by continuous observation with radio- that studied the interaction of the ocean and sondes and various ground-based instruments, atmosphere in the western Pacific. Several sci- including radars and lidars. The ten-to-twelve entific groups pioneered observational and the- year variability of the atmosphere and its rela- oretical investigations of coupling in the equa- tionship with solar activity (e.g., Kodera and torial atmosphere. Among them, extensive field Kuroda 2002) will be studied on the basis of July 2006 S. FUKAO 3 the maintenance of the observational network atmosphere? What are the principal wave established in the CPEA period. sources in the equatorial region? What are There are two transition regions in the the main dissipation mechanisms through Earth’s atmosphere where atmospheric proper- which atmospheric waves interact with ties show significant change across regional the mean flow, wave-breaking shear- boundaries; one is the tropopause and the other instabilities, inertial instabilities, or radia- is the mesopause-homopause region. Varieties tive cooling? of equatorial dynamics (such as tides and atmo- 2. Is development of cloud clusters associated spheric gravity waves) bring their signatures to with convective motion similar in the Indian upper atmospheric phenomena such as airglow Ocean and Indonesian Archipelago? and the equatorial electrojet (EEJ), beyond the 3. Which atmospheric waves drive the QBO mesopause-homopause region (see e.g., Bur- and SAO, and how do they affect atmo- rage et al. 1995 for tidal-origin airglow; Taylor spheric structure in general? How do they et al. 1987 for gravity-wave-origin airglow; Gel- affect meridional and zonal circulation? inas et al. 2002 for EEJ). Momentum transport 4. How do extratropical oscillations and plane- by equatorial waves appears to be particularly tary waves affect the equatorial lower, mid- important in producing the QBO and SAO in dle, and upper
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