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1. Introduction

Total ozone variations in the equatorial region are characterized by the annual cycle, quasi-biennial oscillation (QBO), and those related to El Niño/Southern Oscillation (ENSO) (<References.html#shiotani1992>Shiotani, 1992). The annual maxima take place around August-September period, while the minima are observed in Northern winter. The zonal structure is known to show persistent wave one pattern with the phase of minima located around the date line. The QBO is characterized by zonally uniform variations, while ENSO component has both zonally symmetric and zonal wave components (<References.html#shiotani1994>Shiotani and Hasebe, 1994). Due to lack of observational data, however, the contribution of the tropospheric ozone to these features and those variations at the vicinity of tropopause are not yet well described, making it hard to understand even the fundamental processes responsible for those variations.

Tropical Pacific is characterized by east-west contrast both in dynamical and chemical point of view. Prevailing easterly wind near surface brings about marked contrast in the sea surface temperature (SST) distributions along the equator. In the western Pacific where SST is high, the tropopause is higher and colder than any other places in the world and the ozone poor surface air could be quickly pumped up to the upper troposphere by active convections. Release of nitrogen oxides from biomass burning is intensified during El Niño leading to an enhancement of the ozone concentration in the troposphere. On the other hand, the cold upwelling in the eastern Pacific provides ideal condition to nurture marine algae and biologically produced methyl halide may effectively destroy surface ozone while air in the marine boundary layer is trapped close to the surface due to strong temperature inversion over the cold sea.

The Soundings of Ozone and Water in the Equatorial Region/Pacific Mission (SOWER/Pacific) is intended to improve our knowledge on the ozone and water vapor distributions by making coordinated radio sonde observations in the Pacific region along the equator (SOWER Project, 1998). What is unique in SOWER/Pacific is that it is trying to make organized ozone and water vapor sonde observations together with GPS sonde soundings simultaneously at stations in the eastern, central, and western tropical Pacific.

Figure 1 shows the location of the three SOWER/Pacific stations, namely San Cristóbal, Galápagos in Ecuador (0.90ºS, 89.62ºW), Christmas Island of Republic of Kiribati (2.00ºN, 157.38ºW), and Watukosek in Indonesia (7.57ºS, 112.63ºE). The operational rawin sonde data at Singapore (1.37ºN, 103.98ºE) are also used. Due to limited fund available, the observations have been started as a basis of campaigns, the first of which was realized in March 1998 (Hasebe et al., 1998; 1999). Since then, roughly two-week long campaigns have been conducted repeatedly in the period of February-March and September-October to capture the atmospheric behavior during local rainy/dry seasons and strong/weak pumping from the extratropical stratosphere. Efforts have been made to fill the spatial gaps between these stations by launching sondes from a cruising research vessel (Shiotani et al., 2000a).

In addition to establishing the climatology and variabilities in ozone and water vapor, it is also intended to explore the atmospheric processes related to the troposphere-stratosphere exchange. Collection of correlative data for satellite validation is also within the scope of SOWER/Pacific.

This paper briefly describes an overview of the SOWER/Pacific 1998-2000 campaigns. During this period, ENSO status changed quickly from the final phase of the intense El Niño in March 1998 to La Niña in September 1998 and stayed in this side with short period of neutral condition. Some of the issues could be found also in Hasebe et al. (2000), Fujiwara and Takahashi (2000), Oltmans et al. (2000), Shiotani et al. (2000b), and Vömel et al. (2000).

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