Total ozone in the tropics shows temporal variations at various time scales (e.g., intraseasonal, annual, and those associated with the Quasi-Biennial Oscillation (QBO) and El Niño-Southern Oscillation (ENSO)) with a unique zonal structure (the so-called wave-one structure). Both dynamical and photochemical processes need to be considered for a full understanding of tropical ozone. Although satellite ozone measurements had revealed various characteristics of ozone variations, vertically resolved in-situ measurements were needed particularly around the tropical tropopause to understand even the fundamental processes responsible for those variations.

Water vapor plays key roles in the Earth’s radiative balance and in the stratospheric ozone photochemistry. The water vapor transport and dehydration processes around the tropical tropopause are the major factors to determine the stratospheric water vapor, affecting the stratospheric ozone changes. Again, vertically resolved in-situ measurements of water vapor in the tropics were needed.

Figure 1. Stratosphere, troposphere, and the tropical tropopause layer (TTL).

The need for vertically resolved in-situ ozone and water vapor measurements described above was the primary motivation when the Soundings of Ozone and Water in the Equatorial Region (SOWER) was started in 1998 by using ozonesondes and various balloon-borne frostpoint hygrometers (e.g., NOAA FPH, Snow White, CFH, FLASH-B, and FINEDEW). The first two SOWER stations were located at San Cristobal Island in the Galapagos Islands (Ecuador) in the equatorial eastern Pacific and at Christmas Island (Kiribati) in the equatorial central Pacific (Figure 2). Together with the regular ozonesounding station at Watukosek (Indonesia), the early SOWER covered the three key regions in the tropical Pacific Ocean to characterize the east-west contrast in ozone and water vapor in the tropical Pacific.

Later, the SOWER’s scientific focus was placed more on the dehydration processes in the Tropical Tropopause Layer (TTL) over the tropical western Pacific in northern winter season, where and when the tropopause is coldest, creating driest air for the stratosphere. The SOWER stations now include Tarawa (Kiribati), Bandung, Biak, and Kototabang (Indonesia), Hanoi (Vietnam), and various research vessels (Figure 2). The so-called match observation campaigns have been conducted where Lagrangian water vapor changes along air trajectories are directly measured. Also, cirrus cloud measurements are conducted using the Mie-scattering depolarization lidars and various balloon-borne particle sensors simultaneously with the ozone and water vapor measurements.

Figure 2. Location of the SOWER stations during 1998-2014.

The SOWER activities are made possible by strong, long-term support from the local institutes, i.e., the Instituto Nacionál de Meteorología e Hidrología (INAMHI), Ecuador, the Indonesian National Institute of Aeronautics and Space (LAPAN), Indonesia, the National Hydro-Meteorological Service, Ministry of Natural Resources and Environment, Vietnam, and the Kiribati Meteorological Service, Kiribati.