Ozonesonde observations at Christmas Island (2°N, 157°W) in the equatorial central Pacific

[1] Ozone and water vapor observations have been conducted at Christmas/Kiritimati Island (2°N, 157°W) in the equatorial central Pacific as a part of the Soundings of Ozone and Water in the Equatorial Region (SOWER)/Pacific mission. We launched 33 ozonesondes and 33 chilled-mirror hygrometers in nine observation campaigns from 1999 to 2003 for various seasons. We found that ozone concentrations at Christmas Island are low in the whole troposphere (∼10–35 ppbv), particularly in the marine boundary layer (MBL). Ozone variation is small throughout the year compared with other tropical stations, though annual and interannual variations of meteorological fields are large over the equatorial central Pacific. Just below the tropopause, during the August 2002 campaign, we observed substantially reduced ozone concentrations (<10 ppbv) similar to those found in the MBL, which are maintained at least for the observation period. From meteorological conditions, we found that air mass was advected from the Inter-Tropical Convergence Zone (ITCZ), located to the north of Christmas Island, in accordance with the northeasterly wind that is only observed during northern summer in the upper troposphere. The origin of air mass is supposed to be from the MBL in the ITCZ.