Ozonesonde Observations in the Equatorial Eastern Pacific. The Shoyo-Maru Survey.

Abstract

We have conducted GPS radiosonde and ozonesonde observations on board the research vessel ''Shoyo-Maru'' in the equatorial eastern Pacific. These observations took place in September and October 1999 as a part of the Soundings of Ozone and Water in the Equatorial Region (SOWER)/Pacific mission. The mean profile of ozone is similar to that for the dry season (September to October) at San Cristóbal, Galá pagos (0.9 S, 89.6 W) located in the equatorial eastern Pacific. The mean tropospheric ozone concentration is about 40 ppbv with a maximum in the mid-troposphere. Compared with the mean profile during the dry season at Watukosek, Indonesia (7.5 S, 112.6 E), this mid-tropospheric maximum is larger, and a sharp increase of ozone below the tropopause begins at a lower altitude for the Shoyo-Maru profile. We frequently observed layers in which ozone and humidity are highly anti-correlated. These layers have vertical scales from several kilometers to several hundred meters. Horizontal scales of these layers are roughly 1000 km, which may correspond to time scales of about 2 days, since the vessel sailed about 500 km/day. These layers are related to northerly winds, which bring in wet and ozone-poor air from the inter-tropical convergence zone situated in the northern side of the main cruise track. Similar layers were observed in ozone profiles at San Cristóbal and Watukosek, mostly during the dry season, suggesting the existence of layers advected without vertical mixing.

Numerical Experiments on the Layered Structures in the Mid-Troposphere over the Equatorial Pacific

“…The experiments also showed that a large part of the air in the wet layer comes from the marine boundary layer, lifted by convections. These results support the speculation by Shiotani et al (2002) that northerly winds bring in wet air from the ITCZ.…”

“…On the other hand, the particles in the dry parts above and below the layered structure come from southeast almost horizontally. These results are consistent with the observational fact that wet parts of the layered structures corresponded to northerly winds, and give a support to the suggestion by Shiotani et al (2002) that wet air came from the ITCZ by horizontal advection.…”

“…Our result shows similar longitudinal dependence in relative humidity to the Shoyo-Maru observations shown in Fig. 7 of Shiotani et al (2002) and Fig. 3a of Fujiwara et al (2003).…”

“…In 1999, ozonesonde observations were carried out in the equatorial eastern Pacific on board the research vessel "Shoyo-Maru" as a part of Soundings of Ozone and Water in the Equatorial Region (SOWER)/Pacific mission (Shiotani et al 2002). The ship sailed eastward along 2°N from 140°W on September 21st to 110°W on September 28th, and sailed northeastward to 95°W and 10°N on October 7th (a green line in Fig.…”

Stratospheric ozone climatology and variability over a southern subtropical site: Reunion Island (21° S; 55° E)

“…most likely caused by biomass burning. Their results are in accordance with the observations made over the equatorial eastern pacific, Cristobal, Galapagos (0.9 • S; 89.6 • W) by Shiotani et al (2002). A review by Staehelin et al (2001) addressed that the satellite and ground-based measurements show no significant ozone trends in the tropics but significant long-term negative trends of 2-4 % in the northern and southern mid-latitudes.…”