Abstract
Abstract. This paper mainly focuses on the trends and variability of the UT-LS temperature using radiosonde observations carried out over 16 years (January 1993 to December 2008) from a southern subtropical site, Reunion (20.8° S, 55.5° E), using a linear-regression fitting model. Two kinds of tropopause definitions, namely, cold point tropopause (CPT) and lapse rate tropopause (LRT) are used. In order to characterize and quantify the relationship between regional oceanic forcing and temperature at UT-LS, we took into account the Indian Ocean Dipole (IOD) for the estimation of temperature trends. Results show that the main component is the Annual Cycle (AC), particularly at tropopause (CPT, LRT) and in the lower stratosphere (LS) where more than 26.0±2.4% of temperature variability can be explained by AC. The influence of IOD on the variability of the temperature is at highest ratio at CPT and LS, with respectively 12.3±7.3% and 13.1±5.9%. The correlations between IOD and temperature anomalies at UT-LS are barely significant, which are found to be in close agreement with the results obtained by Rosenlof et al. (2008) over the western tropical Pacific Ocean. The temperature trend in the LS reveals a cooling of about −0.90±0.40 K per decade. The cooling trend at LS is found to be in close agreement with the others studies. Trend estimates in the LS suggest that IOD forcing contributes to increasing cooling by about 0.16±0.05 K per decade. Past works have shown that the additional carbon dioxide increase has a minor effect in the LS, and suggested that other effects than ozone and carbon dioxide changes have to be considered, in order to explain the observed temperature changes in the LS. From this study, we can suggest that the SST changes can be considered also, in addition to effects due to ozone and carbon dioxide changes, in order to explain the observed temperature changes in the LS. As a consequence, our results support the assumption that the Indian Ocean may have a slight impact on temperature variability and on temperature change in the LS over Reunion.
Highlights
Troposphere-lower stratosphere temperature in the Earth’s atmosphere plays an important role in radiation budget and understanding of various chemical elements (Rosenlof et al, 2001; Bethan et al, 1996; Pan et al, 2004)
This paper deals with trends and variability of the UTLS temperatures over a southern subtropical site
The study mainly focuses on analysis of temperature profiles derived from balloon-sonde experiments at Reunion (20.8◦ S, 55.5◦ E) during the January 1993-December 2008 period
Summary
Troposphere-lower stratosphere temperature in the Earth’s atmosphere plays an important role in radiation budget and understanding of various chemical elements (Rosenlof et al, 2001; Bethan et al, 1996; Pan et al, 2004). Over the southern tropics, a study by Bencherif et al (2006) showed a significant cooling in the lower stratosphere They used 22-years of observational upper-air data recorded from 1980 to 2001 over Durban (30.0◦ S, 30.9◦ E), a South African site, and found a cooling of about −1.09±0.41 K per decade in the lower stratosphere. A tropospheric ozone climatology and trend study over Reunion was carried out by Clain et al (2009) using this radiosonde-ozonesonde dataset. They found a positive ozone trend (1.31±0.62 DU per decade) in the upper troposphere (10–16 km), between late 1992 and 2008.
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