New estimates of tropicalmean temperature trend profiles from zonalmean historical radiosonde and pilot balloon wind shear observations

Abstract

Estimation of global scale trends and low-frequency variability directly from radiosonde temperature records is challenging particularly in the Tropics due to large biases in the earlier parts of the records that need to be adjusted. Zonal mean zonal winds and wind shear observations are less prone to time varying biases. Together with a zonal mean reference temperature trend profile in the extratropics, the wind data can be used to estimate zonal mean temperature trends in the tropics using the thermal wind relationship. Earlier estimates using this approach for the periods 1970-2005 and 1979-2005 showed strong trend amplification in the tropical upper troposphere but also very large uncertainties because the latitude for the reference profile was chosen very far north and due to a lack of data in the tropics. Both sources of uncertainties have been addressed by (i) using reference temperature profiles from homogenized radiosonde temperature data sets, (ii) using the new Global historical Radiosondes and Tracked Balloons Archive on standard pressure levels (GRASP) as data source that includes not only radiosondes but also pilot balloons, and (iii) more accurate vertical discretization and sampling of wind data. The new trend estimates are extended back to 1958 and are considerably less noisy. Trend amplification factors between 1.3 and 2.6 were found in the intervals 1979-2005 and 1958-2010, respectively, when using wind shear information. The best estimates for the warming maxima in the upper tropical troposphere agree well with those directly from temperatures but tend to be located at higher altitudes.