Understanding and predicting recent climate changes due to human activities requires reliable, climate quality observations. One of the best records of atmospheric temperatures comes from nearly 3 decades of satellite Microwave Sounding Unit (MSU) data starting in 1979. A new retrieval algorithm for tropospheric temperatures was developed to remove the influence of stratospheric cooling from the MSU mid-tropospheric channel. The performance of this retrieval algorithm is evaluated by calculating residual stratospheric influence from auxiliary datasets. Small uncertainty is found, indicating that it successfully represents variations in tropospheric temperatures.

MSU data have global coverage and are advantageous for temperature trend analysis in regions with sparse upper air observations. For example, few records exist of Antarctic atmospheric temperatures. Our analysis of the MSU data indicates that the Antarctic troposphere cooled in the summer and fall seasons since 1979, in agreement with previous findings. We also find that winter tropospheric warming was not as large or as homogeneous as previously reported. While MSU stratospheric temperatures exhibit the expected pronounced cooling during the spring and summer seasons associated with ozone depletion, we also identified large, unexpected regions of strong polar stratospheric warming in winter and spring.

An examination of global MSU temperature trends revealed changes in meridional temperature gradients, which are suggestive of Hadley cell widening. The MSU and other observations indicate a widening of ∼2–5° over both hemispheres since 1979. General circulation model (GCM) simulations show that the Hadley cell may widen in response to global warming, warming of the west Pacific, or polar stratospheric cooling. We assess Hadley cell widening in current GCMs from historical simulations of the 20^th century as well as future climate projections and pre-industrial control runs. We find that the observed widening cannot be interpreted as natural variability. However, model simulations of 20th century climate are unable to reproduce the observations and are therefore unable to explain them. Widening is found in projections of 21^st century climate but at a smaller rate than the observations. The results illustrate the need for further investigation of the discrepancy between observed and simulated widening of the Hadley cell.