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Temperature changes in the lower and middle stratosphere during 2001-2016 are evaluated using measurements from GPS Radio Occultation (RO) and Advanced Microwave Sounding Unit (AMSU) aboard the Aqua satellite. After downsampling of GPS-RO profiles according to the AMSU weighting functions, the spatially and seasonally resolved trends from the two data sets are in excellent agreement. The observations indicate that the middle stratosphere has cooled in the time period 2002-2016 at an average rate of -0.14 +/- 0.12 to -0.36 +/- 0.14 K/decade, while no significant change was found in the lower stratosphere. The meridionally and vertically resolved trends from high-resolution GPS-RO data exhibit a marked interhemispheric asymmetry and highlight a distinct boundary between tropospheric and stratospheric temperature change regimes matching the tropical thermal tropopause. The seasonal pattern of trend reveals significant opposite-sign structures at high and low latitudes, providing indication of seasonally varying change in stratospheric circulation.

Plain Language Summary Stratosphere is an important part of the Earth climate system. Long-term change in stratospheric temperature is a key indicator of global climate change, reflecting both natural and anthropogenic forcings. A variety of long-term observations and models indicate a cooling throughout the stratosphere in response to ozone depletion and growing emissions of greenhouse gases. This study exploits two independent sets of satellite observations by Advanced Microwave Sounding Unit (AMSU) and GPS radio occultation (RO). We find excellent agreement between spatially and seasonally resolved trends from the two data sets, which provides confidence in our estimates. The observations indicate a statistically significant global cooling in the middle stratosphere since 2001 at a mean rate of -0.14 to -0.36 K/decade and insignificant change in the lower stratosphere. The seasonal and spatial patterns of trend provide indication of seasonally varying change in stratospheric circulation. We argue that GPS-RO technique, featuring certain advantages over other atmospheric observing systems, provides a more detailed view of the stratospheric temperature change thereby allowing a better understanding of the underlying forcing mechanisms. Future RO missions will continue the existing record, potentially becoming the primary source of information on the lower stratospheric temperature in the 21st century.