Radiative forcing and temperature responses associated with abrupt drops in water vapor and ozone in the tropical tropopause layer

The chemical composition of the tropical tropopause layer (TTL) determines its radiative budget, and is simultaneously controlled by temperatures and dynamics in balance with this radiation. In 2001 an “abrupt drop” in TTL temperatures and water vapor mixing ratios was observed. A previous study showed that this ~20% reduction in water vapor led to negative radiative forcing nearly half the magnitude of the positive radiative forcing contributed by carbon dioxide growth from 1996 to 2005. Thus such an abrupt drop in water vapor can have large transient impacts on surface climate. In 2011, another abrupt drop in TTL temperature and water vapor has been observed and was correlated with reductions in TTL ozone. This study uses the Microwave Limb Sounder (MLS) and the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) datasets along with the Community Atmosphere Model's offline radiative driver to study the characteristics and radiative impacts of these abrupt drops in TTL ozone and water vapor. We examine the spatial structure of the 2001 and 2011 abrupt drops and show that reductions in water vapor are more spatially homogenous than reductions in ozone. Radiative forcing calculations, using the fixed-dynamical heating (FDH) assumption, show that the 2011 TTL water vapor reductions are associated with global negative (cooling) radiative forcing, similar to the 2001 abrupt drop; the effect on climate will depend on how long reductions are sustained. Radiative forcing magnitudes associated with the abrupt drops in ozone are also negative, but smaller and more localized due to ozone's higher spatial variability. The negative radiative forcing from TTL water vapor and ozone reductions could be, in part, related to the recently observed “slow down” in the rate of global warming. In addition, calculated temperature responses using FDH show that the tropical upper troposphere radiatively cools due to reductions in both TTL ozone and water vapor.