Transport of ice into the stratosphere and the humidification of the stratosphere over the 21st century

Stratospheric water vapor affecting atmospheric circulation

Water vapor plays an important role in many aspects of the climate system, by affecting radiation, cloud formation, atmospheric chemistry and dynamics. Even the low stratospheric water vapor content provides an important climate feedback, but current climate models show a substantial moist bias in the lowermost stratosphere. Here we report crucial sensitivity of the atmospheric circulation in the stratosphere and troposphere to the abundance of water vapor in the lowermost stratosphere. We show from a mechanistic climate model experiment and inter-model variability that lowermost stratospheric water vapor decreases local temperatures, and thereby causes an upward and poleward shift of subtropical jets, a strengthening of the stratospheric circulation, a poleward shift of the tropospheric eddy-driven jet and regional climate impacts. The mechanistic model experiment in combination with atmospheric observations further shows that the prevailing moist bias in current models is likely caused by the transport scheme, and can be alleviated by employing a less diffusive Lagrangian scheme. The related effects on atmospheric circulation are of similar magnitude as climate change effects. Hence, lowermost stratospheric water vapor exerts a first order effect on atmospheric circulation and improving its representation in models offers promising prospects for future research.

Response of stratospheric water vapour to warming constrained by satellite observations

Future increases in stratospheric water vapour risk amplifying climate change and slowing down the recovery of the ozone layer. However, state-of-the-art climate models strongly disagree on the magnitude of these increases under global warming. Uncertainty primarily arises from the complex processes leading to dehydration of air during its tropical ascent into the stratosphere. Here we derive an observational constraint on this longstanding uncertainty. We use a statistical-learning approach to infer historical co-variations between the atmospheric temperature structure and tropical lower stratospheric water vapour concentrations. For climate models, we demonstrate that these historically constrained relationships are highly predictive of the water vapour response to increased atmospheric carbon dioxide. We obtain an observationally constrained range for stratospheric water vapour changes per degree of global warming of 0.31 ± 0.39 ppmv K-1. Across 61 climate models, we find that a large fraction of future model projections are inconsistent with observational evidence. In particular, frequently projected strong increases (>1 ppmv K-1) are highly unlikely. Our constraint represents a 50% decrease in the 95th percentile of the climate model uncertainty distribution, which has implications for surface warming, ozone recovery and the tropospheric circulation response under climate change.

Quantitative detection of iodine in the stratosphere

Oceanic emissions of iodine destroy ozone, modify oxidative capacity, and can form new particles in the troposphere. However, the impact of iodine in the stratosphere is highly uncertain due to the lack of previous quantitative measurements. Here, we report quantitative measurements of iodine monoxide radicals and particulate iodine (I_y,part) from aircraft in the stratosphere. These measurements support that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (I_y) is injected to the stratosphere. These high I_y amounts are indicative of active iodine recycling on ice in the upper troposphere (UT), support the upper end of recent I_y estimates (0 to 0.8 pptv) by the World Meteorological Organization, and are incompatible with zero stratospheric iodine injection. Gas-phase iodine (I_y,gas) in the UT (0.67 ± 0.09 pptv) converts to I_y,part sharply near the tropopause. In the stratosphere, IO radicals remain detectable (0.06 ± 0.03 pptv), indicating persistent I_y,part recycling back to I_y,gas as a result of active multiphase chemistry. At the observed levels, iodine is responsible for 32% of the halogen-induced ozone loss (bromine 40%, chlorine 28%), due primarily to previously unconsidered heterogeneous chemistry. Anthropogenic (pollution) ozone has increased iodine emissions since preindustrial times (ca. factor of 3 since 1950) and could be partly responsible for the continued decrease of ozone in the lower stratosphere. Increasing iodine emissions have implications for ozone radiative forcing and possibly new particle formation near the tropopause.

Water Vapor, Clouds, and Saturation in the Tropical Tropopause Layer

The goal of this investigation is to understand the mechanism behind the observed high relative humidity with respect to ice (RHi) in the tropical region between ~14 km (150 hPa) and the tropopause, often referred to as the tropical tropopause layer (TTL). As shown by satellite, aircraft and balloon observations, high (>80%) RHi regions are widespread within the TTL. Regions with the highest RHi are co-located with extensive cirrus. During boreal winter, the TTL RHi is highest over the Tropical Western Pacific (TWP) with a weaker maximum over South America and Africa. In the winter, TTL temperatures are coldest and upward motion is the greatest in the TWP. It is this upward motion, driving humid air into the colder upper troposphere that produces the persistent high RHi and cirrus formation. Back trajectory calculations show that comparable adiabatic and diabatic processes contribute to this upward motion. We construct a bulk model of TWP TTL water vapor transport that includes cloud nucleation and ice microphysics that quantifies how upward motion drives the persistent high RHi in the TTL region. We find that atmospheric waves triggering cloud formation regulate the RHi, and that convection dehydrates the TTL. Our forward domain-filling trajectory (FDF) model is used to more precisely simulate the TTL spatial and vertical distribution of RHi. The observed RHi distribution is reproduced by the model and we show that convection increases RHi below the base of the TTL with little impact on the RHi in the TTL region.