Amanda C. Maycock

A large ozone-circulation feedback and its implications for global warming assessments

State-of-the-art climate models now include more climate processes which are simulated at higher spatial resolution than ever1. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in climate simulations1,2. Here we present evidence that how stratospheric ozone is represented in climate models can have a first order impact on estimates of effective climate sensitivity. Using a comprehensive atmosphere-ocean chemistry-climate model, we find an increase in global mean surface warming of around 1°C (~20%) after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4×CO2 forcing. The difference is primarily attributed to changes in longwave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies1,2 in which participating models often use simplified treatments of atmospheric composition changes that are neither consistent with the specified greenhouse gas forcing scenario nor with the associated atmospheric circulation feedbacks3-5.

Regional climate impacts of a possible future grand solar minimum

Any reduction in global mean near-surface temperature due to a future decline in solar activity is likely to be a small fraction of projected anthropogenic warming. However, variability in ultraviolet solar irradiance is linked to modulation of the Arctic and North Atlantic Oscillations, suggesting the potential for larger regional surface climate effects. Here, we explore possible impacts through two experiments designed to bracket uncertainty in ultraviolet irradiance in a scenario in which future solar activity decreases to Maunder Minimum-like conditions by 2050. Both experiments show regional structure in the wintertime response, resembling the North Atlantic Oscillation, with enhanced relative cooling over northern Eurasia and the eastern United States. For a high-end decline in solar ultraviolet irradiance, the impact on winter northern European surface temperatures over the late twenty-first century could be a significant fraction of the difference in climate change between plausible AR5 scenarios of greenhouse gas concentrations.

The Circulation Response to Idealized Changes in Stratospheric Water Vapor

AbstractObservations show that stratospheric water vapor (SWV) concentrations increased by ~30% between 1980 and 2000. SWV has also been projected to increase by up to a factor of 2 over the twenty-first century. Trends in SWV impact stratospheric temperatures, which may lead to changes in the stratospheric circulation. Perturbations in temperature and wind in the stratosphere have been shown to influence the extratropical tropospheric circulation. This study investigates the response to a uniform doubling in SWV from 3 to 6 ppmv in a comprehensive stratosphere-resolving atmospheric GCM. The increase in SWV causes stratospheric cooling with a maximum amplitude of 5–6 K in the polar lower stratosphere and 2–3 K in the tropical lower stratosphere. The zonal wind on the upper flanks of the subtropical jets is more westerly by up to ~5 m s−1. Changes in resolved wave drag in the stratosphere result in an increase in the strength of tropical upwelling associated with the Brewer–Dobson circulation of ~10% throu...

Processes Controlling Tropical Tropopause Temperature and Stratospheric Water Vapor in Climate Models

A warm bias in tropical tropopause temperature is found in the Met Office Unified Model (MetUM), in common with most models from phase 5 of CMIP (CMIP5). Key dynamical, microphysical, and radiative processes influencing the tropical tropopause temperature and lower-stratospheric water vapor concentrations in climate models are investigated using the MetUM. A series of sensitivity experiments are run to separate the effects of vertical advection, ice optical and microphysical properties, convection, cirrus clouds, and atmospheric composition on simulated tropopause temperature and lower-stratospheric water vapor concentrations in the tropics. The numerical accuracy of the vertical advection, determined in the MetUM by the choice of interpolation and conservation schemes used, is found to be particularly important. Microphysical and radiative processes are found to influence stratospheric water vapor both through modifying the tropical tropopause temperature and through modifying upper-tropospheric water vapor concentrations, allowing more water vapor to be advected into the stratosphere. The representation of any of the processes discussed can act to significantly reduce biases in tropical tropopause temperature and stratospheric water vapor in a physical way, thereby improving climate simulations.

Recommendations for diagnosing effective radiative forcing from climate models for CMIP6

The usefulness of previous Coupled Model Intercomparison Project (CMIP) exercises has been hampered by a lack of radiative forcing information. This has made it difficult to understand reasons for differences between model responses. Effective radiative forcing (ERF) is easier to diagnose than traditional radiative forcing in global climate models (GCMs) and is more representative of the eventual temperature response. Here we examine the different methods of computing ERF in two GCMs. We find that ERF computed from a fixed sea-surface temperature (SST) method (ERF_fSST) has much more certainty than regression based methods. Thirty-year integrations are sufficient to reduce the 5-95% confidence interval in global ERF_fSST to 0.1 W m-2. For 2xCO2 ERF, 30 year integrations are needed to ensure that the signal is larger than the local confidence interval over more than 90% of the globe. Within the ERF_fSST method there are various options for prescribing SSTs and sea-ice. We explore these and find that ERF is only weakly dependent on the methodological choices. Prescribing the monthly-averaged seasonally varying model’s preindustrial climatology is recommended for its smaller random error and easier implementation. As part of CMIP6, the Radiative Forcing Model Intercomparison Project (RFMIP) asks models to conduct 30-year ERF_fSST experiments using the model’s own preindustrial climatology of SST and sea-ice. The Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) will also mainly use this approach. We propose this as a standard method for diagnosing ERF and recommend that it be used across the climate modelling community to aid future comparisons.

Possible impacts of a future grand solar minimum on climate: Stratospheric and global circulation changes.

Abstract It has been suggested that the Sun may evolve into a period of lower activity over the 21st century. This study examines the potential climate impacts of the onset of an extreme “Maunder Minimum‐like” grand solar minimum using a comprehensive global climate model. Over the second half of the 21st century, the scenario assumes a decrease in total solar irradiance of 0.12% compared to a reference Representative Concentration Pathway 8.5 experiment. The decrease in solar irradiance cools the stratopause (∼1 hPa) in the annual and global mean by 1.2 K. The impact on global mean near‐surface temperature is small (∼−0.1 K), but larger changes in regional climate occur during the stratospheric dynamically active seasons. In Northern Hemisphere wintertime, there is a weakening of the stratospheric westerly jet by up to ∼3–4 m s−1, with the largest changes occurring in January–February. This is accompanied by a deepening of the Aleutian Low at the surface and an increase in blocking over Northern Europe and the North Pacific. There is also an equatorward shift in the Southern Hemisphere midlatitude eddy‐driven jet in austral spring. The occurrence of an amplified regional response during winter and spring suggests a contribution from a top‐down pathway for solar‐climate coupling; this is tested using an experiment in which ultraviolet (200–320 nm) radiation is decreased in isolation of other changes. The results show that a large decline in solar activity over the 21st century could have important impacts on the stratosphere and regional surface climate.

Do split and displacement sudden stratospheric warmings have different annular mode signatures

Sudden stratospheric warmings (SSWs) contribute to intraseasonal tropospheric forecasting skill due to their surface impacts. Recent studies suggest these impacts depend upon whether the polar vortex splits or is displaced during the SSW. We analyse the annular mode signatures of SSWs in a 1000 year IPSL-CM5A-LR simulation. Although small differences in the mean surface Northern Annular Mode (NAM) index following splits and displacements are found, the sign is not consistent for two independent SSW algorithms, and over 50 events are required to distinguish the responses. We use the winter-time correlation between extratropical lower stratospheric wind anomalies and the surface NAM index as a metric for two-way stratosphere-troposphere coupling, and find that the differences between splits and displacements, and between classification methodologies, can be simply understood in terms of their mean stratospheric wind anomalies. Predictability studies should therefore focus on understanding the factors that determine the persistence of these anomalies following SSWs.

Stratospheric water vapor and climate: Sensitivity to the representation in radiation codes

There has been considerable interest in the climate impact of trends in stratosphericwater vapor (SWV). However, the representation of the radiative properties of water vaporunder stratospheric conditions remains poorly constrained across different radiation codes.This study examines the sensitivity of a detailed line-by-line (LBL) code, a Malkmusnarrow-band model and two broadband GCM radiation codes to a uniform perturbation inSWV in the longwave spectral region. The choice of sampling rate in wave numberspace (Dn) in the LBL code is shown to be important for calculations of the instantaneouschange in heating rate (DQ) and the instantaneous longwave radiative forcing (DF

Diagnosing the radiative and chemical contributions to future changes in tropical column ozone with the UM-UKCA chemistry–climate model

Chemical and dynamical drivers of trends in tropical total-column ozone (TCO3) for the recent past and future periods are explored using the UM-UKCA (Unified Model HadGEM3-A (Hewitt et al., 2011) coupled with the United Kingdom Chemistry and Aerosol scheme) chemistry–climate model. A transient 1960–2100 simulation is analysed which follows the representative concentration pathway 6.0 (RCP6.0) emissions scenario for the future. Tropical averaged (10 S–10 N) TCO3 values decrease from the 1970s, reach a minimum around 2000 and return to their 1980 values around 2040, consistent with the use and emission of halogenated ozone-depleting substances (ODSs), and their later controls under the Montreal Protocol. However, when the ozone column is subdivided into three partial columns (PCO3) that cover the upper stratosphere (PCO3US ), lower stratosphere (PCO3LS ) and troposphere (PCO3T ), significant differences in the temporal behaviour of the partial columns are seen. Modelled PCO3T values under the RCP6.0 emissions scenario increase from 1960 to 2000 before remaining approximately constant throughout the 21st century. PCO3LS values decrease rapidly from 1960 to 2000 and remain constant from 2000 to 2050, before gradually decreasing further from 2050 to 2100 and never returning to their 1980s values. In contrast, PCO3US values decrease from 1960 to 2000, before increasing rapidly throughout the 21st century and returning to 1980s values by ∼ 2020, and reach significantly higher values by 2100. Using a series of idealised UM-UKCA time-slice simulations with concentrations of well-mixed greenhouse gases (GHGs) and halogenated ODS species set to either year 2000 or 2100 levels, we examine the main processes that drive the PCO3 responses in the three regions and assess how these processes change under different emission scenarios. Finally, we present a simple, linearised model to describe the future evolution of tropical stratospheric column ozone values based on terms representing time-dependent abundances of GHG and halogenated ODS.

Revisiting the relationship among metrics of tropical expansion

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined.