The response of the quasi-biennial oscillation (QBO) to an imposed mean upwelling with a periodic... more The response of the quasi-biennial oscillation (QBO) to an imposed mean upwelling with a periodic modulation is studied, by modeling the dynamics of the zero wind line at the equator using a class of equations known as descent rate models. These are simple mathematical models that capture the essence of QBO synchronization by focusing on the dynamics of the height of the zero wind line. A heuristic descent rate model for the zero wind line is described and is shown to capture many of the synchronization features seen in previous studies of the QBO. It is then demonstrated using a simple transformation that the standard Holton-Lindzen model of the QBO can itself be put into the form of a descent rate model if a quadratic velocity profile is assumed below the zero wind line. The resulting nonautonomous ordinary differential equation captures much of the synchronization behavior observed in the full Holton-Lindzen partial differential equation. The new class of models provides a novel framework within which to understand synchronization of the QBO, and we demonstrate a close relationship between these models and the circle map well known in the mathematics literature. Finally, we analyze reanalysis datasets to validate some of the predictions of our descent rate models and find statistically significant evidence for synchronization of the QBO that is consistent with model behavior.
A sample of 1.53×10 9 cosmic-ray-induced single muon events has been recorded at 225 meterswater-... more A sample of 1.53×10 9 cosmic-ray-induced single muon events has been recorded at 225 meterswater-equivalent using the MINOS Near Detector. The underground muon rate is observed to be highly correlated with the effective atmospheric temperature. The coefficient αT , relating the change in the muon rate to the change in the vertical effective temperature, is determined to be 0.428±0.003(stat.)±0.059(syst.). An alternative description is provided by the weighted effective temperature, introduced to account for the differences in the temperature profile and muon flux as a function of zenith angle. Using the latter estimation of temperature, the coefficient is determined
Figure S1: Height profile (left panel) and principal component time-series (right panel) of the f... more Figure S1: Height profile (left panel) and principal component time-series (right panel) of the first two Empirical Orthogonal Components (EOFs) of the FUB zonally-averaged equatorial zonal winds between 10-70 hPa for the period 1958-2016. EOF-1 is in 5 dark blue and EOF-2 is in light blue.
The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using 5... more The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using 500year pre-industrial control simulations from the Met Office Hadley Centre model. Robust precipitation responses to the phase of the QBO are diagnosed in the model, which show zonally asymmetric patterns that resemble the El Niño-Southern Oscillation (ENSO) impacts. These patterns are found because the frequency of ENSO events for each QBO phase is significantly different in these simulations, with more El Niño events found under the westerly phase of the QBO (QBOW) and more La Niña events for the easterly phase (QBOE). The QBO-ENSO relationship is nonstationary and subject to decadal variability in both models and observations. In addition, regression analysis shows that there is a QBO signal in precipitation that is independent of ENSO. No evidence is found to suggest that these QBO-ENSO relationships are caused by ENSO modulating the QBO in the simulations. A relationship between the QBO and a dipole of precipitation in the Indian Ocean is also found in models and observations in boreal fall, characterised by a wetter western Indian Ocean and drier conditions in the eastern part for QBOW and the opposite under QBOE conditions. The Walker circulation is significantly weaker during QBOW compared to QBOE, which could explain the observed and simulated zonally asymmetric precipitation responses at equatorial latitudes, as well as the more frequent El Niño events during QBOW. Further work, including targeted model experiments, is required to better understand the mechanisms causing these relationships between the QBO and tropical convection.
The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillati... more The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) aims to improve the fidelity of tropical stratospheric variability in general circulation and Earth system models by conducting coordinated numerical experiments and analysis. In the equatorial stratosphere, the QBO is the most conspicuous mode of variability. Five coordinated experiments have therefore been designed to (i) evaluate and compare the verisimilitude of modelled QBOs under present-day conditions, (ii) identify robustness (or, alternatively the spread/uncertainty) in the simulated 5 QBO response to commonly imposed changes in model climate forcings (e.g., a doubling of CO 2 amounts) and, (iii) examine
Journal of Applied Meteorology and Climatology, May 1, 2017
In recent years, the idea of geoengineering, artificially modifying the climate to reduce global ... more In recent years, the idea of geoengineering, artificially modifying the climate to reduce global temperatures, has received increasing attention because of the lack of progress in reducing global greenhouse gas emissions. Stratospheric sulfate injection (SSI) is a geoengineering method proposed to reduce planetary warming by reflecting a proportion of solar radiation back into space that would otherwise warm the surface and lower atmosphere. The authors analyze results from the Met Office Hadley Centre Global Environment Model, version 2, Carbon Cycle Stratosphere (HadGEM2-CCS) climate model with stratospheric emissions of 10 Tg yr 21 of SO 2 , designed to offset global temperature rise by around 18C. A reduction in concentrating solar power output of 5.9% on average over land is shown under SSI relative to a baseline future climate change scenario (RCP4.5) caused by a decrease in direct radiation. Solar photovoltaic energy is generally less affected as it can use diffuse radiation, which increases under SSI, at the expense of direct radiation. The results from HadGEM2-CCS are compared with the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) from the Geoengineering Model Intercomparison Project (GeoMIP), with 5 Tg yr 21 emission of SO 2. In many regions, the differences predicted in solar energy output between the SSI and RCP4.5 simulations are robust, as the sign of the changes for both HadGEM2-CCS and GEOSCCM agree. Furthermore, the sign of the total and direct annual mean radiation changes evaluated by HadGEM2-CCS agrees with the sign of the multimodel mean changes of an ensemble of GeoMIP models over the majority of the world.
Journal Of Geophysical Research: Atmospheres, Mar 19, 2020
Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key h... more Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere-troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO 2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO 2 (4xCO 2 ) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO 2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. As for the near-surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO 2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex and thus a longer season of stratosphere-troposphere coupling.
This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscilla... more This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscillation (QBO) in the tropical stratosphere and the tropical troposphere and surface. The impact of the QBO on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Centre Unified Model (UM). The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The simulation of convection and precipitation in the atmosphere-only simulations is not statistically significantly different between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and SST relationships with the QBO phase disappear when nudging is applied. Evidence from the nudging experiments shows that the QBO influence over lower stratospheric static stability is not sufficient to produce tropical surface impacts. The nudging also reduced the influence of the lower troposphere to the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper troposphere has been slightly decoupled from the surface by the nudging. These results suggest that nudging all grid-points might mute relevant feedback processes, including high cloud radiative effects and wave mean flow interactions, occurring at the tropopause level.
Sudden Stratospheric Warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) strato... more Sudden Stratospheric Warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) stratospheric polar vortex and occur on average approximately 6 times per decade in observation based records. However, within these records, intervals of significantly higher and lower SSW rates are observed suggesting the possibility of low frequency variations in event occurrence. A better understanding of factors that influence this decadal variability may help to improve predictability of NH mid-latitude surface climate, through stratosphere-troposphere coupling. In this work, multi-decadal variability of SSW events is examined in a 1000-yr pre-industrial simulation of a coupled Atmosphere-Ocean-Land-Sea ice model. Using a wavelet spectral decomposition method, we show that hiatus events (intervals of a decade or more with no SSWs) and consecutive SSW events (extended intervals with at least one SSW in each year) vary on multi-decadal timescales of period between 60 and 90 years. Signals on these timescales are present for approximately 450 years of the simulation. We investigate the possible source of these long-term signals and find that the direct impact of variability in tropical sea surface temperatures, as well as the associated Aleutian Low, can account for only a small portion of the SSW variability. Instead, the major influence on long-term SSW variability is associated with long-term variability in amplitude of the stratospheric quasi biennial oscillation (QBO). The QBO influence is consistent with the well known Holton-Tan relationship, with SSW hiatus intervals associated with extended periods of particularly strong, deep QBO westerly phases. The results support recent studies that have highlighted the role of vertical coherence in the QBO when considering coupling between the QBO, the polar vortex and tropospheric circulation.
The scientific understanding of the Earth's climate system, including the central question of how... more The scientific understanding of the Earth's climate system, including the central question of how the climate system is likely to respond to human-induced perturbations, is comprehensively captured in GCMs and Earth System Models (ESM). Diagnosing the simulated climate response, and comparing responses across different models, is crucially dependent on transparent assumptions of how the GCM/ESM has been drivenespecially because the implementation can involve subjective decisions and may differ between modelling groups performing the same experiment. This paper outlines the climate forcings and setup of the Met Office Hadley Centre ESM, HadGEM2-ES for the CMIP5 set of centennial experiments. We document the prescribed greenhouse gas concentrations, aerosol precursors, stratospheric and tropospheric ozone assumptions, as well as implementation of land-use change and natural forcings for the HadGEM2-ES historical and future experiments following the Representative Concentration Pathways. In addition, we provide details of how HadGEM2-ES ensemble members were initialised from the control run and how the palaeoclimate and AMIP experiments, as well as the "emission-driven" RCP experiments were performed.
The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using a... more The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using a 500-yr preindustrial control simulation of the Met Office Hadley Centre model. Robust precipitation responses to the phase of the QBO are diagnosed in the model which show zonally asymmetric features, consistent with observational studies. The response in precipitation resembles the El Niño-Southern Oscillation (ENSO) impacts, however, regression analysis shows that there is a QBO signal in precipitation that is independent from ENSO. Moreover, the observed uneven frequency of ENSO events for each QBO phase is also found in these simulations, with more El Niño events found under the westerly phase of the QBO (QBOW) and more La Niña events for the easterly phase (QBOE). No evidence is found to suggest that these QBO-ENSO relationships are caused by ENSO modulating the QBO in the simulations. A previously unknown relationship between the QBO and a dipole of precipitation in the Indian Ocean is found in models and observations in boreal fall, characterized by a wetter western Indian Ocean and drier conditions in the eastern part for QBOW and the opposite under QBOE conditions. QBO WE differences show a stronger East Pacific Inter-tropical Convergence Zone (ITCZ) in boreal winter and a northward shift of the Atlantic ITCZ in boreal spring and summer. The Walker circulation is found to be significantly weaker during QBOW compared to QBOE, explaining the observed and simulated zonally asymmetric responses at equatorial latitudes. Further work, including targeted model experiments, is required to betters understand the mechanisms causing these relationships between the QBO and tropical convection. 1 Introduction Long-distance effects or teleconnections associated with the stratospheric quasi-biennial oscillation (QBO) have been well documented in the subtropics and extratropics, including for example: the stratospheric polar vortex (Holton and Tan, 1980;
Quarterly Journal of the Royal Meteorological Society, Apr 1, 2022
The processes occurring in the tropical tropopause layer (TTL) are of great importance for strato... more The processes occurring in the tropical tropopause layer (TTL) are of great importance for stratosphere-troposphere exchanges and the variability of the Earth's climate. Previous studies demonstrated the increasing ability of atmospheric general circulation models (AGCMs) in simulating the TTL, depending on factors such as the horizontal and vertical resolution, with the major role for physical parametrizations. In this work we assess the mean state and variability of the tropical upper troposphere and lower stratosphere simulated by 13 AGCMs of the Stratosphere-troposphere Processes And their Role in Climate Quasi-Biennial Oscillation initiative (QBOi) for the historical period. As these models internally generate quasi-biennial oscillations (QBOs) of the stratospheric zonal wind, we can analyse the simulated QBO influence on the TTL on interannual timescales. We find that model biases in temperature near the tropopause are strongly related to water vapour concentrations in the lower stratosphere. A source of intermodel spread derives from stratospheric aerosols, as the responses to eruptions differ between those models prescribing volcanic aerosol forcing. The QBO influence on the thermal structure is generally realistic in the equatorial region, but the subtropical response is weak compared with the reanalysis. This is associated with a limited downward penetration of QBO winds, generally smaller QBO meridional widths, and weaker temperature anomalies, which disappear above the tropopause for most models. We discuss the QBO impacts on tropopause pressure and precipitation, characterized by large uncertainties due to the small signal in the observational records and sampling uncertainty. Realistic QBO connection with the troposphere in some models suggests that the underlying physical processes can be correctly simulated. Overall, we find that the QBOi models have limited ability to reproduce the observed modulation of the TTL processes, which is consistent with biases in the vertical and latitudinal extent of the simulated QBOs degrading this connection. K E Y W O R D S internal variability, quasi-biennial oscillation, stratosphere-troposphere coupling, tropical tropopause layer This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Observations show that the seasonal cycle of precipitation in parts of southern Mexico and Centra... more Observations show that the seasonal cycle of precipitation in parts of southern Mexico and Central America exhibits a bimodal signal, known as the Midsummer drought (MSD), but there is no consensus on which processes are most relevant for the two-peak structure of the rainy season. This paper evaluates three hypotheses that could explain the MSD: the SST cloud-radiative feedback, the solar declination angle and the Caribbean Low-Level Jet (CLLJ) moisture transport hypotheses. Model experiments produced by the Met Office Hadley Centre (MOHC) for CMIP6 as well as ERA5 reanalysis data are used to critically assess the predictions of each hypothesis. The simulations capture the double peak signal of precipitation well and reasonably simulate the spatial and temporal variations of the MSD and other relevant climate features such as the CLLJ. Evidence from our analysis suggests that the Eastern Pacific SSTs do not increase in late summer in ERA5 data and only slightly increase in the simulations. More importantly, the Eastern Pacific SST variability in ERA5 and in the model experiments cannot explain the differences in the seasonality of precipitation. The net shortwave radiation at the surface shows a two-peak seasonal cycle; however, this behaviour appears to result from a strong anti-correlation of the incoming shortwave and convective activity due to cloud radiative-effects. There was no evidence found by this study of a causal link in which absorption of shortwave energy forces precipitation variations, as suggested by the solar declination angle hypothesis. The moisture convergence, CLLJ and the precipitable water vapor variations best explain the characteristics of the observed and simulated MSD, particularly for the onset of the MSD. The diagnosed variations of moisture convergence, which are synchronous with the timing of the MSD, point to a dynamic mechanism in which the low-level inflow from the Caribbean is more important for the MSD than other radiative mechanisms.
The UK Chemistry and Aerosols (UKCA) model is a new aerosol-chemistry model coupled to the Met Of... more The UK Chemistry and Aerosols (UKCA) model is a new aerosol-chemistry model coupled to the Met Office Unified Model capable of simulating composition and climate from the troposphere to the mesosphere. Here we introduce the model and assess its performance with a particular focus on the stratosphere. A 20-year perpetual year-2000 simulation forms the basis of our analysis. We assess basic and derived dynamical and chemical model fields and compare to ERA-40 reanalyses and satellite climatologies. Polar temperatures and the lifetime of the southern polar vortex are well captured, indicating that the model is suitable for assessing the ozone hole. Ozone and long-lived tracers compare favourably to observations. Chemical-dynamical coupling, as evidenced by the anticorrelation between winterspring northern polar ozone columns and the strength of the polar jet, is also well captured. Remaining problems relate to a warm bias at the tropical tropopause, slow ascent in the tropical pipe with implications for the lifetimes of long-lived species, and a general overestimation of ozone columns in middle and high latitudes.
This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscilla... more This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscillation (QBO) in the tropical stratosphere and the tropical troposphere and surface. The impact of the QBO on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Centre Unified Model (UM). The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The simulation of convection and precipitation in the atmosphere-only simulations is not statistically significantly different between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and SST relationships with the QBO p...
The World Climate Research Programme (WCRP) envisions a world “that uses sound, relevant, and tim... more The World Climate Research Programme (WCRP) envisions a world “that uses sound, relevant, and timely climate science to ensure a more resilient present and sustainable future for humankind.” This bold vision requires the climate science community to provide actionable scientific information that meets the evolving needs of societies all over the world. To realize its vision, WCRP has created five Lighthouse Activities to generate international commitment and support to tackle some of the most pressing challenges in climate science today. The overarching goal of the Lighthouse Activity on Explaining and Predicting Earth System Change is to develop an integrated capability to understand, attribute, and predict annual to decadal changes in the Earth system, including capabilities for early warning of potential high impact changes and events. This article provides an overview of both the scientific challenges that must be addressed, and the research and other activities required to achi...
The response of the quasi-biennial oscillation (QBO) to an imposed mean upwelling with a periodic... more The response of the quasi-biennial oscillation (QBO) to an imposed mean upwelling with a periodic modulation is studied, by modeling the dynamics of the zero wind line at the equator using a class of equations known as descent rate models. These are simple mathematical models that capture the essence of QBO synchronization by focusing on the dynamics of the height of the zero wind line. A heuristic descent rate model for the zero wind line is described and is shown to capture many of the synchronization features seen in previous studies of the QBO. It is then demonstrated using a simple transformation that the standard Holton-Lindzen model of the QBO can itself be put into the form of a descent rate model if a quadratic velocity profile is assumed below the zero wind line. The resulting nonautonomous ordinary differential equation captures much of the synchronization behavior observed in the full Holton-Lindzen partial differential equation. The new class of models provides a novel framework within which to understand synchronization of the QBO, and we demonstrate a close relationship between these models and the circle map well known in the mathematics literature. Finally, we analyze reanalysis datasets to validate some of the predictions of our descent rate models and find statistically significant evidence for synchronization of the QBO that is consistent with model behavior.
A sample of 1.53×10 9 cosmic-ray-induced single muon events has been recorded at 225 meterswater-... more A sample of 1.53×10 9 cosmic-ray-induced single muon events has been recorded at 225 meterswater-equivalent using the MINOS Near Detector. The underground muon rate is observed to be highly correlated with the effective atmospheric temperature. The coefficient αT , relating the change in the muon rate to the change in the vertical effective temperature, is determined to be 0.428±0.003(stat.)±0.059(syst.). An alternative description is provided by the weighted effective temperature, introduced to account for the differences in the temperature profile and muon flux as a function of zenith angle. Using the latter estimation of temperature, the coefficient is determined
Figure S1: Height profile (left panel) and principal component time-series (right panel) of the f... more Figure S1: Height profile (left panel) and principal component time-series (right panel) of the first two Empirical Orthogonal Components (EOFs) of the FUB zonally-averaged equatorial zonal winds between 10-70 hPa for the period 1958-2016. EOF-1 is in 5 dark blue and EOF-2 is in light blue.
The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using 5... more The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using 500year pre-industrial control simulations from the Met Office Hadley Centre model. Robust precipitation responses to the phase of the QBO are diagnosed in the model, which show zonally asymmetric patterns that resemble the El Niño-Southern Oscillation (ENSO) impacts. These patterns are found because the frequency of ENSO events for each QBO phase is significantly different in these simulations, with more El Niño events found under the westerly phase of the QBO (QBOW) and more La Niña events for the easterly phase (QBOE). The QBO-ENSO relationship is nonstationary and subject to decadal variability in both models and observations. In addition, regression analysis shows that there is a QBO signal in precipitation that is independent of ENSO. No evidence is found to suggest that these QBO-ENSO relationships are caused by ENSO modulating the QBO in the simulations. A relationship between the QBO and a dipole of precipitation in the Indian Ocean is also found in models and observations in boreal fall, characterised by a wetter western Indian Ocean and drier conditions in the eastern part for QBOW and the opposite under QBOE conditions. The Walker circulation is significantly weaker during QBOW compared to QBOE, which could explain the observed and simulated zonally asymmetric precipitation responses at equatorial latitudes, as well as the more frequent El Niño events during QBOW. Further work, including targeted model experiments, is required to better understand the mechanisms causing these relationships between the QBO and tropical convection.
The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillati... more The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) aims to improve the fidelity of tropical stratospheric variability in general circulation and Earth system models by conducting coordinated numerical experiments and analysis. In the equatorial stratosphere, the QBO is the most conspicuous mode of variability. Five coordinated experiments have therefore been designed to (i) evaluate and compare the verisimilitude of modelled QBOs under present-day conditions, (ii) identify robustness (or, alternatively the spread/uncertainty) in the simulated 5 QBO response to commonly imposed changes in model climate forcings (e.g., a doubling of CO 2 amounts) and, (iii) examine
Journal of Applied Meteorology and Climatology, May 1, 2017
In recent years, the idea of geoengineering, artificially modifying the climate to reduce global ... more In recent years, the idea of geoengineering, artificially modifying the climate to reduce global temperatures, has received increasing attention because of the lack of progress in reducing global greenhouse gas emissions. Stratospheric sulfate injection (SSI) is a geoengineering method proposed to reduce planetary warming by reflecting a proportion of solar radiation back into space that would otherwise warm the surface and lower atmosphere. The authors analyze results from the Met Office Hadley Centre Global Environment Model, version 2, Carbon Cycle Stratosphere (HadGEM2-CCS) climate model with stratospheric emissions of 10 Tg yr 21 of SO 2 , designed to offset global temperature rise by around 18C. A reduction in concentrating solar power output of 5.9% on average over land is shown under SSI relative to a baseline future climate change scenario (RCP4.5) caused by a decrease in direct radiation. Solar photovoltaic energy is generally less affected as it can use diffuse radiation, which increases under SSI, at the expense of direct radiation. The results from HadGEM2-CCS are compared with the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) from the Geoengineering Model Intercomparison Project (GeoMIP), with 5 Tg yr 21 emission of SO 2. In many regions, the differences predicted in solar energy output between the SSI and RCP4.5 simulations are robust, as the sign of the changes for both HadGEM2-CCS and GEOSCCM agree. Furthermore, the sign of the total and direct annual mean radiation changes evaluated by HadGEM2-CCS agrees with the sign of the multimodel mean changes of an ensemble of GeoMIP models over the majority of the world.
Journal Of Geophysical Research: Atmospheres, Mar 19, 2020
Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key h... more Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere-troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO 2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO 2 (4xCO 2 ) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO 2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. As for the near-surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO 2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex and thus a longer season of stratosphere-troposphere coupling.
This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscilla... more This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscillation (QBO) in the tropical stratosphere and the tropical troposphere and surface. The impact of the QBO on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Centre Unified Model (UM). The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The simulation of convection and precipitation in the atmosphere-only simulations is not statistically significantly different between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and SST relationships with the QBO phase disappear when nudging is applied. Evidence from the nudging experiments shows that the QBO influence over lower stratospheric static stability is not sufficient to produce tropical surface impacts. The nudging also reduced the influence of the lower troposphere to the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper troposphere has been slightly decoupled from the surface by the nudging. These results suggest that nudging all grid-points might mute relevant feedback processes, including high cloud radiative effects and wave mean flow interactions, occurring at the tropopause level.
Sudden Stratospheric Warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) strato... more Sudden Stratospheric Warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) stratospheric polar vortex and occur on average approximately 6 times per decade in observation based records. However, within these records, intervals of significantly higher and lower SSW rates are observed suggesting the possibility of low frequency variations in event occurrence. A better understanding of factors that influence this decadal variability may help to improve predictability of NH mid-latitude surface climate, through stratosphere-troposphere coupling. In this work, multi-decadal variability of SSW events is examined in a 1000-yr pre-industrial simulation of a coupled Atmosphere-Ocean-Land-Sea ice model. Using a wavelet spectral decomposition method, we show that hiatus events (intervals of a decade or more with no SSWs) and consecutive SSW events (extended intervals with at least one SSW in each year) vary on multi-decadal timescales of period between 60 and 90 years. Signals on these timescales are present for approximately 450 years of the simulation. We investigate the possible source of these long-term signals and find that the direct impact of variability in tropical sea surface temperatures, as well as the associated Aleutian Low, can account for only a small portion of the SSW variability. Instead, the major influence on long-term SSW variability is associated with long-term variability in amplitude of the stratospheric quasi biennial oscillation (QBO). The QBO influence is consistent with the well known Holton-Tan relationship, with SSW hiatus intervals associated with extended periods of particularly strong, deep QBO westerly phases. The results support recent studies that have highlighted the role of vertical coherence in the QBO when considering coupling between the QBO, the polar vortex and tropospheric circulation.
The scientific understanding of the Earth's climate system, including the central question of how... more The scientific understanding of the Earth's climate system, including the central question of how the climate system is likely to respond to human-induced perturbations, is comprehensively captured in GCMs and Earth System Models (ESM). Diagnosing the simulated climate response, and comparing responses across different models, is crucially dependent on transparent assumptions of how the GCM/ESM has been drivenespecially because the implementation can involve subjective decisions and may differ between modelling groups performing the same experiment. This paper outlines the climate forcings and setup of the Met Office Hadley Centre ESM, HadGEM2-ES for the CMIP5 set of centennial experiments. We document the prescribed greenhouse gas concentrations, aerosol precursors, stratospheric and tropospheric ozone assumptions, as well as implementation of land-use change and natural forcings for the HadGEM2-ES historical and future experiments following the Representative Concentration Pathways. In addition, we provide details of how HadGEM2-ES ensemble members were initialised from the control run and how the palaeoclimate and AMIP experiments, as well as the "emission-driven" RCP experiments were performed.
The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using a... more The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using a 500-yr preindustrial control simulation of the Met Office Hadley Centre model. Robust precipitation responses to the phase of the QBO are diagnosed in the model which show zonally asymmetric features, consistent with observational studies. The response in precipitation resembles the El Niño-Southern Oscillation (ENSO) impacts, however, regression analysis shows that there is a QBO signal in precipitation that is independent from ENSO. Moreover, the observed uneven frequency of ENSO events for each QBO phase is also found in these simulations, with more El Niño events found under the westerly phase of the QBO (QBOW) and more La Niña events for the easterly phase (QBOE). No evidence is found to suggest that these QBO-ENSO relationships are caused by ENSO modulating the QBO in the simulations. A previously unknown relationship between the QBO and a dipole of precipitation in the Indian Ocean is found in models and observations in boreal fall, characterized by a wetter western Indian Ocean and drier conditions in the eastern part for QBOW and the opposite under QBOE conditions. QBO WE differences show a stronger East Pacific Inter-tropical Convergence Zone (ITCZ) in boreal winter and a northward shift of the Atlantic ITCZ in boreal spring and summer. The Walker circulation is found to be significantly weaker during QBOW compared to QBOE, explaining the observed and simulated zonally asymmetric responses at equatorial latitudes. Further work, including targeted model experiments, is required to betters understand the mechanisms causing these relationships between the QBO and tropical convection. 1 Introduction Long-distance effects or teleconnections associated with the stratospheric quasi-biennial oscillation (QBO) have been well documented in the subtropics and extratropics, including for example: the stratospheric polar vortex (Holton and Tan, 1980;
Quarterly Journal of the Royal Meteorological Society, Apr 1, 2022
The processes occurring in the tropical tropopause layer (TTL) are of great importance for strato... more The processes occurring in the tropical tropopause layer (TTL) are of great importance for stratosphere-troposphere exchanges and the variability of the Earth's climate. Previous studies demonstrated the increasing ability of atmospheric general circulation models (AGCMs) in simulating the TTL, depending on factors such as the horizontal and vertical resolution, with the major role for physical parametrizations. In this work we assess the mean state and variability of the tropical upper troposphere and lower stratosphere simulated by 13 AGCMs of the Stratosphere-troposphere Processes And their Role in Climate Quasi-Biennial Oscillation initiative (QBOi) for the historical period. As these models internally generate quasi-biennial oscillations (QBOs) of the stratospheric zonal wind, we can analyse the simulated QBO influence on the TTL on interannual timescales. We find that model biases in temperature near the tropopause are strongly related to water vapour concentrations in the lower stratosphere. A source of intermodel spread derives from stratospheric aerosols, as the responses to eruptions differ between those models prescribing volcanic aerosol forcing. The QBO influence on the thermal structure is generally realistic in the equatorial region, but the subtropical response is weak compared with the reanalysis. This is associated with a limited downward penetration of QBO winds, generally smaller QBO meridional widths, and weaker temperature anomalies, which disappear above the tropopause for most models. We discuss the QBO impacts on tropopause pressure and precipitation, characterized by large uncertainties due to the small signal in the observational records and sampling uncertainty. Realistic QBO connection with the troposphere in some models suggests that the underlying physical processes can be correctly simulated. Overall, we find that the QBOi models have limited ability to reproduce the observed modulation of the TTL processes, which is consistent with biases in the vertical and latitudinal extent of the simulated QBOs degrading this connection. K E Y W O R D S internal variability, quasi-biennial oscillation, stratosphere-troposphere coupling, tropical tropopause layer This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Observations show that the seasonal cycle of precipitation in parts of southern Mexico and Centra... more Observations show that the seasonal cycle of precipitation in parts of southern Mexico and Central America exhibits a bimodal signal, known as the Midsummer drought (MSD), but there is no consensus on which processes are most relevant for the two-peak structure of the rainy season. This paper evaluates three hypotheses that could explain the MSD: the SST cloud-radiative feedback, the solar declination angle and the Caribbean Low-Level Jet (CLLJ) moisture transport hypotheses. Model experiments produced by the Met Office Hadley Centre (MOHC) for CMIP6 as well as ERA5 reanalysis data are used to critically assess the predictions of each hypothesis. The simulations capture the double peak signal of precipitation well and reasonably simulate the spatial and temporal variations of the MSD and other relevant climate features such as the CLLJ. Evidence from our analysis suggests that the Eastern Pacific SSTs do not increase in late summer in ERA5 data and only slightly increase in the simulations. More importantly, the Eastern Pacific SST variability in ERA5 and in the model experiments cannot explain the differences in the seasonality of precipitation. The net shortwave radiation at the surface shows a two-peak seasonal cycle; however, this behaviour appears to result from a strong anti-correlation of the incoming shortwave and convective activity due to cloud radiative-effects. There was no evidence found by this study of a causal link in which absorption of shortwave energy forces precipitation variations, as suggested by the solar declination angle hypothesis. The moisture convergence, CLLJ and the precipitable water vapor variations best explain the characteristics of the observed and simulated MSD, particularly for the onset of the MSD. The diagnosed variations of moisture convergence, which are synchronous with the timing of the MSD, point to a dynamic mechanism in which the low-level inflow from the Caribbean is more important for the MSD than other radiative mechanisms.
The UK Chemistry and Aerosols (UKCA) model is a new aerosol-chemistry model coupled to the Met Of... more The UK Chemistry and Aerosols (UKCA) model is a new aerosol-chemistry model coupled to the Met Office Unified Model capable of simulating composition and climate from the troposphere to the mesosphere. Here we introduce the model and assess its performance with a particular focus on the stratosphere. A 20-year perpetual year-2000 simulation forms the basis of our analysis. We assess basic and derived dynamical and chemical model fields and compare to ERA-40 reanalyses and satellite climatologies. Polar temperatures and the lifetime of the southern polar vortex are well captured, indicating that the model is suitable for assessing the ozone hole. Ozone and long-lived tracers compare favourably to observations. Chemical-dynamical coupling, as evidenced by the anticorrelation between winterspring northern polar ozone columns and the strength of the polar jet, is also well captured. Remaining problems relate to a warm bias at the tropical tropopause, slow ascent in the tropical pipe with implications for the lifetimes of long-lived species, and a general overestimation of ozone columns in middle and high latitudes.
This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscilla... more This study evaluates the main hypotheses to explain a coupling between the quasi-biennial oscillation (QBO) in the tropical stratosphere and the tropical troposphere and surface. The impact of the QBO on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Centre Unified Model (UM). The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The simulation of convection and precipitation in the atmosphere-only simulations is not statistically significantly different between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and SST relationships with the QBO p...
The World Climate Research Programme (WCRP) envisions a world “that uses sound, relevant, and tim... more The World Climate Research Programme (WCRP) envisions a world “that uses sound, relevant, and timely climate science to ensure a more resilient present and sustainable future for humankind.” This bold vision requires the climate science community to provide actionable scientific information that meets the evolving needs of societies all over the world. To realize its vision, WCRP has created five Lighthouse Activities to generate international commitment and support to tackle some of the most pressing challenges in climate science today. The overarching goal of the Lighthouse Activity on Explaining and Predicting Earth System Change is to develop an integrated capability to understand, attribute, and predict annual to decadal changes in the Earth system, including capabilities for early warning of potential high impact changes and events. This article provides an overview of both the scientific challenges that must be addressed, and the research and other activities required to achi...
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