Tropospheric Response Research Articles

Robustness of the Simulated Tropospheric Response to Ozone Depletion

Recent studies have shown large intermodel differences in the magnitude of the simulated response of the Southern Hemisphere tropospheric circulation to ozone depletion. This inconsistency may be a result of different model dynamics, different ozone forcing, or statistical uncertainty. Here the summertime tropospheric response to ozone depletion is analyzed in an array of climate model simulations with incrementally increasing complexity. This allows the sensitivity of the response to a range of factors to be carefully tested, including the choice of model, the prescribed sea surface temperatures and greenhouse gas concentrations, the inclusion of a coupled ocean, the temporal resolution of the prescribed ozone concentrations, and the inclusion of interactive chemistry. A consistent poleward shift of the extratropical jet is found in all simulations. All simulations also show a strengthening of the extratropical jet and a widening of the southern edge of the Hadley cell, but the magnitude of these responses is much less consistent. However, in all simulations statistical uncertainty due to interannual variability is found to be large relative to the size of the response, despite considering long (100 yr) annually repeating simulations. It is therefore proposed that interannual variability is a dominant cause of intermodel differences in past studies, which have generally analyzed shorter, transient simulations.

Downward Wave Reflection as a Mechanism for the Stratosphere–Troposphere Response to the 11-Yr Solar Cycle

The effects of solar activity on the stratospheric waveguides and downward reflection of planetary waves during NH early to midwinter are examined. Under high solar (HS) conditions, enhanced westerly winds in the subtropical upper stratosphere and the associated changes in the zonal wind curvature led to an altered waveguide geometry across the winter period in the upper stratosphere. In particular, the condition for barotropic instability was more frequently met at 1 hPa near the polar-night jet centered at about 55°N. In early winter, the corresponding change in wave forcing was characterized by a vertical dipole pattern of the Eliassen–Palm (E–P) flux divergent anomalies in the high-latitude upper stratosphere accompanied by poleward E–P flux anomalies. These wave forcing anomalies corresponded with negative vertical shear of zonal mean winds and the formation of a vertical reflecting surface. Enhanced downward E–P flux anomalies appeared below the negative shear zone; they coincided with more frequent occurrence of negative daily heat fluxes and were associated with eastward acceleration and downward group velocity. These downward-reflected wave anomalies had a detectable effect on the vertical structure of planetary waves during November–January. The associated changes in tropospheric geopotential height contributed to a more positive phase of the North Atlantic Oscillation in January and February. These results suggest that downward reflection may act as a “top down” pathway by which the effects of solar ultraviolet (UV) radiation in the upper stratosphere can be transmitted to the troposphere.

Reduced Southern Hemispheric circulation response to quadrupled CO2 due to stratospheric ozone feedback

AbstractDue to computational constraints, interactive stratospheric ozone chemistry is commonly neglected in most climate models participating in intercomparison projects. The impact of this simplification on the modeled response to external forcings remains unexplored. In this work, we examine the importance of including interactive stratospheric ozone chemistry on the Southern Hemispheric circulation response to an abrupt quadrupling of CO2. We find that including interactive ozone significantly reduces (by 20%) the response of the midlatitude jet to CO2, even though it does not alter the surface temperature response. The reduction of the tropospheric circulation response is due to CO2 induced ozone changes and their effects on the meridional temperature gradient near the tropopause. Our findings suggest that neglecting this stratospheric ozone feedback results in an overestimate of the circulation response to increased CO2. This has important implications for climate projections of the Southern Hemispheric circulation response to CO2.

Seasonal variations in atmospheric responses to oceanic eddies in the Kuroshio Extension

Analyses using high-resolution satellite observations reveal distinct seasonal variations in atmospheric responses to oceanic eddies in the Kuroshio Extension (KE) region, characterised by much stronger surface wind speed and heat flux responses in the cold seasons (winter and spring) than in the warm seasons (summer and autumn). Cloud liquid water and rain rate also display seasonally dependent characteristics, with more deficit (surplus) in winter than in summer over the cold (warm) oceanic eddies. CFSR (Climate Forecast System Reanalysis) data can well reproduce these seasonal variations in surface atmospheric responses to the eddies in the KE region, albeit with much weaker responses in surface wind speed and with stronger responses in latent heat flux in comparison with the results based on satellite observations. In addition, the CFSR data also reveal remarkable seasonal variations in tropospheric responses, with eddy-induced wind speed (vertical velocity) anomalies reaching as high as 900 hPa (800 hPa) in winter, while they only occur near the sea surface in summer. The Weather Research and Forecast (WRF) model is applied to study the seasonal variations in atmospheric responses to idealised oceanic eddies. The model successfully simulates the seasonal variations in atmospheric responses to an idealised warm eddy in terms of wind speed, heat flux, marine atmospheric boundary layer (MABL) height and vertical velocity in both seasons. Both the CFSR data and the model simulations indicate that the seasonal variations in atmospheric responses to oceanic eddies can be attributed to the variations in background atmospheric stability during different seasons.

On the atmospheric response experiment to a Blue Arctic Ocean

AbstractWe demonstrated atmospheric responses to a reduction in Arctic sea ice via simulations in which Arctic sea ice decreased stepwise from the present‐day range to an ice‐free range. In all cases, the tropospheric response exhibited a negative Arctic Oscillation (AO)‐like pattern. An intensification of the climatological planetary‐scale wave due to the present‐day sea ice reduction on the Atlantic side of the Arctic Ocean induced stratospheric polar vortex weakening and the subsequent negative AO. Conversely, strong Arctic warming due to ice‐free conditions across the entire Arctic Ocean induced a weakening of the tropospheric westerlies corresponding to a negative AO without troposphere‐stratosphere coupling, for which the planetary‐scale wave response to a surface heat source extending to the Pacific side of the Arctic Ocean was responsible. Because the resultant negative AO‐like response was accompanied by secondary circulation in the meridional plane, atmospheric heat transport into the Arctic increased, accelerating the Arctic amplification.

Quantifying Eddy Feedbacks and Forcings in the Tropospheric Response to Stratospheric Sudden Warmings

Abstract The equatorward shift of the zonal-mean midlatitude tropospheric jet following a stratospheric sudden warming in a comprehensive stratosphere-resolving model is found to be well quantified by the simple model of tropospheric eddy feedbacks proposed by Lorenz and Hartmann. This permits a decomposition of the shift into a component driven by the stratospheric anomalies and a component driven by tropospheric feedbacks. This is done by extending the simple model to include three effective forcing mechanisms by which the stratosphere may influence the tropospheric jet. These include 1) the zonally symmetric adjustments associated with the mean meridional circulation and the direct influence of the stratospheric anomalies on 2) the tropospheric synoptic-scale or 3) the tropospheric planetary-scale eddies. Although the anomalous tropospheric winds are primarily maintained against surface friction by the synoptic-scale eddies, this response can be entirely attributed to the eddy feedback term. The response of the planetary-scale eddies, in contrast, can be directly attributed to the stratosphere. The zonally symmetric tropospheric circulation associated with downward control is found to play little role in driving the tropospheric response. The prospects of applying this methodology to reanalysis data are also considered, but statistical limitations and the relatively weak projection of the vertically integrated composite wind anomalies onto the leading EOF preclude any conclusions from being drawn.

Reconciling the observed and modeled Southern Hemisphere circulation response to volcanic eruptions

AbstractConfusion exists regarding the tropospheric circulation response to volcanic eruptions, with models and observations seeming to disagree on the sign of the response. The forced Southern Hemisphere circulation response to the eruptions of Pinatubo and El Chichón is shown to be a robust positive annular mode, using over 200 ensemble members from 38 climate models. It is demonstrated that the models and observations are not at odds, but rather, internal climate variability is large and can overwhelm the forced response. It is further argued that the state of the El Niño–Southern Oscillation can at least partially explain the sign of the observed anomalies and may account for the perceived discrepancy between model and observational studies. The eruptions of both El Chichón and Pinatubo occurred during El Niño events, and it is demonstrated that the Southern Annular Mode anomalies following volcanic eruptions are weaker during El Niño events compared to La Niña events.

Classification of stratospheric extreme events according to their downward propagation to the troposphere

AbstractThis study presents a classification of stratospheric extreme events during northern winter into events with or without a consistent downward propagation of anomalies to the troposphere. Anomalous strong and weak stratospheric polar vortex events are detected from daily time series of the polar cap averaged (60°–90°N) geopotential height anomaly. The method is applied to chemistry‐climate model data (E39CA and WACCM3.5) and reanalyses data (ERA40). The analyses show that in about 80% of all events no significant tropospheric response can be detected. The stratospheric perturbation of both weak and strong events with a significant tropospheric response persists significantly longer throughout the stratosphere compared to the events without a tropospheric response. The strength of the stratospheric perturbation determines the strength of the tropospheric response only to a small degree. Results are consistent across all three data sets.

Robust Wind and Precipitation Responses to the Mount Pinatubo Eruption, as Simulated in the CMIP5 Models

Abstract The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño–Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.

Tropospheric column ozone response to ENSO in GEOS-5 assimilation of OMI and MLS ozone data

Abstract. We use GEOS-5 analyses of Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) ozone observations to investigate the magnitude and spatial distribution of the El Niño Southern Oscillation (ENSO) influence on tropospheric column ozone (TCO) into the middle latitudes. This study provides the first explicit spatially resolved characterization of the ENSO influence and demonstrates coherent patterns and teleconnections impacting the TCO in the extratropics. The response is evaluated and characterized by both the variance explained and sensitivity of TCO to the Niño 3.4 index. The tropospheric response in the tropics agrees well with previous studies and verifies the analyses. A two-lobed response symmetric about the Equator in the western Pacific/Indonesian region seen in some prior studies and not in others is confirmed here. This two-lobed response is consistent with the large-scale vertical transport. We also find that the large-scale transport in the tropics dominates the response compared to the small-scale convective transport. The ozone response is weaker in the middle latitudes, but a significant explained variance of the TCO is found over several small regions, including the central United States. However, the sensitivity of TCO to the Niño 3.4 index is statistically significant over a large area of the middle latitudes. The sensitivity maxima and minima coincide with anomalous anti-cyclonic and cyclonic circulations where the associated vertical transport is consistent with the sign of the sensitivity. Also, ENSO related changes to the mean tropopause height can contribute significantly to the midlatitude response. Comparisons to a 22-year chemical transport model simulation demonstrate that these results from the 9-year assimilation are representative of the longer term. This investigation brings insight to several seemingly disparate prior studies of the El Niño influence on tropospheric ozone in the middle latitudes.

On the link between Barents‐Kara sea ice variability and European blocking

AbstractThis study examines the connection between the variability of sea ice concentration in the Barents and Kara (B‐K) seas and winter European weather on an intraseasonal time scale. Low sea ice regimes in autumn and early winter over the B‐K seas are shown to affect the strength and position of the polar vortex, and increase the frequency of blocking regimes over the Euro‐Atlantic sector in late winter. A hypothesis is presented on the mechanism that links sea ice over the B‐K seas and circulation regimes in the North Atlantic, and is investigated considering 34 years of European Centre for Medium‐Range Weather Forecasts reanalysis data. Four key steps have been identified, starting from a local response of the near‐surface fluxes and modification of the upper tropospheric wave pattern, to the stratospheric adjustment and the tropospheric response in the North Atlantic. The proposed mechanism explains the delayed, late winter response of the North Atlantic Oscillation to the late autumn sea ice reduction, which has been found both in observations and model experiments. It also provides valuable insights on how the reduction of Arctic sea ice can influence the position of the tropospheric jet in the Euro‐Atlantic sector.

Influence of the Quasi-Biennial Oscillation and Sea Surface Temperature Variability on Downward Wave Coupling in the Northern Hemisphere

Abstract Downward wave coupling occurs when an upward-propagating planetary wave from the troposphere decelerates the flow in the upper stratosphere and forms a downward reflecting surface that redirects waves back to the troposphere. To test this mechanism and potential factors influencing the downward wave coupling, three 145-yr sensitivity simulations with NCAR’s Community Earth System Model [CESM1(WACCM)], a state-of-the-art high-top chemistry–climate model, are analyzed. The results show that the quasi-biennial oscillation (QBO) and SST variability significantly impact downward wave coupling. Without the QBO, the occurrence of downward wave coupling is significantly suppressed. In contrast, stronger and more persistent downward wave coupling occurs when SST variability is excluded. The above influence on the occurrence of downward wave coupling is mostly due to a direct influence of the QBO and SST variability on stratospheric planetary wave source and propagation. The strengths of the tropospheric circulation and surface responses to a given downward wave coupling event, however, behave differently. The surface anomaly is significantly weaker (stronger) in the experiment with fixed SSTs (without QBO), even though the statistical signal of downward wave coupling is strongest (weakest) in this experiment. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy–mean flow feedback and the possible contribution of SST anomalies in the North Atlantic during the downward wave coupling event. The weaker synoptic-scale eddy–mean flow feedback and the absence of the positive NAO-related SST-tripole pattern in the fixed SST experiment are consistent with a weaker tropospheric response to downward wave coupling. The results highlight the importance of synoptic-scale eddies in setting the tropospheric response to downward wave coupling.

The stratospheric pathway for Arctic impacts on midlatitude climate

AbstractRecent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic‐midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary‐scale waves. Those results and consistency with observation‐based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase.

The role of planetary waves in the tropospheric jet response to stratospheric cooling

AbstractAn idealized general circulation model is used to assess the importance of planetary‐scale waves in determining the position of the tropospheric jet, specifically its tendency to shift poleward as winter stratospheric cooling is increased. Full model integrations are compared against integrations in which planetary waves are truncated in the zonal direction, and only synoptic‐scale waves are retained. Two series of truncated integrations are considered, using (i) a modified radiative equilibrium temperature or (ii) a nudged‐bias correction technique. Both produce tropospheric climatologies that are similar to the full model when stratospheric cooling is weak. When stratospheric cooling is increased, the results indicate that the interaction between planetary‐ and synoptic‐scale waves plays an important role in determining the structure of the tropospheric mean flow and rule out the possibility that the jet shift occurs purely as a response to changes in the planetary‐ or synoptic‐scale wave fields alone.

Drivers of changes in stratospheric and tropospheric ozone between year 2000 and 2100

Abstract. A stratosphere-resolving configuration of the Met Office's Unified Model (UM) with the United Kingdom Chemistry and Aerosols (UKCA) scheme is used to investigate the atmospheric response to changes in (a) greenhouse gases and climate, (b) ozone-depleting substances (ODSs) and (c) non-methane ozone precursor emissions. A suite of time-slice experiments show the separate, as well as pairwise, impacts of these perturbations between the years 2000 and 2100. Sensitivity to uncertainties in future greenhouse gases and aerosols is explored through the use of the Representative Concentration Pathway (RCP) 4.5 and 8.5 scenarios. The results highlight an important role for the stratosphere in determining the annual mean tropospheric ozone response, primarily through stratosphere–troposphere exchange (STE) of ozone. Under both climate change and reductions in ODSs, increases in STE offset decreases in net chemical production and act to increase the tropospheric ozone burden. This opposes the effects of projected decreases in ozone precursors through measures to improve air quality, which act to reduce the ozone burden. The global tropospheric lifetime of ozone (τO3) does not change significantly under climate change at RCP4.5, but it decreases at RCP8.5. This opposes the increases in τO3 simulated under reductions in ODSs and ozone precursor emissions. The additivity of the changes in ozone is examined by comparing the sum of the responses in the single-forcing experiments to those from equivalent combined-forcing experiments. Whilst the ozone responses to most forcing combinations are found to be approximately additive, non-additive changes are found in both the stratosphere and troposphere when a large climate forcing (RCP8.5) is combined with the effects of ODSs.

Observed and modeled tropospheric cold anomalies associated with sudden stratospheric warmings

AbstractSurface weather patterns related to 35 major sudden stratospheric warmings (SSWs) in 1958–2010 are analyzed based on reanalysis data. Similar analyses are conducted with data from seven stratosphere‐resolving Earth system models. The analyses are carried out separately for displacement and splitting SSWs. On the basis of the observational analysis, it is shown that in northern Eurasia, the cold anomalies linked to the SSWs tend to be stronger and more widespread before the central date of the SSWs than during the first 2 months after the event central dates. This is particularly true for the displacement events. The cold anomalies preceding the SSWs are coupled to atmospheric blocking events which trigger the SSWs. While the role of SSWs as important predictors of cold air outbreaks in the Northern Hemisphere is well recognized, our results indicate that the impact of the preceding blocking on near‐surface temperatures is, in fact, widely more significant than the downward impact of the SSWs. Thus, stratosphere‐troposphere coupling provides only limited predictability for cold air outbreaks in Eurasia. The models reproduce qualitatively well the typical large‐scale surface weather patterns following the SSWs, but they largely miss the cooling preceding the SSWs over Europe and western Siberia. Hence, the strongest modeled temperature anomalies related to the SSWs occur after the events. Moreover, the model results indicate that the tropospheric response to SSWs is stronger following split events. At the same time, many models simulate too few splitting SSWs.

Mechanisms of Stratospheric and Tropospheric Circulation Response to Projected Arctic Sea Ice Loss*

Abstract The impact of projected Arctic sea ice loss on the atmospheric circulation is investigated using the Whole Atmosphere Community Climate Model (WACCM), a model with a well-resolved stratosphere. Two 160-yr simulations are conducted: one with surface boundary conditions fixed at late twentieth-century values and the other with identical conditions except for Arctic sea ice, which is prescribed at late twenty-first-century values. Their difference isolates the impact of future Arctic sea ice loss upon the atmosphere. The tropospheric circulation response to the imposed ice loss resembles the negative phase of the northern annular mode, with the largest amplitude in winter, while the less well-known stratospheric response transitions from a slight weakening of the polar vortex in winter to a strengthening of the vortex in spring. The lack of a significant winter stratospheric circulation response is shown to be a consequence of largely cancelling effects from sea ice loss in the Atlantic and Pacific sectors, which drive opposite-signed changes in upward wave propagation from the troposphere to the stratosphere. Identical experiments conducted with Community Atmosphere Model, version 4, WACCM’s low-top counterpart, show a weaker tropospheric response and a different stratospheric response compared to WACCM. An additional WACCM experiment in which the imposed ice loss is limited to August–November reveals that autumn ice loss weakens the stratospheric polar vortex in January, followed by a small but significant tropospheric response in late winter and early spring that resembles the negative phase of the North Atlantic Oscillation, with attendant surface climate impacts.

Ocean mediation of tropospheric response to reflecting and absorbing aerosols

Abstract. Radiative forcing by reflecting (e.g., sulfate, SO4) and absorbing (e.g., black carbon, BC) aerosols is distinct: the former cools the planet by reducing solar radiation at the top of the atmosphere and the surface, without largely affecting the atmospheric column, while the latter heats the atmosphere directly. Despite the fundamental difference in forcing, here we show that the structure of the tropospheric response is remarkably similar between the two types of aerosols, featuring a deep vertical structure of temperature change (of opposite sign) at the Northern Hemisphere (NH) mid-latitudes. The deep temperature structure is anchored by the slow response of the ocean, as a large meridional sea surface temperature (SST) gradient drives an anomalous inter-hemispheric Hadley circulation in the tropics and induces atmospheric eddy adjustments at the NH mid-latitudes. The tropospheric warming in response to projected future decline in reflecting aerosols poses additional threats to the stability of mountain glaciers in the NH. Additionally, robust tropospheric response is unique to aerosol forcing and absent in the CO2 response, which can be exploited for climate change attribution.

Tropospheric Ozone Response on Climate Change in Malaysia

Ozone at the tropospheric level is considered as pollutants and harms the human health. As the chemistry of tropospheric ozone is sensitive to solar radiation and temperature, one may expect significant changes to the tropospheric ozone variability due to climate change. In this study, the authors have explored the impact of regional climate change to the tropospheric ozone in a number of urban, rural and remote areas in Malaysia. This paper aims to develop a climate change scenario for A2 emissions over Malaysia using RCM PRECIS model and used the climate output as input to CiTTyCAT, a tropospheric chemistry model. At the end of this century, the surface temperature and precipitation were found to increase both during wet season (winter monsoon) and dry season (summer monsoon). In response to climate change relative to the observed ozone concentrations in year 2008, tropospheric ozone concentrations have been found to increase in urban areas (Kuala Lumpur, Sg. Petani and Kota Kinabalu), while in rural and remote areas (Kapit and Danum Valley) concentrations have been found to decrease. These projections were observed in both seasons. This study has suggested that tropospheric ozone sensitivity to climate change shows a degree of response variability.

Separating the Mechanisms of Transient Responses to Stratospheric Ozone Depletion–Like Cooling in an Idealized Atmospheric Model

Abstract Previous studies have suggested that Southern Hemisphere (SH) summertime trends in the atmospheric circulation in the second half of the twentieth century are mainly driven by stratospheric ozone depletion in spring. Here, the authors show that the pattern and timing of observed trends, characterized by downward propagation of signals, can be approximately captured in an idealized atmospheric global circulation model (AGCM) by imposing ozone depletion–like radiative cooling. It is further shown that the synoptic eddies dominantly contribute to the transient tropospheric response to polar stratospheric cooling. The authors examine three possible mechanisms on the downward influence of polar stratospheric cooling. The polar stratospheric cooling affects tropospheric synoptic eddies via (i) the direct influences on the lower-stratospheric synoptic eddies, (ii) the planetary wave–induced residual circulation, and (iii) the planetary eddy–synoptic eddy nonlinear interaction. It is argued that the planetary wave–induced residual circulation is not the dominant mechanism and that the planetary eddies and further nonlinear interaction with synoptic eddies are more likely the key to the downward influence of the ozone depletion–like cooling.