IGCM4: a fast, parallel and flexible intermediate climate model

The IGCM4 (Intermediate Global Circulation Model version 4) is a global spectral primitive equation climate model whose predecessors have extensively been used in areas such as climate research, process modelling and atmospheric dynamics. The IGCM4’s niche and utility lies in its speed and flexibility allied with the complexity of a primitive equation climate model. Moist processes such as clouds, evaporation, atmospheric radiation and soil moisture are simulated in the model, though in a simplified manner compared to state-of-the-art global circulation models (GCMs). IGCM4 is a parallelised model, enabling both very long integrations to be conducted and the effects of higher resolutions to be explored. It has also undergone changes such as alterations to the cloud and surface processes and the addition of gravity wave drag. These changes have resulted in a significant improvement to the IGCM’s representation of the mean climate as well as its representation of stratospheric processes such as sudden stratospheric warmings. The IGCM4’s physical changes and climatology are described in this paper

Springtime arctic ozone depletion forces northern hemisphere climate anomalies

Large-scale chemical depletion of ozone due to anthropogenic emissions occurs over Antarctica as well as, to a lesser degree, the Arctic. Surface climate predictability in the Northern Hemisphere might be improved due to a previously proposed, albeit uncertain, link to springtime ozone depletion in the Arctic. Here we use observations and targeted chemistry–climate experiments from two models to isolate the surface impacts of ozone depletion from complex downward dynamical influences. We find that springtime stratospheric ozone depletion is consistently followed by surface temperature and precipitation anomalies with signs consistent with a positive Arctic Oscillation, namely, warm and dry conditions over southern Europe and Eurasia and moistening over northern Europe. Notably, we show that these anomalies, affecting large portions of the Northern Hemisphere, are driven substantially by the loss of stratospheric ozone. This is due to ozone depletion leading to a reduction in short-wave radiation absorption, when in turn causing persistent negative temperature anomalies in the lower stratosphere and a delayed break-up of the polar vortex. These results indicate that the inclusion of interactive ozone chemistry in atmospheric models can considerably improve the predictability of Northern Hemisphere surface climate on seasonal timescales

How can Brewer–Dobson circulation trends be estimated from changes in stratospheric water vapour and methane?

The stratospheric meridional overturning circulation, also referred to as the Brewer–Dobson circulation (BDC), controls the composition of the stratosphere, which, in turn, affects radiation and climate. As the BDC cannot be directly measured, one has to infer its strength and trends indirectly. For instance, trace gas measurements allow the calculation of average transit times. Satellite measurements provide information on the distributions of trace gases for the entire stratosphere, with measurements of particularly long temporal and dense spatial coverage available for stratospheric water vapour (H2O). Although chemical processes and boundary conditions confound interpretation, the influence of methane (CH4) oxidation on H2O in the stratosphere is relatively straightforward, and thus H2O is an appealing tracer for transport analysis despite these caveats. In this work, we explore how mean age of air trends can be estimated from the combination of stratospheric H2O and CH4 data, by carrying out a proof of concept within the model environment of the Chemical Lagrangian Model of the Stratosphere (CLaMS). In particular, we assess the methodological uncertainties related to the two commonly used approximations of (i) instantaneous stratospheric entry mixing ratio propagation and (ii) constant correlation between mean age and the fractional release factor of CH4. Performing various sensitivity studies with CLaMS, we test different methods of the mean age of air trend estimation, and we aim to provide simple and practical advice on the adjustment of the used approximations for obtaining more reliable mean age of air trends from the measurements of H2O and CH4. Our results show that the estimated mean age of air trends from the combination of stratospheric H2O and CH4 changes may be significantly affected by the assumed approximations. Depending on the investigated stratospheric region and the considered period, the error in estimated mean age of air trends can be large, especially in the lower stratosphere. For particular periods, the errors from the two approximations can lead to opposite effects, which may even cancel out. Finally, for a more reliable estimate of the mean age of air trends, we propose adjusting the approximation method by using an idealized age spectrum to propagate stratospheric entry mixing ratios. The findings of this work can be used for assessing the uncertainties in stratospheric BDC trend estimation from global satellite measurements

Stratospheric aerosol - Observations, processes, and impact on climate

Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes

Stepwise changes in stratospheric water vapor?

The sparse data available of stratospheric water vapor since the 1950s suggests a positive long‐term trend that cannot be explained by the methane increase and what is known about temperature trends around the tropical tropopause, which constrain the amount of water entering the stratosphere. Here, we discuss the 1991–2005 time series of stratospheric water (and methane) measurements from the Halogen Occultation Experiment (HALOE). The high sampling, global coverage and measurement of methane render HALOE data ideal to check the data for self‐consistency and to pinpoint the time of changes in entry mixing ratios. In addition to the well‐known ‘drop’ in October 2000, the HALOE data at 10 hPa and less suggest a steep increase in entry mixing ratios shortly before the beginning of the HALOE measurements. Model calculations using simple representations of the stratospheric age of air spectrum in the tropics show that the very dry phase may be explained by a range of scenarios: A long (several years) dry phase followed by a step increase with amplitude 0.3 ppmv; a shorter (≥1 year) dry pulse with amplitude 0.6 ppmv; or steep linear trends over about 2 years with total increases similar to the step scenarios. The drop in October 2000 coincides with anomalously large eddy heat fluxes in the Southern hemisphere and low tropopause temperatures, but no such relation is found for the situation around 1991. The coincidence with the eruption of Mt. Pinatubo is discussed. The evidence for the results presented here is circumstantial, but they would imply that decoupling between stratospheric water trends and tropical tropopause temperatures can occur on short timescales

Mean-Flow Damping Forms the Buffer Zone of the Quasi-Biennial Oscillation: 1D Theory

The quasi-biennial oscillation (QBO) is a descending pattern of alternating easterly and westerly winds in the tropical stratosphere. Upwelling is generally understood to counteract the descent of the QBO. The upwelling hypothesis holds that where upwelling exceeds the intrinsic descent rate of the QBO, the QBO cannot descend and a buffer zone forms. Descent-rate models of the QBO, which represent a highly simplified evolution of a QBO wind contour, support the upwelling hypothesis. Here, we show that the upwelling hypothesis and descent-rate models only correctly describe buffer zone formation in the absence of wave dissipation below critical levels. When there is wave dissipation below critical levels, the 1D QBO response to upwelling can be either to 1) reform below the upwelling, 2) undergo period-lengthening collapse, or 3) expand a preexisting buffer zone. The response depends on the location of the upwelling and the lower boundary condition. Mean-flow damping always forms a buffer zone. A previous study of reanalyses showed that there is mean-flow damping in the buffer zone due to horizontal momentum flux divergence. Therefore, the 1D model implicates lateral terms in buffer zone formation that it cannot self-consistently include

On the Causal Relationship Between the Moist Diabatic Circulation and Cloud Rapid Adjustment to Increasing CO2

General circulation models predict that clouds in the atmosphere rapidly adjust to the radiative perturbation of an abrupt increase in atmospheric CO2 concentration on a short time scale of about 10 days. This rapid adjustment consists of an increase of clouds in the boundary layer and a decrease of clouds in the free troposphere. Our focus is the mechanism for the decrease of clouds in the free troposphere, which is the dominating component of cloud rapid adjustment in most general circulation models. We propose that the decrease in clouds in the free troposphere arises from the causal relationship between the moist diabatic circulation and the production of condensates that forms clouds in moist processes. As CO2 concentration increases, tropospheric radiative cooling is reduced, resulting in weakening of the moist diabatic circulation and a decrease in precipitation. As the hydrologic cycle weakens and the moist processes involving phase change of water vapor to form the condensates in the atmosphere lessen, the mass of cloud condensates decreases. This decrease in cloud condensates can be predicted from the decrease in the radiative subsidence mass flux, which is a metric for the strength of the moist diabatic circulation in the free troposphere

Anomalous Dynamics of QBO Disruptions Explained by 1D Theory with External Triggering

The quasi-biennial oscillation (QBO) is an alternating, descending pattern of zonal winds in the tropical stratosphere with a period averaging 28 months. The QBO was disrupted in 2016, and arguably again in 2020, by the formation of an anomalous easterly shear zone, and unprecedented stagnation and ascent of shear zones aloft. Several mechanisms have been implicated in causing the 2016 disruption, most notably triggering by horizontal eddy momentum flux divergence, but also anomalous upwelling and wave stress. In this paper, the 1D theory of the QBO is used to show how seemingly disparate features of disruptions follow directly from the dynamics of the QBO response to triggering. The perturbed QBO is interpreted using a heuristic version of the 1D model, which establishes that 1) stagnation of shear zones aloft resulted from wave dissipation in the shear zone formed by the triggering, and 2) ascent of shear zones aloft resulted from climatological upwelling advecting the stagnant shear zones. Obstacles remain in the theory of triggering. In the 1D theory, the phasing of the triggering is key to determining the response, but the dependence on magnitude is less steep. Yet in MERRA-2, there are triggering events only 20% weaker than the 2016 triggering and equal to the 2020 triggering that did not lead to disruptions. Complicating matters further, MERRA-2 has record-large analysis tendencies during the 2016 disruption, reducing confidence in the resolved momentum budget

How Tropical Convection Couples High Moist Static Energy Over Land and Ocean

We show that in the tropics, tropical atmospheric dynamics force the subcloud moist static energy (MSE) over land and ocean to be very similar in, and only in, regions of deep convection. Using observed rainfall as a proxy for convection and reanalysis data to calculate MSE, we show that subcloud MSE in the nonconvective regions may differ substantially between land and ocean but is uniform across latitudes in convective regions even on a daily timescale. This result holds also in CMIP5 model simulations of past cold and future warm climates. Furthermore, the distribution of rainfall amount in subcloud MSE is very similar over land and ocean with the peak at 343 J/g and a half width at half maximum of 3 J/g. Our results demonstrate that the horizontally uniform free tropospheric temperature forces the highest subcloud MSE values to be similar over land and ocean