Hemispheric asymmetries in recent changes of the stratospheric circulation

Despite the expected opposite effects of ozone recovery, the stratospheric Brewer-Dobson circulation (BDC) has been found to weaken in the Northern hemisphere (NH) relative to the Southern hemisphere (SH) in recent decades, inducing substantial effects on chemical composition. We investigate hemispheric asymmetries in BDC changes since about 2000 in simulations with the transport model CLaMS driven with different reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and contrast those to a suite of free-running climate model simulations. We find that age of air increases robustly in the NH stratosphere relative to the SH in all reanalyses considered. Related nitrous oxide changes agree well between reanalysis-driven simulations and satellite measurements, providing observational evidence for the hemispheric asymmetry in BDC changes. Residual circulation metrics further show that the composition changes are caused by structural BDC changes related to an upward shift and strengthening of the deep BDC branch, resulting in longer transit times, and a downward shift and weakening shallow branch in the NH relative to the SH. All reanalyses agree on this mechanism. Although climate model simulations show that ozone recovery will lead to overall reduced circulation and age of air trends, the hemispherically asymmetric signal in circulation trends is small compared to internal variability. Therefore, the observed circulation trends over the recent past are not in contradiction to expectations from climate models. Furthermore, the hemispheric asymmetry in BDC trends imprints on the composition of the lower stratosphere and the signal might propagate into the troposphere, potentially affecting composition down to the surface.

Another Record: Ocean Warming Continues through 2021 despite La Niña Conditions

The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.

Assessing the consequences of including aerosol absorption in potential Stratospheric Aerosol Injection Climate Intervention Strategies

Theoretical Stratospheric Aerosol Intervention (SAI) strategies model the deliberate injection of aerosols or their precursors into the stratosphere thereby reflecting incident sunlight back to space and counterbalancing a fraction of the warming due to increased concentrations of greenhouse gases. This cooling mechanism is known to be relatively robust through analogues from explosive volcanic eruptions which have been documented to cool the climate of the Earth. However, a practical difficulty of SAI strategies is how to deliver the injection high enough to ensure dispersal of the aerosol within the stratosphere on a global scale. Recently, it has been suggested that including a small amount of absorbing material in a dedicated 10-day intensive deployment might enable aerosols or precursor gases to be injected at significantly lower, more technologically-feasible altitudes. The material then absorbs sunlight causing a localised heating and ‘lofting’ of the particles, enabling them to penetrate into the stratosphere. Such self-lofting has recently been observed following the intensive wildfires in 2019–2020 in south east Australia, where the resulting absorbing aerosol penetrated into the stratosphere and was monitored by satellite instrumentation for many months subsequent to emission. This study uses the fully coupled UKESM1 climate model simulations performed for the Geoengineering Model Intercomparison Project (GeoMIP) and new simulations where the aerosol optical properties have been adjusted to include a moderate degree of absorption. The results indicate that partially absorbing aerosols i) reduce the cooling efficiency per unit mass of aerosol injected, ii) increase deficits in global precipitation iii) delay the recovery of the stratospheric ozone hole, iv) disrupt the Quasi Biennial Oscillation when global mean temperatures are reduced by as little as 0.1 K, v) enhance the positive phase of the wintertime North Atlantic Oscillation which is associated with floods in Northern Europe and droughts in Southern Europe. While these results are dependent upon the exact details of the injection strategies and our simulations use ten times the ratio of black carbon to sulfate that is considered in the recent intensive deployment studies, they demonstrate some of the potential pitfalls of injecting an absorbing aerosol into the stratosphere to combat the global warming problem.

Extreme precipitation on consecutive days occurs more often in a warming climate

Extreme precipitation occurring on consecutive days may substantially increase the risk of related impacts, but changes in such events have not been studied at a global scale. Here we use a unique global dataset based on in situ observations and multi-model historical and future simulations to analyse the changes in the frequency of extreme precipitation on consecutive days (EPCD). We further disentangle the relative contributions of variations in precipitation intensity and temporal correlation of extreme precipitation, to understand the processes that drive the changes in EPCD. Observations and climate model simulations show that the frequency of EPCD is increasing in most land regions, in particular in North America, Europe and the Northern Hemisphere high latitudes. These increases are primarily a consequence of increasing precipitation intensity, but changes in the temporal correlation of extreme precipitation regionally amplify or reduce the effects of intensity changes. Changes are larger in simulations with a stronger warming signal, suggesting that further increases in EPCD are expected for the future under continued climate warming.

Parameterized orographic gravity wave drag in CMIP6 models, distribution, variability, trends and intermodel spread

<p>Internal gravity waves (GWs) are a naturally occurring and intermittent phenomenon in the atmosphere. GWs can propagate horizontally and vertically and are important for atmospheric dynamics, influencing the atmospheric thermal and dynamical structure. Research on GWs is connected with some of the most challenging issues of Earth climate and atmospheric science. Consideration of GW-related processes is necessary for a proper description and modelling of the middle and upper atmosphere. However, as GWs exist on scales from a few to thousands of kilometers, they cannot be fully resolved by general circulation models (GCMs) and hence have to be parameterized. Although recently satellite and reanalysis datasets with improved resolution and novel analysis methods together with high-resolution atmospheric models have been tightening the constraints for GW parameterizations in GCMs, the parameterized GW effects still bear a significant margin of uncertainty.</p> <p>To quantify this uncertainty, we analyze the three-dimensional distribution and interannual variability of orographic gravity wave drag (OGWD) in chemistry-climate model simulations. For this, we use a set of AMIP simulations produced within the CMIP6 activity. In particular, we focus on the intermodel spread in the vertical and horizontal OGWD distribution. The different models generaly agree on the areas of the OGWD hotspots. However, in all these regions we find considerable intermodel differences in OGWD magnitude as well as in the altitude of the strongest GW dissipation. In this presentation, we show our findings and discuss possible explanations for the intermodel differences, like different parametrization schemes and choices of tunable parameters.</p>

Stratosphere-Mesosphere exchange: Long term changes and drivers

<p>The meridional overturning mass circulation in the middle atmosphere, i.e. the Brewer-Dobson circulation (BDC), was first discovered before decades based on the distribution of trace gases and a basic analytical concept of BDC has been derived using the transformed Eulerian mean equations. Since then, BDC is usually defined as consisting of a diffusive part, and an advective, residual mean circulation. In the vertical, BDC is separated into two branches – a shallow branch in the lower stratosphere and a deep branch higher in the middle atmosphere.<br />Climate model simulations robustly show that the advective BDC part accelerates in connection to the greenhouse gas-induced climate change and this acceleration dominates the middle atmospheric changes in climate model projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwelling across the tropopause, which measures the amount of mass advected by residual circulation from the troposphere to the stratosphere per unit of time. The upper BDC branch received considerably less research attention than its shallow part, but features some striking phenomenon in the terrestrial atmosphere. It couples the stratosphere and mesosphere and is also responsible for a large portion of interhemispheric transport and coupling in the middle atmosphere.<br />In our research, for the first time, we produce a conceptual study of the advective stratosphere-mesosphere exchange. The analysis of advective exchange of mass between the stratosphere and mesosphere, i.e. the advective mass transport across the stratopause represents another step towards a better understanding of the structure of the upper BDC part and at the same time provides valuable insights into the relatively little-explored stratopause region. We investigate the variability and trends in mass fluxes from the stratosphere to the mesosphere and vice versa based on data from the EMAC-L90 model CCMI-1 simulation for the period 1960-2100. We develop an analytical method that allows us to attribute the changes of transport to causative factors such as acceleration of residual circulation, variable height of the stratopause, change of a geometric shape of the stratopause and changes in width of the upwelling and downwelling regions. The main driver of the increasing mass exchange between the stratosphere and the mesosphere is the faster circulation, however, the other terms are not negligible. The derived methodology offers the possibility of using an analogous procedure also for the tropopause in the future.</p>

El Niño-like conditions and seasonal aridity in the Indo-Pacific Warm Pool during the Younger Dryas

Island South-East Asia (ISEA) is a highly humid region and hosts the world’s largest tropical peat deposits. Most of this peat accumulated relatively recently during the Holocene, suggesting a generally drier and/or more seasonal climate during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period has received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the very end of the last deglacial period, at 12 ka BP. A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy-evidence, CESM1 simulates overall drier conditions at 12 ka BP. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality at 12 ka BP is mainly the result of a combination of three factors: 1) large orbital insolation difference between summer and winter in contrast to the LGM and the present day; 2) a stronger winter monsoon caused by a larger interhemispheric thermal gradient in boreal winters; and 3) a major reorganization of the Walker Circulation with an inverted land-sea circulation with a complete breakdown of deep convection over ISEA. The altered atmospheric circulation mean state during winters led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Inter-Tropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period.

Estimating unprecedented extremes in UK summer daily rainfall

The UNSEEN (UNprecedented Simulated Extremes using ENsembles) method involves using a large ensemble of climate model simulations to increase the sample size of rare events. Here we extend UNSEEN to focus on intense summertime daily rainfall, estimating plausible rainfall extremes in the current climate. To address modelling limitations simulations from two climate models were used; an initialised 25km global model that uses parametrized convection, and a dynamically downscaled 2.2km model that uses explicit convection. In terms of the statistical characteristics that govern very rare return periods, the models are not significantly different from the observations across much of the UK. Our analysis provides more precise estimates of 1000-year return levels for extreme daily rainfall, reducing sampling uncertainty by 70-90% compared to using observations alone. This framework enables observed daily storm profiles to be adjusted to more statistically robust estimates of extreme rainfall. For a damaging storm in July 2007 which led to surface water flooding, we estimate physically plausible increases in the total daily rainfall of 50 – 100%. For much of the UK the annual chance of record-breaking daily summertime rainfall is estimated to be around 1% per year in the present-day climate. Analysis of the dynamical states in our UNSEEN events indicates that heavy daily rainfall is associated with a southward displaced and meandering North Atlantic jet stream, increasing the advection of warm moist air from across Southern Europe and the Mediterranean, and intensifying extratropical storms. This work represents an advancement in the use of climate modelling for estimating present-day climate hazards and outlines a framework for applying UNSEEN at higher spatial and temporal resolutions.