Epidemic waves in relief against a structured climatic background

<p>For directly transmissible infectious diseases, seasonality in the course of epidemics may manifest in four major ways: susceptibility of the hosts, their individual and collective behavior, transmissibility of the pathogen, and survival of the latter under evolving environmental conditions. Mechanisms and concepts are not finally settled, notably in a pandemic setting. Climatic seasonality by itself is an aggregate, structured phenomenon that provides a spatially distributed background to the epidemic outbreak and its evolution at multiple timescales. Using advanced methods of data and systems analysis, including epidemiological and climate modeling, the RKI data of the COVID-19 epidemic curve for Berlin and a five-parameter climate data set of the nearby station Lindenberg (Mark) are analyzed in daily resolution over the period March 2020 to October 2021. Aimed to identify extrinsic impacts due to organized intraseasonal climate dynamics, as seen in sunshine duration and surface climate (pressure, temperature, humidity, wind), on intrinsic dynamics of the epidemic system, an established (SEIR) model of the latter and modifications thereof have been subjected to in-depth studies with a view on both genesis and timing of epidemic waves and their potential synchronization with climatic background dynamics. Starting with a case study for the spring 2020 period of shutdown, which unveils remarkable synchronies with the seasonal transition, estimates are given and applied to the follow-up period of individual and combined impacts of climate variables on the SEIR model in different oscillatory (non-equilibrium, lately endemic) regimes of operation.</p>

Stratospheric Ozone Response to Sulfate Aerosol and Solar Dimming Climate Interventions based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) Simulations

This study assesses the impacts of sulfate aerosol intervention (SAI) and solar dimming on stratospheric ozone based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) experiments, called G6sulfur and G6solar. For G6sulfur the stratospheric sulfate aerosol burden is increased to reflect some of the incoming solar radiation back into space in order to cool the surface climate, while for G6solar the global solar constant is reduced to achieve the same goal. The high emissions scenario SSP5-8.5 is used as the baseline experiment and surface temperature from the medium emission scenario SSP2-4.5 is the target. Based on three out of six Earth System Models (ESMs) that include interactive stratospheric chemistry, we find significant differences in the ozone distribution between G6solar and G6sulfur experiments compared to SSP5-8.5 and SSP2-4.5, which differ by both region and season. Both SAI and solar dimming methods reduce incoming solar insolation and result in tropospheric temperatures comparable to SSP2-4.5 conditions. G6sulfur increases the concentration of absorbing sulfate aerosols in the stratosphere, which increases lower tropical stratospheric temperatures by between 5 to 13 K for six different ESMs, leading to changes in stratospheric transport. The increase of the aerosol burden also increases aerosol surface area density, which is important for heterogeneous chemical reactions. The resulting changes in ozone include a significant reduction of total column ozone (TCO) in the Southern Hemisphere polar region in October of 10 DU at the onset and up to 20 DU by the end of the century. The relatively small reduction in TCO for the multi-model mean in the first two decades results from variations in the required sulfur injections in the models and differences in the complexity of the chemistry schemes, with no significant ozone loss for 2 out of 3 models. The decrease in the second half of the 21st century counters increasing TCO between SSP2-4.5 and SSP5-8.5 due to the super-recovery resulting from increasing greenhouse gases. In contrast, in the Northern Hemisphere (NH) high latitudes, only a small initial decline in TCO is simulated, with little change in TCO by the end of the century compared to SSP5-8.5. All models consistently simulate an increase in TCO in the NH mid-latitudes up to 20 DU compared to SSP5-8.5, in addition to 20 DU increase resulting from increasing greenhouse gases between SSP2-4.5 and SSP5-8.5. G6solar counters zonal wind and tropical upwelling changes between SSP2-4.5 and SSP5-8.5 but does not change stratospheric temperatures. Solar dimming results in little change in TCO compared to SSP5-8.5 and does not counter the effects of the ozone super-recovery. Only in the tropics, G6solar results in an increase of TCO of up to 8 DU compared to SSP2-4.5, which may counter the projected reduction due to climate change in the high forcing future scenario. This work identifies differences in the response of SAI and solar dimming on ozone, which are at least partly due to differences and shortcomings in the complexity of aerosol microphysics, chemistry, and the description of ozone photolysis in the models. It also identifies that solar dimming, if viewed as an analog to SAI using a predominantly scattering aerosol, would, for the most part, not counter the potential harmful increase in TCO beyond historical values induced by increasing greenhouse gases.

Tropical Pacific relationships with Southern Hemisphere atmospheric circulation and Antarctic climate

<p>Significant trends in high-latitude Southern Hemisphere atmospheric circulation and surface climate have been observed over recent decades, which are likely linked to teleconnections from the tropics. This study investigates how a recent shift in tropical Pacific climate toward increased La Niña conditions has influenced the atmospheric circulation and surface climate across the high southern latitudes, and how variations in the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influence the surface climate of Antarctica. Over 1979-2014, significant cooling of eastern tropical Pacific sea surface temperatures (SSTs) is detected in all seasons. The eastern tropical Pacific cooling is associated with: (1) an intensified Walker Circulation during austral summer and autumn; (2) a weakened South Pacific Hadley cell and sub-tropical jet during autumn; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during both summer and autumn. Observed cooling in the eastern tropical Pacific is linearly congruent with 60-80% of the observed positive zonal-mean zonal wind trend between 50 and 60°S during summer (~35% of the interannual variability), and around half of the positive zonal-mean zonal wind trend during autumn (~15% of the interannual variability), the latter being most marked over the South Pacific. Although previous studies have linked the strengthening of the tropospheric westerlies during summer and autumn to ozone depletion, results from this study indicate poleward momentum fluxes and strengthened lower-tropospheric baroclinicity associated with eastern tropical Pacific cooling also help to maintain a strengthened mid-latitude jet through the 21st century, especially across the South Pacific. The La Niña shift in tropical Pacific SSTs is also significantly related to several changes in Antarctic surface climate. During autumn, a regional pattern of cooling occurred along coastal East Antarctica after 1979, with the rate of cooling increasing at Novolazarevskaya, Syowa, Casey, and Dumont d’Urville stations, while the rate of cooling decreased at Mawson and Davis stations. It is shown that regional circulation changes associated with tropical Pacific teleconnections project strongly onto the regional nature of the cooling trends, with 40% of the cooling at Novolazarevskaya and Syowa linearly congruent with the increased La Niña conditions, and more than 60% of the cooling at Casey and Dumont d’Urville linearly congruent with increased SSTs over the western tropical Pacific. The autumn La Niña pattern is associated with an anomalous anticyclone over the high-latitude South Atlantic that strengthens southwesterly winds and cold air advection across Novolazarevskaya and Syowa. Meanwhile, warming over the western tropical Pacific is associated with a meridional wavetrain stretching from southwest Australia to eastern East Antarctica and anomalous poleward momentum fluxes that locally strengthen westerly / southwesterly winds along and offshore of Casey and Dumont d’Urville, amplifying the cooling seen there. During spring, a physical mechanism linking the West Antarctic warming to the tropical Pacific is identified. Spring warming of West Antarctica and the Antarctic Peninsula is associated with a significant increase in tropical deep convection on the poleward side of the South Pacific Convergence Zone. The increase in deep convection is strongest during September, during which a meridional wavetrain is seen over the western South Pacific with anomalous cyclonic circulation over the Ross Sea and warm, northerly flow to western West Antarctica. During October, the wavetrain propagates east toward the Antarctic Peninsula as the climatological background westerlies strengthen, which leads to increased warm, northerly flow to the western Antarctic Peninsula. Observed increases in deep convection along the South Pacific Convergence Zone during September are linearly congruent with over half of the observed circulation and surface warming trends seen across the West Antarctic region during September and October. Lastly, this study finds a spatial dependency of the ENSO and SAM impact on Antarctic Peninsula climate. Variability in ENSO has a persistent and statistically significant relationship with western Peninsula climate only, which is strongest during the winter and spring seasons. Meanwhile, variability in the SAM dominates climate across the northeastern Peninsula during all seasons through the Föhn effect, and northeast Peninsula relationships with the tropics are relatively weak. In autumn, when widespread warming of the Antarctic Peninsula has been linked to the tropics, this study finds the tropical connection to be weak and statistically insignificant on interannual timescales, and regional circulation associated with the SAM dominates climate variability across the Peninsula during autumn.</p>

Tropical Pacific relationships with Southern Hemisphere atmospheric circulation and Antarctic climate

<p>Significant trends in high-latitude Southern Hemisphere atmospheric circulation and surface climate have been observed over recent decades, which are likely linked to teleconnections from the tropics. This study investigates how a recent shift in tropical Pacific climate toward increased La Niña conditions has influenced the atmospheric circulation and surface climate across the high southern latitudes, and how variations in the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influence the surface climate of Antarctica. Over 1979-2014, significant cooling of eastern tropical Pacific sea surface temperatures (SSTs) is detected in all seasons. The eastern tropical Pacific cooling is associated with: (1) an intensified Walker Circulation during austral summer and autumn; (2) a weakened South Pacific Hadley cell and sub-tropical jet during autumn; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during both summer and autumn. Observed cooling in the eastern tropical Pacific is linearly congruent with 60-80% of the observed positive zonal-mean zonal wind trend between 50 and 60°S during summer (~35% of the interannual variability), and around half of the positive zonal-mean zonal wind trend during autumn (~15% of the interannual variability), the latter being most marked over the South Pacific. Although previous studies have linked the strengthening of the tropospheric westerlies during summer and autumn to ozone depletion, results from this study indicate poleward momentum fluxes and strengthened lower-tropospheric baroclinicity associated with eastern tropical Pacific cooling also help to maintain a strengthened mid-latitude jet through the 21st century, especially across the South Pacific. The La Niña shift in tropical Pacific SSTs is also significantly related to several changes in Antarctic surface climate. During autumn, a regional pattern of cooling occurred along coastal East Antarctica after 1979, with the rate of cooling increasing at Novolazarevskaya, Syowa, Casey, and Dumont d’Urville stations, while the rate of cooling decreased at Mawson and Davis stations. It is shown that regional circulation changes associated with tropical Pacific teleconnections project strongly onto the regional nature of the cooling trends, with 40% of the cooling at Novolazarevskaya and Syowa linearly congruent with the increased La Niña conditions, and more than 60% of the cooling at Casey and Dumont d’Urville linearly congruent with increased SSTs over the western tropical Pacific. The autumn La Niña pattern is associated with an anomalous anticyclone over the high-latitude South Atlantic that strengthens southwesterly winds and cold air advection across Novolazarevskaya and Syowa. Meanwhile, warming over the western tropical Pacific is associated with a meridional wavetrain stretching from southwest Australia to eastern East Antarctica and anomalous poleward momentum fluxes that locally strengthen westerly / southwesterly winds along and offshore of Casey and Dumont d’Urville, amplifying the cooling seen there. During spring, a physical mechanism linking the West Antarctic warming to the tropical Pacific is identified. Spring warming of West Antarctica and the Antarctic Peninsula is associated with a significant increase in tropical deep convection on the poleward side of the South Pacific Convergence Zone. The increase in deep convection is strongest during September, during which a meridional wavetrain is seen over the western South Pacific with anomalous cyclonic circulation over the Ross Sea and warm, northerly flow to western West Antarctica. During October, the wavetrain propagates east toward the Antarctic Peninsula as the climatological background westerlies strengthen, which leads to increased warm, northerly flow to the western Antarctic Peninsula. Observed increases in deep convection along the South Pacific Convergence Zone during September are linearly congruent with over half of the observed circulation and surface warming trends seen across the West Antarctic region during September and October. Lastly, this study finds a spatial dependency of the ENSO and SAM impact on Antarctic Peninsula climate. Variability in ENSO has a persistent and statistically significant relationship with western Peninsula climate only, which is strongest during the winter and spring seasons. Meanwhile, variability in the SAM dominates climate across the northeastern Peninsula during all seasons through the Föhn effect, and northeast Peninsula relationships with the tropics are relatively weak. In autumn, when widespread warming of the Antarctic Peninsula has been linked to the tropics, this study finds the tropical connection to be weak and statistically insignificant on interannual timescales, and regional circulation associated with the SAM dominates climate variability across the Peninsula during autumn.</p>

Decadal climate predictions with the Canadian Earth System Model version 5 (CanESM5)

The Canadian Earth System Model version 5 (CanESM5) developed at Environment and Climate Change Canada's Canadian Centre for Climate Modelling and Analysis (CCCma) is participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). A 40-member ensemble of CanESM5 retrospective decadal forecasts (or hindcasts) is integrated for 10 years from realistic initial states once a year during 1961 to the present using prescribed external forcing. The results are part of CCCma's contribution to the Decadal Climate Prediction Project (DCPP) component of CMIP6. This paper evaluates CanESM5 large ensemble decadal hindcasts against observational benchmarks and against historical climate simulations initialized from pre-industrial control run states. The focus is on the evaluation of the potential predictability and actual skill of annual and multi-year averages of key oceanic and atmospheric fields at regional and global scales. The impact of initialization on prediction skill is quantified from the hindcasts decomposition into uninitialized and initialized components. The dependence of potential and actual skill on ensemble size is examined. CanESM5 decadal hindcasts skillfully predict upper-ocean states and surface climate with a significant impact from initialization that depend on climate variable, forecast range, and geographic location. Deficiencies in the skill of North Atlantic surface climate are identified and potential causes discussed. The inclusion of biogeochemical modules in CanESM5 enables the prediction of carbon cycle variables which are shown to be potentially skillful on decadal timescales, with a strong long-lasting impact from initialization on skill in the ocean and a moderate short-lived impact on land.

The Brewer–Dobson circulation in CMIP6

The Brewer–Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to simulate surface climate variability and change adequately. For the first time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in the sign of BDC trends between observations and models in the middle and upper stratosphere. Nevertheless, the large uncertainty in the observational trend estimates opens the door to compatibility. In particular, when accounting for the limited sampling of the observations, model and observational trend error bars overlap in 40 % of the simulations with available output. The increasing CO2 simulations feature an acceleration of the BDC but reveal a large spread in the middle-to-upper stratospheric trends, possibly related to the parameterized gravity wave forcing. The very close connection between the shallow branch of the residual circulation and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.

A high-resolution downscaled CMIP6 projections dataset of essential surface climate variables over the globe coherent with the ERA5-Land reanalysis for climate change impact assessments

A high-resolution climate projections dataset is obtained by statistically downscaling climate projections from the CMIP6 experiment using the ERA5-Land reanalysis from the Copernicus Climate Change Service. This global dataset has a spatial resolution of 0.1°x 0.1°, comprises 5 climate models and includes two surface daily variables at monthly resolution: air temperature and precipitation. Two greenhouse gas emissions scenarios are available: one with mitigation policy (SSP126) and one without mitigation (SSP585). The downscaling method is a Quantile Mapping method (QM) called the Cumulative Distribution Function transform (CDF-t) method that was first used for wind values and is now referenced in dozens of peer-reviewed publications. The data processing includes quality control of metadata according to the climate modelling community standards and value checking for outlier detection.