September 2025:
New publication:
Savita, A., Kjellsson, J., Latif, M., Nnamchi, H. C., & Wahl, S. (2025). Causes of Eurasian winter-cooling during the late 20th and early 21st century. Geophysical Research Letters, 52, e2024GL114140.
Summary:
Using atmospheric-model experiments, we find that intrinsic atmospheric variability is the main driver of the recent pronounced wintertime surface cooling over Eurasia during 1993鈥�2013. More specifically, the leading mode of atmospheric circulation variability over the Atlantic sector known as the North Atlantic Oscillation (NAO) was responsible for the largest portion of the Eurasian cooling. The sea-surface temperature (SST) of the Pacific Ocean also contributed to the cooling, but to a lesser degree than the NAO, whereas the North Atlantic SST had only a modest influence. This study suggests that decadal predictability of Eurasian surface climate could be limited.
The Probability Density Function of the DJF-surface air temperature (SAT) trend for the period 1993鈥�2014, averaged over the region between 60掳E鈥�120掳E and 45掳N鈥�60掳N (box shown in Figure ). (a) Control ensemble (Ctl), the gray bars represent the estimates from the individual members, the blue line is the fitted distribution based on kernel density estimation, and the solid black line depicts the ERA5-SAT trend. The ensemble mean of the Ctl ensemble is indicated by the solid orange line, with the orange dashed lines representing the 卤standard deviation (sigma). (b), Fitted Probability Density Functions from the Ctl and the noIPV and noAMV ensembles. The numbers in the legend are the ensemble mean and standard deviation obtained by applying a bootstrapping method. Panels (c, d) show the ensemble mean difference between Ctl and noIPV and noAMV, respectively. Dots indicate statistically significance at the 95% confidence level.
September 2025:
New publication:
Liu, Y., Kjellsson, J., Savita, A., and Park, W.: Impact of horizontal resolution and model time step on European precipitation extremes in the OpenIFS 43r3 atmospheric model, Geosci. Model Dev., 18, 5435鈥�5449, , 2025
Abstract:
Events of extreme precipitation pose a hazard to many parts of Europe but are typically not well represented in climate models. Here, we evaluate daily extreme precipita- tion over Europe during 1982鈥�2019 in observations (GPCC), reanalysis (ERA5), and a set of atmosphere-only simulations at low (100 km), medium (50 km), and high (25 km) horizon- tal resolution and also at different time steps (i.e., 60, 30, and 15 min) using low resolution (100 km) with identical verti- cal resolutions using OpenIFS (version 43r3). We find that both OpenIFS simulations and reanalysis underestimate the rates of extreme precipitation compared to observations. The biases are largest for the lowest resolution (100 km) and de- crease with higher-horizontal-resolution (50 and 25 km) sim- ulations in all seasons. The sensitivity to horizontal resolu- tion is particularly high in mountain regions (such as the Alps, Scandinavia, Iberian Peninsula), likely linked to the sensitivity of vertical velocity to the representation of to- pography. The sensitivity of precipitation to model resolu- tion increases dramatically with increasing percentiles, with modest biases in the 70th鈥�80th percentile range and large bi- ases above the 99th percentile range. We also find that pre- cipitation above the 99th percentile mostly consists of large- scale precipitation (鈭� 80 %) in winter, while in summer it is mostly large-scale precipitation in northern Europe (鈭� 70 %) and convective precipitation in southern Europe (鈭�70%). Convective precipitation is more sensitive to model time step than to horizontal resolution. Large-scale precipitation in- creases significantly with both higher horizontal resolution and a shorter model time step.
European averaged precipitation amounts and RMSEs (referenced to GPCC) for European total precipitation at different percentile ranges (70th鈥�80th, 80th鈥�90th, 90th鈥�95th, 95th鈥�99th, 99th鈥�99.5th, 99.5th鈥�99.9th, and > 99.9th percentile) in ERA5 (black) and OpenIFS simulations (LR60m: magenta, LR30m: orange, LR: red, MR: blue, HR: purple) during 1982鈥�2019. Cross marks are the precipitation amounts. Dots are the RMSE values, and error bars are the 95 % CI.
September 2025:
New publication:
Sanderson, B. M., Brovkin, V., Fisher, R. A., Hohn, D., Ilyina, T., Jones, C. D., Koenigk, T., Koven, C., Li, H., Lawrence, D. M., Lawrence, P., Liddicoat, S., MacDougall, A. H., Mengis, N., Nicholls, Z., O'Rourke, E., Romanou, A., Sandstad, M., Schwinger, J., S茅f茅rian, R., Sentman, L. T., Simpson, I. R., Smith, C., Steinert, N. J., Swann, A. L. S., Tjiputra, J., and Ziehn, T.: flat10MIP: an emissions-driven experiment to diagnose the climate response to positive, zero and negative CO2 emissions, Geosci. Model Dev., 18, 5699鈥�5724, , 2025.
Short Summary:
This study investigates how climate models warm in response to simplified carbon emissions trajectories, refining the understanding of climate reversibility and commitment. Metrics are defined for warming response to cumulative emissions and for the cessation of emissions or ramp-down to net-zero and net-negative levels. Results indicate that previous concentration-driven experiments may have overstated the Zero Emissions Commitment due to emissions rates exceeding historical levels.
Summary results for global mean surface temperature (GMST) response in the trial flat10MIP. Colored lines indicate temperature change from (a) pre-industrial levels to (b, c) T100鈥墆r (the average temperature in years 91鈥�110 in esm-flat10) in each of the participating ESMs. Shaded regions refer to the simple climate models' probabilistic distribution ranging from the 10th to 90th percentiles. This distribution is shown as violin plots for the last time step of each scenario, where the shading shows the full range of results and the vertical line indicates the 10th鈥�90th percentiles, with the median in the center. A 20-year moving average is applied to all time series.
September 2025:
Oral Examination of Estela Monteiro鈥檚 doctoral thesis
Summary:
Over the past century, human activities have caused strong alterations to the climate system. Such changes have been confidently attributed to human activities, and international political efforts have attempted to avoid further impacts by limiting global warming levels. In this sense, highly ambitious emissions mitigation scenarios are a desirable future to be aimed for, with a current need for a rapid political and societal move towards such. Despite a clear benefit, and necessity, of reaching such scenarios, uncertainty remains regarding potential system responses as the importance of carbon forcing decreases. Even if these uncertainties should not halt decision-making towards such kinds of future scenarios, a better understanding of the expected responses will allow society to properly assess needed and possible outcomes from our current, and past, decisions.
A first result of this thesis shows that varying aerosols spatial implementation in an intermediate complexity model allows for a considerable surface temperature variability, as well as variability in individual components鈥�, such as land carbon uptake and ocean heat uptake, contributions to the warming or cooling of the system. Triggered feedbacks due to aerosols implementations can have a crucial impact in assessing highly ambitious mitigation scenarios, and should be accounted for by the tools used to study such scenarios.
Furthermore, the presentation introduced the use of a new framework (FROT: Framework for Radiative cOntributions to Temperature responses), to comprehensively assess the temperature variability simulated by distinct models for different ambitious mitigation future scenarios, and to account for individual contributions from different climate components. The findings corroborate the need for including forcings and transient responses when assessing climate development, and show that different sets of climate contributions allow for temperature stabilisation.
Finally, the work here has focused on techniques with the potential to remove carbon dioxide from the atmosphere. It found that for centennial timescales dissolved ocean oxygen, in terms of its total global content, is unable to return to pre-industrial levels. This response, however, is highly dependent on the region and depth considered. While oxygen solubility appears to play a role in controlling the responses in the upper ocean, the findings indicate that apparent oxygen use dominates the simulated oxygen development, especially related to changes in circulation and ventilation.
The reported findings widen the understanding of the range of expected responses and components development under highly ambitious emissions mitigation scenarios, either by reducing or by clarifying different uncertainties expected in the assessment, analysis and potential real-world outcomes of these scenarios. The conclusions here can provide meaningful information to not only improve science understanding, but also inform political and societal decision-making processes.
September 2025:
New publication:
Mojib Latif (Hg). 2025: 240 S
Die vielen Gesichter der Freiheit
Bedroht der Klimawandel die Freiheit? Was bedeuten freie M盲rkte f眉r den Menschen? Ist das geeinte Europa ein Hort der Freiheit? Macht uns die K眉nstliche Intelligenz wirklich frei? Wie frei sind Gedanken in totalit盲ren Systemen? Und l盲sst sich Freiheit 眉berhaupt messen? Diesen und anderen Fragen gehen renommierte Forscherinnen und Forscher aus Natur- und Geisteswissenschaften im neuen Band der Essay-Reihe der Akademie der Wissenschaften in Hamburg nach. Sie zeigen: Der Begriff der Freiheit birgt viele Dimensionen.
August 2025:
New publication:
Cai, W., C. Reason, E. Mohino, B. Rodr铆guez de Fonseca, J. Malherbe, A. Santoso, X. Li,
H. Chikoore, H. C. Nnamchi, M. J. McPhaden, N. Keenlyside, A. Taschetto, L. Wu, B.
Ng, Y. Liu, T. Geng, K. Yang, G. Wang, F. Jia, X. Lin, S. Li, Y. Yan, J. Wang, L. Zhang,
Z. Li, W. Pokam, L Zhou, X. Zhang, F. Engelbrecht (2025). Impact of El Ni帽o-Southern
Oscillation on African climate. Nature Reviews Earth & Environment. 6, 503520.
.
Abstract:
The El Ni帽o鈥揝outhern Oscillation (ENSO) 鈥� describing shifts between warm El Ni帽o and cold La Ni帽a phases 鈥� has a substantial effect on the global climate. In this Review, we outline the mechanisms and climate impacts of ENSO in Africa, focusing on rainfall. ENSO鈥檚 influence varies strongly by season, region, phase, event and decade, highlighting complex dynamics and asymmetries. Although difficult to generalize, key characteristics include: anomalies across the Sahel in July鈥揝eptember, related to the tropospheric temperature mechanism; a strong dipole in anomalies between eastern and southern Africa during October鈥揇ecember (the short rain reason) and December鈥揊ebruary, linked to interactions with the Indian Ocean Dipole and Indian Ocean Basin mode, respectively; and anomalies over southern Africa (with possible indications of opposite anomalies over East Africa) during March鈥揗ay (the long rain season), associated with continuation of the Indian Ocean Basin mode. These teleconnections tend to be most pronounced for East Pacific El Ni帽o and Central Pacific La Ni帽a events, as well as during decades when interbasin interactions are strongest. Although challenging to simulate, climate models suggest that these impacts will strengthen in the future, manifesting as an increased frequency of ENSO-related dry and wet extremes. Given the reliance of much of Africa on rain-fed agriculture, resolving these relationships is vital, necessitating realistic simulation of regional circulations, ENSO and its interbasin interactions.
Mechanisms of impact of the El Ni帽o鈥揝outhern Oscillation (ENSO) on African climate
a, Schematic representation of the tropospheric temperature mechanism, whereby El Ni帽o triggers a Gill鈥揗atsuno response, the atmospheric Kelvin waves (eastward arrows) of which increase atmospheric stability outside the Pacific, suppressing atmospheric convection (and thereby reducing rainfall) over the Sahel (blue downward arrows). b, Schematic representation of the Indian Ocean influence in September鈥揙ctober鈥揘ovember (SON), wherein El Ni帽o weakens the Walker circulation (grey arrows), leading to a positive Indian Ocean Dipole (IOD) that enhances short rains in East Africa through increasing convergence toward East Africa. c, As in panel b, but during December鈥揓anuary鈥揊ebruary (DJF) when El Ni帽o-related Walker circulation weakening causes Indian Ocean Basin mode (IOB) warming that reduces the land鈥搊cean pressure gradient, and lowers the southward extension of seasonal flow convergence and the equatorward shift of the South Indian Convergence Zone equatorward compared with the climatology (light versus dark arrows), in turn, increasing rainfall in northeast southern Africa but decreasing it in southeast southern Africa. d, Schematic representation of the Atlantic Ocean influence in June鈥揓uly鈥揂ugust (JJA), whereby El Ni帽o-related weakening of the Walker circulation drives development of an Atlantic Ni帽a that shifts the tropical rain belt northward, decreasing rainfall over the Guinea coast but increasing rainfall over the Sahel; this increased Sahel rainfall competes with rainfall reductions via the tropospheric temperature mechanism in panel a. Thus, El Ni帽o influences African climate through troposphere temperature warming and concurrent modes of variability in the Indian Ocean and the Atlantic oceans.
