Bjerknes Climate Prediction Unit

Cai, W., Reason, C., Mohino, E., …. , N.S. Keenlyside et al.  2025: Climate impacts of the El Niño–Southern Oscillation in Africa. Nat Rev Earth Environ. https://doi.org/10.1038/s43017-025-00705-7 Summary: The El Niño–Southern Oscillation (ENSO) — describing shifts between warm El Niño and cold La Niña phases — 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’s 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–September, related to the tropospheric temperature mechanism; a strong dipole in anomalies between eastern and southern Africa during October–December (the short rain reason) and December–February, 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–May (the long rain season), associated with continuation of the Indian Ocean Basin mode. These teleconnections tend to be most pronounced for East Pacific El Niño and Central Pacific La Niña 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. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Hu, A., I. Richter, Y. Okumura, N. Burls, N. Keenlyside, R. Parfitt, K. Bellomo, A. Bellucci, R. Farneti, A. Fedorov, B. S. Ferster, C. He, Q. Li, D. Matei 2025: Unraveling the Complexity of Global Climate Dynamics: Interactions among El Niño–Southern Oscillation, Atlantic Meridional Overturning Circulation, and Tropical Basins Across Different Timescales. Ocean-Land-Atmos Res.. https://spj.science.org/doi/10.34133/olar.0096 Summary: Tropical basin interactions and the climatic linkages between mid-to-high latitudes and the tropics are active research areas. These interactions include the influence of El Niño–Southern Oscillation (ENSO) on the tropical Indian and Atlantic oceans, the feedback from these basins on ENSO, the influence of the tropics on mid-to-high-latitude climates, and the feedback from higher latitudes on tropical climate variability. This review summarizes the current understanding of these relationships and key underlying physical processes. In particular, we assessed the current knowledge of tropical variability and the interactions between the tropics and extratropics, including ENSO variability and diversity, the influence of ENSO on the tropical Atlantic and Indian Oceans, interactions among tropical basins on different timescales, variability in the Atlantic meridional overturning circulation (AMOC), the effect of tropical basins on the AMOC, the relationship between the AMOC and Atlantic multidecadal variability, the influence of the AMOC on ENSO and tropical variability, and the impact of other mid-to-high-latitude processes on tropical variability. Although ENSO is the dominant mode of variability on interannual timescales, its characteristics are not stationary and can be influenced by processes from other tropical basins and mid-to-high latitudes. The strength and variations of these interactions among different tropical basins and latitudes can be modulated by changes in external forcing, whether of natural or anthropogenic origin, and may also be shaped by nonlinear interactions between different modes of internal variability. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Chafik, L., Årthun, M., Langehaug, H.R., Nilsson, J., Rossby, T. 2025: The Nordic Seas overturning is modulated by northward-propagating thermohaline anomalies. Commun Earth Environ. https://doi.org/10.1038/s43247-025-02557-x Summary: The inflow of warm waters into the Nordic Seas, crucial for sustaining the climate-regulating Atlantic overturning circulation, can be reconstructed from hydrography using a north-south dynamic height gradient across the Greenland-Scotland Ridge. Variations in this influx are herein linked to northward-propagating thermohaline anomalies, initially observed at the intergyre boundary and likely driven by changes in ocean heat transport. As these anomalies reach the eastern subpolar North Atlantic, they modulate the cross-ridge dynamic height difference, thereby influencing both the Atlantic inflow and the Nordic Seas overflows on multi-year to decadal scales. Thus, these thermohaline anomalies play a dynamically active role in modulating the watermass exchanges across the ridge and downstream along the Atlantic Water path, rather than being a simple passive train of signals. This explains why these thermohaline signals are a key source of climate predictability and provides fresh insights into the functioning of the Nordic Seas overturning circulation from observations. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Richter, I., Chang, P., Chiu, P.-G., Danabasoglu, G., Doi, T., Dommenget, D., Gastineau, G., Gillett, Z. E., Hu, A., Kataoka, T., Keenlyside, N. S., Kucharski, F., Okumura, Y. M., Park, W., Stuecker, M. F., Taschetto, A. S., Wang, C., Yeager, S. G., and Yeh, S.-W. 2025: The Tropical Basin Interaction Model Intercomparison Project (TBIMIP). Geosci. Model Dev.. https://doi.org/10.5194/gmd-18-2587-2025 Summary: Large-scale interaction between the three tropical ocean basins is an area of intense research that is often conducted through experimentation with numerical models. A common problem is that modeling groups use different experimental setups, which makes it difficult to compare results and delineate the role of model biases from differences in experimental setups. To address this issue, an experimental protocol for examining interaction between the tropical basins is introduced. The Tropical Basin Interaction Model Intercomparison Project (TBIMIP) consists of experiments in which sea surface temperatures (SSTs) are prescribed to follow observed values in selected basins. There are two types of experiments. One type, called standard pacemaker, consists of simulations in which SSTs are restored to observations in selected basins during a historical simulation. The other type, called pacemaker hindcast, consists of seasonal hindcast simulations in which SSTs are restored to observations during 12-month forecast periods. TBIMIP is coordinated by the Climate and Ocean – Variability, Predictability, and Change (CLIVAR) Research Focus on Tropical Basin Interaction. The datasets from the model simulations will be made available to the community to facilitate and stimulate research on tropical basin interaction and its role in seasonal-to-decadal variability and climate change. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Sun, X., Y. Gao, X.-Q. Yang, Z. Fang, X. Zhang, S. Yuan, N. S. Keenlyside 2025: Dominant modes of interannual winter SAT covariability between the Arctic and the Tibetan Plateau: spatio-temporal structures and dynamical linkages. Clim Dyn. https://doi.org/10.1007/s00382-025-07711-x Summary: As two highly sensitive climate zones in the world, the Arctic and Tibetan Plateau (TP) regions respectively exhibit significantly uneven spatial variability in surface air temperature (SAT) and greatly influence the Eurasian climate on the interannual timescale. However, despite the synchronized warming trends in these two regions, their interannual spatio-temporal connection remains unclear. In this study, we applied the singular value decomposition (SVD) method to ERA5 wintertime surface air temperature anomalies to explore the dominant modes of SAT covariability between the Arctic and TP. We identified two major interannual modes: the dipolar Arctic-uniform TP (DA-UTP) and the quadrupolar Arctic-dipolar TP (QA-DTP), which together explain 82% of their covariance. The DA-UTP mode resembles the negative phase of the Arctic Oscillation, characterized by a hemispheric-scale pattern of “warm northern North America—cold northern Eurasia—warm TP”, while the QA-DTP mode exhibits a meridional teleconnection in the eastern hemisphere, featuring “warm Barents and Kara Seas—cold Eurasia—warm southern TP”. Both modes primarily draw energy from the North Atlantic Ocean and affect East Asian through the atmospheric Rossby wave train. The corresponding North Atlantic SST anomalies display a tripolar distribution, with the center of the negative SST gradient anomaly in the second mode shifted southward compared to the first. These two climate modes further modulate synoptic and sub-seasonal-to-seasonal winter temperature anomalies in Eurasia by altering the hemispheric-scale temperature gradient. The findings of this study contribute to a deeper knowledge and understanding of the interannual spatial and temporal relationships of wintertime surface temperature anomalies between the Arctic and TP. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Yin, M.,  X.-Q. Yang, L. Sun, L. Tao, N. Keenlyside 2025: Amplified wintertime Arctic warming causes Eurasian cooling via nonlinear feedback of suppressed synoptic eddy activities. Sci. Adv.. https://www.science.org/doi/10.1126/sciadv.adr6336 Summary: The amplified wintertime Arctic warming has accelerated in recent decades. However, whether and how the warming can cause Eurasian cooling remains debated. By identifying daily Arctic warming events, we find direct observational evidence that the Arctic warming tends to cause substantial Eurasian cooling and an increase in occurrence frequency of Eurasian cooling events with a roughly 2-day lag. We propose a mechanism explaining the causality. We find that the Arctic warming causes a large suppression in activities of daily weather disturbances (referred to as synoptic eddies) over high-latitude Eurasia. This produces a meridional dipole in geopotential height anomalies characterized by an equivalent-barotropic anomalous low (high) and a lower-level cooling (warming) over mid-latitude Eurasia (the Arctic) via a nonlinear eddy–to–mean flow feedback. The feedback induces near-surface northeasterly anomalies that enlarge the Eurasian cooling via cold advection. Thus, we conclude that the warm Arctic versus cold Eurasia is essentially an intrinsic dipole determined by synoptic eddy–mean flow interaction. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Xiu, Y., Wang, Y., Luo, H., Garcia-Oliva, L., Yang, Q. 2025: Impact of ocean, sea ice or atmosphere initialization on seasonal prediction of regional Antarctic sea ice. JAMES. https://doi.org/10.1029/2024MS004382 Summary: This study investigates how the atmosphere, ocean, or sea ice observations affect the seasonal prediction of Antarctic sea ice. We analyze three sets of predictions from the Norwegian Climate Prediction Model, each integrating different data sets of the atmosphere, ocean, or sea ice. Initially, we assess the seasonal cycles, trends, and variability of Antarctic sea ice in these data sets. We found that including atmosphere observations gave the best seasonal cycle compared to the observed sea ice. However, the linear trend in sea ice when including atmospheric data is poorly reproduced in the western Southern Ocean. Regarding variability, including the combined ocean and sea ice data gave the best performance. Next, we assess the accuracy of regional Antarctic sea ice prediction. We found that the accuracy varies with region and season. Austral winter predictions in western Antarctic have some skill up to a year in advance, while those in the eastern Antarctic are less reliable. Predictions based on atmosphere data are generally more accurate than those based on ocean or ocean/sea-ice data, especially when predicting from July or October. Interestingly, once ocean data is used, involving additional sea ice data improves sea ice concentration in the reanalysis but not in the predictions. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Passos, L., Langehaug, H.R., Årthun M., Straneo, F. 2024: On the Relation between Thermohaline Anomalies and Water Mass Transformation in the Eastern Subpolar North Atlantic. Journal of Climate. https://doi.org/10.1175/JCLI-D-23-0379.1 Summary: Decadal thermohaline anomalies carried northward by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern subpolar gyre using the reanalyses EN4.2.2 for the ocean and the ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the subpolar North Atlantic and the Norwegian Sea. We use observation-based datasets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply complex empirical orthogonal functions. Our results show that when a warm anomaly enters the eastern subpolar gyre, more SFWMT occurs in light-density classes (27.0–27.2 kg m−3). In contrast, when a cold anomaly enters the eastern subpolar gyre, more SFWMT occurs in denser classes (27.4–27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm–salty and cold–fresh subsurface anomalies, repeating throughout the 74-yr-long record with four warm–salty and cold–fresh periods after the 1950s. The cold–fresh anomaly periods happen simultaneously with the Great Salinity Anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the subpolar North Atlantic (SPNA) than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic meridional overturning circulation and predictability in the North Atlantic region. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Irvalı, N., Ninnemann, U.S., Olsen, A., Rose, N.L., Thornalley, D.J., Mjell, T.L., Counillon, F. 2024: Revising chronological uncertainties in marine archives using global anthropogenic signals: a case study on the oceanic 13C Suess effect. Geochronology. https://doi.org/10.5194/gchron-6-449-2024 Summary: Marine sediments are excellent archives for reconstructing past changes in climate and ocean circulation. Overlapping with instrumental records, they hold the potential to elucidate natural variability and contextualize current changes. Yet, dating uncertainties of traditional approaches (e.g., up to ± 30–50 years for the last 2 centuries) pose major challenges for integrating the shorter instrumental records with these extended marine archives. Hence, robust sediment chronologies are crucial, and most existing age model constraints do not provide sufficient age control, particularly for the 20th century, which is the most critical period for comparing proxy records to historical changes. Here we propose a novel chronostratigraphic approach that uses anthropogenic signals such as the oceanic 13C Suess effect and spheroidal carbonaceous fly-ash particles to reduce age model uncertainties in high-resolution marine archives. As a test, we apply this new approach to a marine sediment core located at the Gardar Drift, in the subpolar North Atlantic, and revise the previously published age model for this site. We further provide a refined estimate of regional reservoir corrections and uncertainties for Gardar Drift. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Wang, X., Zhang, Z., Yu, E., Guo, C., Otterå, O. H., Counillon, F. 2024: Warm Advection as a Cause for Extreme Heat Event in North China. Geophysical Research Letters. https://doi.org/10.1029/2024GL108995 Summary: Nowadays, heat waves have a significant impact on our society and result in substantial economic losses. Climate projections indicate that extreme heat events (EHEs) will become more frequent. However, heat waves have also often occurred in the past 300 years during periods with much lower anthropogenic forcing. One notable example is the severe heat event in the summer of 1743, which killed more than 10 thousand people in North China. The mechanism behind such events remains uncertain, making it exciting and valuable to investigate such heat waves in the past. In this study, we use a global model, a nested regional model, and tree-ring records to explore the mechanisms driving EHEs. The statistical robustness of the connection between EHEs in North China and Northeast China Vortexes is supported by modern observations. Notably, from 1950 to 2021, 63.6% of EHEs in North China coincide with active Northeast China Vortexes. Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.