Rein Haarsma

The winter North Atlantic Oscillation (NAO), key driver of climate variability in the Euro-Atlantic sector, remains unsettled as to its spatio-temporal characteristics in the Northern Hemisphere. A wavenumber decomposition of NAO-related circulation anomalies is performed using reanalysis data to explore its hemispheric signature, which exhibits a non-annular, wave-like structure at all levels. In the troposphere-stratosphere system, the NAO variability yields a wavenumber 2 pattern tilting westward and increasing in amplitude with height, suggesting upward propagation. Wavenumber 1, instead, shows no statistical significance in the middle stratosphere, unlike the El Niño-Southern Oscillation teleconnection where is dominant. In the upper troposphere, the NAO variability displays a wavenumber 5 pattern at middle-subtropical latitudes reminiscent of the Circumglobal Waveguide Pattern, and a wavenumber 3 pattern at middle-subpolar latitudes which is phase-locked with climatology in the lower troposphere. These findings emphasize that interannual NAO dynamics not only include transient, synoptic-scale eddy activity in the North Atlantic but also stationary, large-scale wave activity far beyond, having marked implications for climate predictability.

The downscaling of climate change projections is crucial for impact and adaptation studies, as such studies require higher spatial resolution data than those provided by global CMIP6 models. EC-Earth3 is therefore used as a driving model for dynamic downscaling with the Eta/INPE regional model over SA. In this study, we analyze projected trends under the SSP5-8.5 scenario, focusing on the 2-m air temperature (2 m) and precipitation until the end of the 21st century. Changes are evaluated from 2015 onwards relative to the 1985–2014 climatology and analyzed seasonally (DJF, MAM, JJA, SON). In the near future period, an increasing trend in precipitation is projected over the South Atlantic Convergence Zone (SACZ) during DJF, as well as over northern SA during MAM, while a reduction in precipitation is projected over most of the domain, particularly over the Amazon region. From 2025 onwards, precipitation reductions over the Amazon are projected to predominate in nearly all seasons, while positive changes are projected over parts of northeastern, southeastern, and central SA. This pattern persists in subsequent decades. From 2060 onwards, precipitation reduction over the SACZ exceeds 300 mm per season in SON and DJF. By the end of the century, a strong negative change is projected across the entire SACZ region. In contrast, a persistent positive change is projected over southern Brazil, likely associated with an increase in extreme precipitation events. This contrast tends to enhance the spatial variability of climate over SA and suggests changes in the South American monsoon system and the continental hydrological cycle. Regarding temperature, initial chances do not exceed ±1 °C over most of central SA. From 2025 onwards, positive changes range from +1 to +3 °C across most of the domain. The warming signal intensifies over time, exceeding +4 °C from 2035 onwards during SON. From the 2040s onward, positive temperature changes dominate across the model domain under the SSP5-8.5 scenario. Projected mean temperature increases exceed +6 °C over the central region from the 2050s onwards. By the end of the century, temperature increase range from +6 to +8 °C over most of Brazil. The main agricultural production region in Central Brazil exhibits the highest warming rates. The SON season presents the largest temperature increases and precipitation anomalies.

Precipitation projections over the Guiana region in northern South America are hampered by dry biases over that region in CMIP6 models. These biases are partly due to the wrong position of the inter tropical convergence zone (ITCZ) that is generally located too far to the south and unresolved mesoscale processes that are important for convective precipitation prevailing in that region. To account for that pseudo global warming (PGW) experiments with a regional climate model have been performed. In these experiments a perturbation derived from global climate models is imposed on present day simulations that are forced by reanalyses boundary conditions. The higher spatial resolution of regional climate models allows for a better simulation of mesoscale processes and using reanalyses as boundary conditions for the present climate induces an improved position of the ITCZ. For the present climate, the RACMO regional climate model with a resolution of about 12 KM, driven by ERA5 reanalyses boundary conditions, reveals a strong reduction of the dry bias. For the future climate the PGW experiments, perturbed by an ensemble mean of CMIP6 models, indicate a strong increase in precipitation, in particular during the wet season. This contrasts with CMIP6 models that indicate a drying for that region. This has strong implications for climate adaptation measures as presently this region is already affected by large floods induced by heavy rainfall. Present research is focused on changes in extreme precipitation and on different processes explaining these differences, such the position of the ITCZ, the role of orography and the simulation of mesoscale processes.

The Atlantic Meridional Overturning Circulation (AMOC) is an important feature of the Atlantic Ocean circulation transporting a large amount of heat into the North Atlantic (about 1.2 Pw at 26.5˚N), that strongly affects the Atlantic climate and its adjacent regions, including Western European and the Caribbean.One of the possible tipping points induced by global warming is a collapse of the AMOC. Recent research suggests that an irreversible AMOC collapse might already start this century even under low emission scenarios1,2,3. Using state-of-the-art climate models of the latest IPCC report (CMIP6) we have investigated the impact of such an imminent AMOC collapse for the Caribbean climate for a low emission scenario. On average the time scale for a total collapse is more than a century. In the worst case resulting in a severe reduction in annual mean rainfall for specific regions in the Caribbean of about 70%. Other significant implications of an AMOC collapse are a reduction of the increase in surface temperatures in the order of one degree °C, an increase in sea level rise of about 10 cm, and less favourable conditions for hurricane activity. These findings quantify Caribbean climate change impacts in the event of an AMOC collapse, complementing existing climate scenarios. ReferencesDitlevsen, P., & Ditlevsen, S. (2023). Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nature Communications, 14(1), 1-12.Drijfhout, S., Angevaare, J. R., Mecking, J., van Westen, R. M., & Rahmstorf, S. (2025). Shutdown of northern Atlantic overturning after 2100 following deep mixing collapse in CMIP6 projections. Environmental Research Letters, 20(9), 094062.Rahmstorf, S. (2024). Is the Atlantic overturning circulation approaching a tipping point? Oceanography, 37(3), 16-29.

In the last decades, there has been an ongoing discussion whether the winter North Atlantic Oscillation (NAO) is a zonally-symmetric hemispheric mode of variability driven by transient eddies and strongly coupled to the stratosphere [Arctic Oscillation/Northern Annular Mode (AO/NAM)]; or a regional mode of variability forced locally by transient eddies (NAO) but with associated hemispheric anomalies related to stationary eddies in the Circumglobal Waveguide Pattern (CWP). We revisit this question using zonal wavenumber decomposition of NAO-related circulation anomalies in reanalysis (ERA5, NCEP-NCAR). At upper-tropospheric levels, the NAO exhibits a wave-like structure that resembles the CWP, where wavenumber 3 seems to dominate at subpolar latitudes and wavenumber 5 is more prominent at suptropical latitudes. Wavenumber 4 does not significantly contribute to the NAO pattern in the total field. The wave activity flux of NAO-related variability reveals downstream propagation and splitting of wave energy which appears to be consistent with the meridional component of the wind and with theoretical arguments of stationary wavenumber based on the background flow. These results support the relevance of large-scale stationary waves in the hemispheric signature of the NAO. To further diagnose the tropospheric propagation of NAO-related anomalies, additional analysis using ray tracing and experiments with a linear barotropic model are performed.

We share insights from the communication efforts surrounding the KNMI`23 climate scenarios for the Dutch Caribbean islands of Bonaire, Sint Eustatius, and Saba (the BES islands). The scenarios were published by the Royal Netherlands Meteorological Institute (KNMI) in October 2023. We focus on the approach used, lessons learned, and insights gained. We communicate our scenarios through various approaches, including a report aimed at the general public, active engagement with stakeholders, end-users, policy and decision makers, and local communities through presentations, workshops, and discussions. These interactions aim to increase awareness, understanding, and cooperation. We aim to provide valuable insights for policy and decision makers and scientists across disciplines. As a government institute, we are committed to conducting policy-relevant research that supports the development of climate plans tailored to each BES island. This presentation examines the challenges, successes and lessons learned from our communication initiatives.

Various stakeholders seek effective methods to communicate the potential impacts of tropical cyclone (TC) rainfall and subsequent flood hazards. While current methods, such as Intensity–Duration–Frequency curves, offer insights, they do not fully capture TC rainfall complexity and variability. This research introduces an innovative workflow utilizing GPM-IMERG satellite precipitation estimates to cluster TC rainfall spatial–temporal patterns, thereby illustrating their potential for flood hazard assessment by simulating associated flood responses. The methodology is tested using rainfall time series from a single TC as it traversed a 500 km diameter buffer zone around Dominica. Spatial partitional clustering with K-means identified the spatial clusters of rainfall time series with similar temporal patterns. The optimal value of K = 4 was most suitable for grouping the rainfall time series of the tested TC. Representative precipitation signals (RPSs) from the quantile analysis generalized the cluster temporal patterns. RPSs served as the rainfall input for the openLISEM, an event-based hydrological model simulating related flood characteristics. The tested TC exhibited three spatially distinct levels of rainfall magnitude, i.e., extreme, intermediate, and least intense, each resulting in different flood responses. Therefore, TC rainfall varies in space and time, affecting local flood hazards; flood assessments should incorporate variability to improve response and recovery.

In the last decades, there has been an ongoing discussion about whether the winter NAO has a deep barotropic and annular signature, being the stratosphere a key player of this variability, or, on the contrary, the NAO presents a wave-like behavior with its perturbations originating in the troposphere and propagating upward and downstream.In our work we tackle this discussion through zonal wavenumber decomposition of the NAO-related anomalies in both reanalysis (ERA5 and NCEP-NCAR) and climate models of different complexity and resolution (SPEEDO/L8, CAM5.3/L46 and EC-EARTH3.3/L91). We find that the NAO exhibits a non-annular, wave-like structure in all levels, particularly in the upper troposphere (300 hPa) but also in the middle stratosphere (30 hPa). Indeed, we identify that the troposphere-stratosphere coupling associated with the NAO includes a wavenumber 2 structure tilting westward and increasing in amplitude with height, indicating upward propagation and forcing of the polar vortex. This recurrent NAO-related wavenumber 2 anomaly could contribute to the characteristic wavenumber 2 pattern of the climatological stratospheric circulation. Wavenumber 1, on the other hand, shows no statistical significance in the stratosphere; while in the troposphere it appears to be an artifact of the method, reflecting the prominence of regional dynamics in the North Atlantic. At upper-tropospheric levels, wavenumber 3 seems to dominate the wave-like structure of the total field at mid/high latitudes, whereas wavenumber 5 is more prominent at lower latitudes. Finally, wavenumber 4 does not significantly contribute to the NAO-related anomalies. These results are robust in reanalysis and in the climate models, although some differences are noticeable, likely linked to model biases in the mean flow.To further interpret the vertical propagation of long waves (wavenumbers 1 and 2) and the horizontal propagation of small waves (wavenumber 3 and 5), additional diagnostics such as eddy heat flux and wave activity flux will be analyzed.

Understanding the impact of radiative forcing on climate variability and change in the Tropical Atlantic is crucial for different socio-economic sectors, given their substantial impacts in both local and remote regions. To properly evaluate the effect of a changing climate on the variability, the use of standard transient historical and scenario simulations requires very large ensembles. A computationally cheaper alternative implemented in this study consists of performing two 250-year-long atmosphere-ocean coupled simulations with EC-EARTH 3.3 (CMIP6 version) with fixed radiative forcing at the years 2000 and 2050, representative of present and future climate conditions, respectively. The changes in the leading modes of Tropical Atlantic variability (TAV), including the Atlantic Niño/Niña and the Subtropical North Atlantic pattern, have been assessed in three target seasons: spring (MAM), summer (JJ) and early winter (ND). While the change in sea surface temperature (SST) climatology shows homogeneous warming, the difference between future and present SST variability exhibits a distinct behaviour consistent along the seasonal cycle, with a decrease in the equatorial region and an increase at subtropical latitudes. This study explores the processes associated with the suppressed/enhanced TAV, with a particular focus on the less-explored early winter season. In agreement with previous studies, the Atlantic Meridional Overturning Circulation (AMOC) shows a weakening in strength, but the results also show an increase in variability. The AMOC-related deepening of the equatorial thermocline and the flattening linked to weakened trade winds are consistent with the suppressed SST variability of the Atlantic Niño/Niña. On the other hand, the enhanced SST variability at subtropical latitudes is mainly associated with an increase in turbulent heat flux variability, with a minor contribution of the mixed layer depth variability. Variability in turbulent heat flux is influenced primarily by latent heat flux, connected to changes in precipitation variability.