Benefits of simulating precipitation characteristics over Africa with a regionally-coupled atmosphere–ocean model

High-quality climate information at appropriate spatial and temporal resolution is essential to develop and provide tailored climate services for Africa. A common method to produce regional climate change data is to dynamically downscale global climate projections by means of regional climate models (RCMs). Deficiencies in the representation of the sea surface temperatures (SSTs) in earth system models (ESMs) and missing atmosphere–ocean interactions in RCMs contribute to the precipitation bias. This study analyzes the influence of the regional atmosphere–ocean coupling on simulated precipitation and its characteristics over Africa, and identifies those regions providing an added value using the regionally coupled atmosphere–ocean model ROM. For the analysis, the MPI-ESM-LR historical simulation and emission scenario RCP8.5 were dynamically downscaled with ROM at a spatial resolution of 0.22° × 0.22° for the whole African continent, including the tropical Atlantic and the Southwest Indian Ocean. The results show that reduced SST warm biases in both oceans lead to more realistic simulated precipitation over most coastal regions of Sub-Saharan Africa and over southern Africa to varying degrees depending on the season. In particular, the annual precipitation cycles over the coastal regions of the Atlantic Ocean are closer to observations. Moreover, total precipitation and extreme precipitation indices in the coupled historical simulation are significantly lower and more realistic compared to observations over the majority of the analyzed sub-regions. Finally, atmosphere–ocean coupling can amplify or attenuate climate change signals from precipitation indices or even change their sign in a regional climate projection

Impact of air–sea coupling on the climate change signal over the Iberian Peninsula

In this work we use a regional atmosphere–ocean coupled model (RAOCM) and its stand-alone atmospheric component to gain insight into the impact of atmosphere–ocean coupling on the climate change signal over the Iberian Peninsula (IP). The IP climate is influenced by both the Atlantic Ocean and the Mediterranean sea. Complex interactions with the orography take place there and high-resolution models are required to realistically reproduce its current and future climate. We find that under the RCP8.5 scenario, the generalized 2-m air temperature (T2M) increase by the end of the twenty-first century (2070–2099) in the atmospheric-only simulation is tempered by the coupling. The impact of coupling is specially seen in summer, when the warming is stronger. Precipitation shows regionally-dependent changes in winter, whilst a drier climate is found in summer. The coupling generally reduces the magnitude of the changes. Differences in T2M and precipitation between the coupled and uncoupled simulations are caused by changes in the Atlantic large-scale circulation and in the Mediterranean Sea. Additionally, the differences in projected changes of T2M and precipitation with the RAOCM under the RCP8.5 and RCP4.5 scenarios are tackled. Results show that in winter and summer T2M increases less and precipitation changes are of a smaller magnitude with the RCP4.5. Whilst in summer changes present a similar regional distribution in both runs, in winter there are some differences in the NW of the IP due to differences in the North Atlantic circulation. The differences in the climate change signal from the RAOCM and the driving Global Coupled Model show that regionalization has an effect in terms of higher resolution over the land and ocean

Climate change signal in the ocean circulation of the Tyrrhenian Sea

The Tyrrhenian Sea plays an important role in the winter deep water formation in the northwestern Mediterranean through the water that enters the Ligurian Sea via the Corsica Channel. Therefore, the study of the impact of the changes on the future climate on the Tyrrhenian circulation and its consequences represents an important issue. Furthermore, the seasonally dependent Tyrrhenian circulation, which is rich in dynamical mesoscale structures, is dominated by the interplay of local climate and the basin-wide Mediterranean circulation via the water transport across its major straits, and an adequate representation of its features represents an important modeling challenge. In this work we examine with a regionally coupled atmosphere–ocean model the changes in the Tyrrhenian circulation by the end of the 21st century under the RCP8.5 emission scenario, their driving mechanisms, and their possible impact on winter convection in the NW Mediterranean. Our model successfully reproduces the main features of the Mediterranean Sea and Tyrrhenian Basin present-day circulation. We find that toward the end of the century the winter cyclonic along-slope stream around the Tyrrhenian Basin becomes weaker. This weakening increases the wind work transferred to the mesoscale structures, which become more intense than at present in winter and summer. We also find that, in the future, the northward water transport across the Corsica Channel towards the Liguro-Provençal basin becomes smaller than today. Also, water that flows through this channel presents a stronger stratification because of a generalized warming with a freshening of upper and a saltening of intermediate waters. Both factors may contribute to the interruption of deep water formation in the Gulf of Lions by the end of the century

Demonstrating the asymmetry of the Indian Ocean Dipole response in regional earth system model of CORDEX-SA

An accurate representation of the Indian Ocean Dipole (IOD) is crucial for the reliable projection of Indian summer monsoon rainfall, making it necessary to improve the understanding of the response of the IOD in the warming climate. For the first time, a high-resolution regional earth system model (RESM) over the CORDEX-SA domain is used to investigate the IOD characteristics. The model performance is evaluated in simulating the IOD and associated mechanism. RESM shows a good resemblance in simulating IOD phases (positive and negative). However, the systematic discrepancy is observed in magnitude. Additionally, RESM well represented the positive IOD's inter-event variability. For example, the stronger event dominated by significant cold anomalies over Sumatra with enhanced westward-extended while a moderate event shows weak cooling confined to the region of Sumatra. Additionally, RESM shows potential to distinguish the ENSO and non-ENSO years with more remarkable skill in representing the spatial pattern of SST over IOD region during non-ESNO years than ENSO years. The RESM realistically simulated the IOD amplitude with greater skill than CMIP5/6 models reported in the earlier studies, indicating reliability towards the projection of the Indian summer monsoon. The weaker IOD-ENSO relationship is caused by producing the more significant number of IOD during non-ENSO years. Despite this reliable fidelity, IOD's slightly earlier peak is driven by the early establishment of low-level equatorial easterly wind. This study provided valuable insight into the IOD's different phases, responsible forcings, and limitations of the RESM in accounting for the role of internal climate variability that can be useful for further improvement in the model physics

Impact of ocean–atmosphere coupling on future projection of Medicanes in the Mediterranean sea

Cyclones with tropical characteristics called medicanes (“Mediterranean Hur-ricanes”) eventually develop in the Mediterranean Sea. They have large harm-ful potential and a correct simulation of their evolution in climate projections is important for an adequate adaptation to climate change. Different studies suggest that ocean–atmosphere coupled models provide a better representation of medicanes, especially in terms of intensity and frequency. In this work, we use the regionally-coupled model ROM to study how air-sea interactions affect the evolution of medicanes in future climate projections. We find that under the RCP8.5 scenario our climate simulations show an overall frequency decrease which is more pronounced in the coupled than in the uncoupled con-figuration, whereas the intensity displays a different behaviour depending on the coupling. In the coupled run, the relative frequency of higher-intensity medicanes increases, but this is not found in the uncoupled simulation. Also, this study indicates that the coupled model simulates better the summer mini-mum in the occurrence of medicanes, avoiding the reproduction of unrealisti-cally intense events that can be found in summer in the uncoupled model

The present and future offshore wind resource in the Southwestern African region

In the last decades, offshore wind harvesting has increased enormously, and is seen as a renewable energy resource with great potential in many regions of the world. Therefore, it is crucial to understand how this resource will evolve in a warming climate. In the present study, offshore wind resource in the Southwestern African region is analysed for the present and future climates. A ROM (REMO-OASIS-MPIOM) climate simulation in uncoupled and coupled atmosphere–ocean mode, at 25 km horizontal resolution, and a multi-model ensemble built with a set of regional climate models from the CORDEX-Africa experiment at 0.44° resolution were used. The projected changes of the offshore wind energy density throughout the twenty-first century are examined following the RCP4.5 and RCP8.5 greenhouse gas emissions scenarios. Characterised by strong coastal-parallel winds, the Southwestern African offshore region shows high values of wind energy density at 100 m, up to 1500 Wm⁻² near the coast, particularly offshore Namibia and west South Africa. Conversely, along Angola’s coast the available offshore wind energy density is lower. Throughout the twenty-first century, for the weaker climate mitigation scenario (RCP8.5), an increase of the offshore wind resource is projected to occur along Namibia and South African western coasts, more pronounced at the end of the century (+ 24%), while a decrease is projected along Angola’s coasts, reaching a negative anomaly of about − 32%. Smaller changes but with the same pattern are projected for the stronger climate mitigation scenario (RCP4.5). The future deployment of offshore floating hub turbines placed at higher heights may allow higher production of energy in this region. Along offshore Namibia and west South Africa, the wind energy density at 250 m showed differences that range between 30 and 50% relative to wind energy density at 100 m

Surface and Intermediate Water Changes Triggering the Future Collapse of Deep Water Formation in the North Western Mediterranean

Deep water formation (DWF) in the North Western Mediterranean (NWMed) is a key feature of Mediterranean overturning circulation. DWF changes under global warming may have an impact on the Mediterranean biogeochemistry and marine ecosystem. Here we analyze the deep convection in the Gulf of Lions (GoL) in a changing climate using a regional climate system model with a horizontal resolution high enough to represent DWF. We find that under the RCP8.5 scenario the NWMed DWF collapses by 2040–2050, leading to a 92% shoaling in the winter mixed layer by the end of the century. The collapse is related to a strengthening of the vertical stratification in the GoL caused by changes in properties of Modified Atlantic Water and Levantine Intermediate Water, being their relative contribution to the increase of the stratification 57.8% and 42.2%, respectively. The stratification changes also alter the Mediterranean overturning circulation and the exchange with the Atlantic