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Direct measurements of the Atlantic Meridional Overturning Circulation (AMOC) are necessary to understand its evolution in a context of climate change. International programs monitored its recent evolution at different latitudes. However, the AMOC coherence over the North Atlantic remains unclear. Here, we explore the potential of the Overturning in the Subpolar North Atlantic Programme (OSNAP) array to inform us on mid‐latitude AMOC strength on interannual‐to‐multiannual timescales in two high‐resolution coupled models. We find that the AMOC strength measured at OSNAP is not related to the variability of the AMOC strength at mid‐latitudes. Instead, our study reveals that the density at which the maximum overturning occurs at OSNAP is a key precursor of the mid‐latitude AMOC strength. The causal relationship between AMOC density at the OSNAP array and AMOC strength at 45°N is linked to an atmospherically driven shoaling of isopycnal surfaces that propagates along the western boundary in a year. Plain Language Summary Warm and salty water flows northward at surface into the high latitudes of the North Atlantic. This water is made heavier as it becomes colder and fresher and forms a return southward flow at depth. Direct measurements of this so‐called Atlantic Meridional Overturning Circulation (AMOC) have been ongoing in the subtropical North Atlantic since 2004, and more recently in the subpolar North Atlantic. These measurements tell us that the AMOC in these two geographically adjacent regions do vary from daily to interannual timescales. However, the coherence of the varying AMOC between these two observational systems remains unclear. Our study shows that density changes in the subpolar North Atlantic propagate around the boundary of the subpolar North Atlantic within 1 year to modify the AMOC strength at mid‐latitudes. We further show that these density changes are linked to the dominant atmospheric mode of variability known as the North Atlantic Oscillation (NAO). The NAO affects sea level pressure, altering the wind stress and buoyancy exchange at the sea surface, therefore changing surface density. These results will help inform how we design future ocean observing systems. Key Points The density at which the subpolar overturning streamfunction is maximum is a key precursor of the mid‐latitude overturning strength The density anomaly propagates from the subpolar gyre to 45°N by following the western boundary within a year The density anomaly over the subpolar gyre is atmospherically driven by changes in buoyancy and wind forcings

A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI‐ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low‐level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two‐layer model. Key Points An updated version of the Max Planck Institute for Meteorology Earth System Model (MPI‐ESM1.2) is presented The model includes both code corrections and parameterization improvements Despite this, the model maintains an equilibrium climate sensitivity, which rises with warming

Waves critically modulate the air‐sea fluxes, and upper‐ocean thermodynamics in a Tropical Cyclone (TC) system. This study improves the modeling of TC intensification by incorporating non‐breaking wave‐induced turbulence and sea spray from breaking waves into an atmosphere‐ocean‐wave coupled model. Notably, wind forecast error decreased by around 10% prior to TCs' peak intensity. The positive feedback of sea spray along with compensatory negative feedback from non‐breaking waves, overall enhanced TCs' intensity. These breaking and non‐breaking wave‐coupled processes consistently cool sea surface temperature, resulting in improvement of the modeled SST. Observed improvements in full‐year TC cases ranging from Categories I to IV in this study suggest that an accurate characterization of ocean wave‐coupled processes is crucial for improving TCs' intensity forecasts and advancing our understanding of severe weather events in both, the atmosphere and ocean. Plain Language Summary Tropical Cyclones (TCs), such as hurricanes and typhoons, are destructive natural disasters that can cause extensive damage. Our study focused on understanding the role of ocean waves and related processes in TCs. Through numerical modeling, we found that ocean waves, specifically breaking and non‐breaking waves, have a substantial influence on TCs' intensity. Breaking waves contribute positively through the production of sea spray droplets, while non‐breaking wave‐induced turbulence has a compensatory negative effect, resulting in an enhancement in TCs' intensity. Incorporating both wave mechanisms into the models improved the accuracy of TCs' intensity and their underlying sea surface temperature. By highlighting the importance of ocean wave‐coupled physics, we aim to enhance our understanding of TCs and improve disaster preparedness to mitigate their impacts on coastal communities. Key Points Full‐year regional hindcast of Tropical Cyclones (TCs) at the North West Australia Inclusion of wave‐coupled processes improves TCs modeling, by reducing forecast errors and enhancing rapid intensification simulations Sea spray increases TC development while nonbreaking wave turbulence has the opposite effect with the first process dominating

This study investigates the effects of sea spray on extreme precipitation forecast in Beijing of China between 28 July and 2 August 2023 as a case test. In this case, fully coupled model increased upward moisture in the Bohai and Yellow Seas and increased accumulated rainfall by 21% in North China. For the extreme precipitation events with the 5‐day accumulated precipitation exceeding 500 mm, the atmosphere‐only model did not forecast the events; the coupled model without sea spray performed well with the 0.29 threat score (TS) and 88 mm root mean square error (RMSE); in the fully coupled model, the effects of sea spray increased atmospheric instability, which increased the precipitation around Beijing and yielded a more accurate forecast with the 0.37 TS and 65 mm RMSE. This paper suggests a scientific clue to improve numerical simulation for extreme rainfall events, however, more cases are still needed for statistical evaluation. Plain Language Summary Although meteorological forecasting ability has been improved considerably during the past decades, precipitation during extreme events remains typically underestimated. Improving model accuracy for heavy rainfall events is, indeed, a ground challenge. In this study, we conducted three numerical experiments under different conditions to evaluate the effects of sea spray on extreme precipitation forecasts for the Beijing extreme rainfall event between 28 July and 2 August 2023. Our results indicated that for this 5‐day forecasting case, including sea spray in the simulations enhanced precipitation in North China by transporting more moisture from the Bohai and Yellow Seas upward into the atmosphere. The moister and warmer air caused by sea spray effects was transported to Beijing through northwestward wind and lifted by the local terrain, then caused a more unstable atmosphere and higher intensity of precipitation around Beijing. By using statistical indicators and setting different precipitation thresholds for this extreme rainfall case, we determined that in this case, the fully coupled model including sea spray yielded more accurate forecast. To statistically evaluate the effects of sea spray in extreme precipitation forecasts, more cases are still needed. Key Points Fully coupled model led more upward moisture in the Bohai and Yellow Seas and increased 5‐day accumulated rainfall by 21% in North China Fully coupled model improved the forecasting threat score from 0 to 0.37 with the 5‐day accumulated precipitation exceeding 500 mm Sea spray led to more unstable atmosphere for the Beijing extreme rainfall event, yielding a more accurate forecast

The impacts of parameterized upper-ocean wave mixing on global climate simulations are assessed through modification to Large et al.’sK-profile ocean boundary layer parameterization (KPP) in a coupled atmosphere–ocean–wave global climate model. The authors consider three parameterizations and focus on impacts to high-latitude oceanmixed layer depths and related ocean diagnostics. The McWilliams and Sullivan parameterization (MS2000) adds a Langmuir turbulence enhancement to the nonlocal component of KPP. It is found that the Langmuir turbulence–induced mixing provided by this parameterization is too strong in winter, producing overly deep mixed layers, and of minimal impact in summer. The later Smyth et al. parameterization modifies MS2000 by adding a stratification effect to restrain the turbulence enhancement under weak stratification conditions (e.g., winter) and to magnify the enhancement under strong stratification conditions. The Smyth et al. scheme improves the simulated winter mixed layer depth in the simulations herein, with mixed layer deepening in the Labrador Sea and shoaling in the Weddell and Ross Seas. Enhanced vertical mixing through parameterized Langmuir turbulence, coupled with enhanced lateral transport associated with parameterized mesoscale and submesoscale eddies, is found to be a key element for improving mixed layer simulations. Secondary impacts include strengthening the Atlantic meridional overturning circulation and reducing the Antarctic Circumpolar Current. The Qiao et al. nonbreaking wave parameterization is the third scheme assessed here. It adds a wave orbital velocity to the Reynolds stress calculation and provides the strongest summer mixed layer deepening in the Southern Ocean among the three experiments, but with weak impacts during winter.

Key questions remain about the atmospheric response to variability in the oceanic western boundary currents (WBCs). Here we exploit a unique high‐resolution slab‐ocean coupled climate model to investigate how ocean heat transport (OHT) anomalies in the major WBCs of both hemispheres affect the atmospheric circulation. Prescribed OHT anomalies lead to robust changes in convective precipitation anomalies equatorward of the maximum surface warming. The response is deepest and most pronounced over the Northern Hemisphere (NH) WBCs, where it is associated with significant changes in upper tropospheric vertical motion, condensational heating and geopotential heights. The response is relatively shallow over the Southern Hemisphere (SH) WBCs. The findings reveal the robustness of the atmospheric response to OHT anomalies and highlight key hemispheric differences: in the NH, OHT anomalies are balanced by deep atmospheric vertical motion; in the SH, they are balanced primarily by shallow horizontal temperature advection. Plain Language Summary We study how ocean heat transport (OHT) influences the atmospheric circulation in the major western boundary currents (WBCs) of both hemispheres, including the Gulf Stream, Kuroshio‐Oyashio Extension, Brazil‐Malvinas Confluence, and Agulhas Currents. We find that the heating due to anomalous ocean heat transport causes air to rise on the equatorward side of the largest surface heating in all WBC regions. The regions of rising air are also associated with more intense convective precipitation. The effect is strongest in the Northern Hemisphere (NH) where the atmospheric response extends to the upper troposphere, leading to significant heating and atmospheric circulation anomalies aloft. The findings highlight the robustness of the atmospheric response to ocean dynamical processes in the western boundary currents, although differences in the hemispheric responses are noteworthy. In the NH WBCs, the atmospheric response to OHT anomalies is balanced primarily through vertical air movement, whereas in the Southern Hemisphere, the response is balanced primarily by low‐level horizontal temperature advection. Key Points The atmospheric response to ocean heat transport (OHT) anomalies in the western boundary currents (WBC) is examined Anomalous OHT drives robust changes in the atmospheric circulation and convective precipitation over the WBCs of both hemispheres The Northern Hemisphere responses extend to the upper troposphere; the Southern Hemisphere responses are limited to the lower troposphere

The effect of atmosphere–ocean coupling on intensity changes of tropical cyclones (TCs) under global warming was examined using a regional high‐resolution three‐dimensional atmosphere–ocean coupled model. A storyline event attribution approach was applied to four historical intense TCs in the western North Pacific. The results indicate that atmosphere–ocean coupling buffers TC intensification as global warming progresses. This buffering effect increased as storms traveled northward. Moreover, the effect intensified as warming progressed, because reductions in sea surface temperature induced by the storm increased as the storm strengthened in future warmer climates. The magnitude of the buffering effect depended on the storm's size and translation speed; a large, slow‐moving storm had significant resilience against global warming, whereas a compact, fast‐moving storm was sensitive to global warming. A high‐resolution atmosphere–ocean coupled model is important for more reliable future projections of TC intensity under the changing climate. Plain Language Summary Intense tropical cyclones (TCs) often cause extreme destruction. Therefore, to prevent future disasters, it is essential to understand how warmer environmental conditions will affect intense TCs. Atmosphere–ocean interaction during an intense TC can significantly reduce the sea surface temperature (SST). However, most projections of future TC intensity have been made either with a high‐resolution atmosphere model that did not take account of atmosphere–ocean interaction or with a relatively coarse‐resolution atmosphere–ocean coupled model that could not adequately reproduce intense TC intensity. We used a regional high‐resolution three‐dimensional atmosphere–ocean coupled model to quantitively assess how atmosphere–ocean coupling affected intensity changes of four historical intense typhoons under four different warming conditions. We found that the atmosphere–ocean coupling buffered changes in storm intensity associated with global warming by modulating the storm‐induced SST‐cooling in the vicinity of the storm center. We also found that the magnitude of the buffering effect depended on the storm size and translation speed and differed greatly among storms. Our results indicate that a high‐resolution atmosphere–ocean coupled model that can represent storm intensity and size and the resultant SST‐cooling should be used for reliable projections of future TC intensity under a changing climate. Key Points Intensity changes of tropical cyclones (TCs) under four different warming conditions are examined by a regional high‐resolution atmosphere–ocean coupled model An atmosphere–ocean coupling effect buffers changes in storm intensity under a changing climate The buffering effect intensifies as global warming progresses, because sea surface temperature‐cooling induced by a TC increases as the TC strengthens

Coupled atmosphere–ocean phenomena known as northeast Pacific marine heatwaves (MHWs) simulated in a climate model with an eddy-permitting ocean model were examined. During the analyzed 270-years of the preindustrial control run, 13 events of MHW (defined here by warm annual-mean SST anomalies with over 1.5 times of the standard deviation) were detected. The simulated MHWs are linked to the decadal-scale climate modes of PDO, inverted NPGO (IV-NPGO), and the central Pacific El Niño (CP-El-Niño): IV-NPGO and then PDO changed the signs from negative to positive a few years before the MHWs at around which PDO took the maxima, when CP-El-Niño occurred. Air–sea interactions between subtropical-tropical and within mid-latitudes suggest playing crucial roles in the evolution of the MHWs.

Recent studies have started to explore coupled data assimilation (CDA) in coupled ocean–atmosphere models because of the great potential of CDA to improve climate analysis and seamless weather–climate prediction on weekly-to-decadal time scales in advanced high-resolution coupled models. In this review article, we briefly introduce the concept of CDA before outlining its potential for producing balanced and coherent weather–climate reanalysis and minimizing initial coupling shocks. We then describe approaches to the implementation of CDA and review progress in the development of various CDA methods, notably weakly and strongly coupled data assimilation. We introduce the method of coupled model parameter estimation (PE) within the CDA framework and summarize recent progress. After summarizing the current status of the research and applications of CDA-PE, we discuss the challenges and opportunities in high-resolution CDA-PE and nonlinear CDA-PE methods. Finally, potential solutions are laid out.

The monsoon system in India plays a pivotal role in shaping the country’s climate. Recent studies have indicated that the increasing variability of monsoons is attributable to climate change, resulting in prolonged periods of drought and excessive rainfall. Understanding, analyzing, and forecasting monsoons is crucial for socioeconomic sustainability and communities’ overall well-being. Climate forecasts, which project future Earth climates typically up to 2100, rely on models such as the Couple Model Intercomparison Project (CMIP). However, confidence in these forecasts remains low due to the limitations of global climate models, particularly in terms of capturing the intricacies of monsoon dynamics, notably from June to September. To address this issue, researchers have examined precipitation simulations under various future scenarios using both CMIP5 and the latest CMIP6 models. Evaluating the performance of these models from 1979 to 2014, particularly in simulating mean precipitation and temperature, has revealed improvements in multi-model ensembles (MME), highlighting advancements in monsoon characteristics. By comparing the CMIP5 and CMIP6 models, researchers have identified the most reliable models for climate downscaling research, which can provide more accurate predictions of regional climate changes, thereby offering valuable insights for enhancing climate modeling in the Indian subcontinent.