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We describe a new, state‐of‐the‐art, Earth System Regional Climate Model (RegCM‐ES), which includes the coupling between the atmosphere, ocean, and land surface, as well as a hydrological and ocean biogeochemistry model, with the capability of using a variety of physical parameterizations. The regional coupled model has been implemented and tested over some of the COordinated Regional climate Downscaling Experiment (CORDEX) domains and more regional settings featuring climatically important coupled phenomena. Regional coupled ocean‐atmosphere models can be especially useful tools to provide information on the mechanisms of air‐sea interactions and feedbacks occurring at fine spatial and temporal scales. RegCM‐ES shows a good representation of precipitation and SST fields over the domains tested, as well as realistic simulations of coupled air‐sea processes and interactions. The RegCM‐ES model, which can be easily implemented over any regional domain of interest, is open source, making it suitable for usage by the broad scientific community. Plain Language Summary The increasing availability of observational data sets of high temporal and spatial resolution is providing a more complete view of the ocean and atmosphere, revealing strong air‐sea coupling processes. In order to obtain an accurate representation and better understanding of the climate system, its variability, and possible future change, the inclusion of all mechanisms of interaction among the different climate components becomes ever more desirable. Regional coupled ocean‐atmosphere models can be especially useful tools to provide information on the mechanisms of air‐sea interactions and feedback occurring at regional fine spatial and temporal scales. Here we present a new, state‐of‐the‐art, Earth System Regional Climate Model (RegCM‐ES). Key Points A new Regional Earth System Model (RegCM‐ES) successfully simulates climate features in regions where coupled air‐sea processes are important RegCM‐ES shows reduction of precipitation biases and good performance simulating the effects of air‐sea interactions over frontal regions RegCM‐ES is an open source community model, making it suitable for use by a large scientific community on any regional domain of interest

The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia.

The zonal oscillation of the western Pacific subtropical high (WPSH) significantly influences the weather and climate over East Asia. This study investigates characteristics and mechanisms of the zonal variability of the WPSH on subseasonal time scales during summer by using a subseasonal WPSH (Sub-WPSH) index. Accompanied with the Sub-WPSH index, strong anticyclonic (cyclonic) anomalies are found over East Asia and coastal region south of 30°N on both 850 hPa and 500 hPa. During the positive period of the Sub-WPSH index, the WPSH extends more westward with enhanced precipitation over the Yangtze–Huaihe river basin and suppressed precipitation over the south of the Yangtze River in China. These precipitation anomalies can last for at least 1 week. While the subseasonal zonal variability of the WPSH is found to be closely associated with atmospheric teleconnections and local air- sea interaction, the mechanisms of the variability are different before and after mid-July (early and late summer). In both early and late summer, the East Asia/Pacific (EAP) wave train pattern affects the zonal shift of the WPSH by inducing a low-level anomalous anticyclonic/cyclonic circulation over the subtropical western Pacific, and this mechanism is stronger in late summer. In constrast, the influence of the Silk-Road pattern wave train is more important in the early summer. Meanwhile, in late summer, a stronger SST forcing on the atmosphere and a faster cycle of subseasonal variations of the WPSH are observed before the westward stretch of the WPSH, which could be related to the colder local SST anomalies. The westward stretch of the WPSH is accompanied by stronger anticyclonic anomalies in late summer.

In climate models, the subgrid‐scale orography (SSO) parameterization imposes a blocked flow drag at low levels that is opposed to the local flow. In IPSL‐CM6A‐LR, an SSO lift force is also applied perpendicular to the local flow to account for the effect of locally blocked air in narrow valleys. Using IPSL‐CM6A‐LR sensitivity experiments, it is found that the tuning of both effects strongly impacts the atmospheric circulation. Increasing the blocking and reducing the lift lead to an equatorward shift of the Northern Hemisphere subtropical jet and a reduction of the midlatitude eddy‐driven jet speed. It also improves the simulated synoptic variability, with a reduced storm‐track intensity and increased blocking frequency over Greenland and Scandinavia. Additionally, it cools the polar lower troposphere in boreal winter. Transformed Eulerian Mean diagnostics also show that the low‐level eddy‐driven subsidence over the polar region is reduced consistent with the simulated cooling. The changes are amplified in coupled experiments when compared to atmosphere‐only experiments, as the low‐troposphere polar cooling is further amplified by the temperature and albedo feedbacks resulting from the Arctic sea ice growth. In IPSL‐CM6A‐LR, this corrects the warm winter bias and the lack of sea ice that were present over the Arctic before adjusting the SSO parameters. Our results, therefore, suggest that the adjustment of SSO parameterization alleviates the Arctic sea ice bias in this case. However, the atmospheric changes induced by the parametrized SSO also impact the ocean, with an equatorward shift of the Northern Hemisphere oceanic gyres and a weaker Atlantic meridional overturning circulation. Plain Language Summary Some of the processes responsible for the impacts of orography on the mean flow, such as low‐level flow blocking, or mountain waves, are unresolved in climate models at standard horizontal resolution. Such processes are accounted for using subgrid‐scale orography parameterization in climate models. Adjusting such parameterization is well known to improve the simulation of the mean climate in midlatitudes and to increase the skill of operational forecasts. In this study, the impact on the Arctic climate is studied in a climate model. It is found that adjusting the subgrid‐scale orography parameterization modulates both the atmospheric variability and mean state, with a large impact on the atmospheric momentum, heat, and moisture transport from the midlatitude to the Arctic. In particular, increasing the low‐level flow blocking leads to decreased atmospheric transport to the Arctic. Such impacts are found in both atmosphere‐only and coupled ocean‐atmosphere sensitivity simulations designed to investigate the influence of the parametrized orography. The coupled climate simulations further illustrate the impact of the subgrid‐scale orography adjustment for the sea ice and oceanic circulation. Increasing low‐level flow blocking is found to increase substantially the winter sea ice growth, while it reduces the Atlantic meridional overturning circulation. Key Points The adjustment of the parametrized orography in climate models can alleviate the near‐surface winter biases over the Arctic Increasing low‐level drag reduces Northern Hemisphere stationary wave amplitudes and shifts the subtropical jet equatorward Increasing low‐level drag increases the Arctic sea ice coverage and reduces the Atlantic meridional overturning circulation

During 2016 boreal summer and fall, a strong negative Indian Ocean Dipole (IOD) event occurred, which led to large climate impacts such as the drought over East Africa. In this study, efforts are made to understand the dynamics of this IOD event and to evaluate real-time IOD predictions from current operational seasonal forecast systems. We show that both the wind-evaporation-SST and thermocline feedback lead to fast IOD growth in boreal summer 2016. Anomalous westerlies over the tropical Indian Ocean warmed the sea surface temperature (SST) over the tropical southeastern Indian Ocean (TSEIO) by reducing local evaporation; and wind induced thermocline deepening increased TSEIO SST by vertical advection. The intraseasonal disturbances in May induced the early subsurface warming and initiated the 2016 IOD. Due to negative cloud-radiation-SST feedback, the 2016 IOD event decayed quickly after October. We also demonstrate the successful real-time IOD predictions by the operational Hadley Center Global seasonal forecasting system version 5 (GloSea5) and the Beijing Climate Center Climate System Model (BCC-CSM1.1m). Resulting from the realistic representation of observed air–sea interactions, both models successfully predicted the evolution of the 2016 IOD up to 2 seasons ahead. The skillful prediction is also due to the precursor of the early subsurface warming in the eastern Indian Ocean, which increases intrinsic predictability of the 2016 IOD event. It is also demonstrated that IOD amplitude biases can be reduced by the joint-model prediction. The successful prediction of the 2016 IOD event allowed the East African drought to be predicted 4–6 months ahead. Our study reveals that current operational climate models can give useful warning of impending IOD events and impending climate extremes.

In this work we present a unique set of coincident and collocated high‐resolution observations of surface currents and directional properties of surface waves collected from an airborne instrument, the Modular Aerial Sensing System, collected off the coast of Southern California. High‐resolution observations of near surface current profiles and shear are obtained using a new instrument, “DoppVis”, capable of capturing horizontal spatial current variability down to 128 m resolution. This data set provides a unique opportunity to examine how currents at scales ranging from 1 to 100 km modulate bulk (e.g., significant wave height), directional and spectral properties of surface gravity waves. Such observations are a step toward developing better understanding of the underlying physics of submesoscale processes (e.g., frontogenesis and frontal arrest) and the nature of transitions between mesoscale and submesoscale dynamics. Plain Language Summary In recent years, through improvement of computational resolution of global ocean models, scientists have begun to suspect that kilometer‐scale eddies, whirlpools and fronts, called “submesoscale” variability, make important contributions to horizontal and vertical exchange of climate and biological variables in the upper ocean. Such features are challenging to analyze, because of their size (and how quickly they evolve; within hours), they are too large to study from a research vessel but smaller than regions typically studied with satellite measurements. In this work, we use a research aircraft instrumented to characterize ocean currents, temperature, color (in turn chlorophyll concentration) and the properties of surface waves over an area large enough to capture submesoscale processes. This approach is a step forward in understanding and quantifying the underlying physics of submesoscale processes, and in turn develops parameterization that can help improve the fidelity of weather and climate models. Key Points Unique coincident and collocated airborne observations of Sea Surface Temperature, surface currents and properties of surface waves across submesoscales features A new airborne instrument enables observations of surface currents, vertical and horizontal shear to capture quickly evolving ocean features Such observations are crucial to develop better understanding of the physics of submesoscale processes and wave‐current interaction

Despite the significant socioeconomic implications in the link between atmospheric waviness and extreme weather events, future atmospheric waviness trends remain elusive due to uncertainties arising from diverse definitions and insufficient dynamical formalism in existing metrics. This study employs a local wave activity (LWA) metric, whose prognostic equation links wave activity changes to forcing mechanisms, to assess wintertime Northern Hemisphere waviness in ERA5 and HighResMIP data sets. The models generally exhibit high fidelity in reproducing observed waviness, while biases stem primarily from biases in the LWA source, low‐level meridional heat flux, which tend to improve with higher resolutions. Future projections exhibit reduction in LWA, primarily due to suppressed LWA generation, which is mitigated by higher‐resolution models. We found that both biases and reduction of the LWA source are closely associated with sensible heat fluxes from the ocean to the atmosphere, highlighting the potential impacts of resolving ocean currents. Plain Language Summary This study investigates atmospheric waviness in the Northern Hemisphere during winter, using a measure called local wave activity, which enables identifying causes behind waviness changes. Applying this metric to various climate models with different resolutions and a reanalysis data set, we found that historical simulations by the models generally well capture the observed climatological waviness. Biases are found over Europe and mid‐latitudes in Eurasia and the Pacific, which generally improve with higher‐resolution models. In the future simulation, reduction in waviness over much of the Northern Hemisphere, especially in the Atlantic and Pacific sectors, due to a decreased waviness source is found. Higher‐resolution models mitigate this reduction over North Atlantic. Our findings emphasize the connection between waviness and heat given from the ocean to the atmosphere, highlighting the importance of higher ocean resolution for accurate future waviness projections. This study underscores the significant role of the ocean in shaping atmospheric waviness and its implications for climate predictions. Key Points Biases in Northern Hemisphere wintertime climatological waviness in historical runs are mostly ameliorated with high‐resolution models Future Northern Hemisphere winter waviness will undergo a general reduction, which is mitigated by high‐resolution models over North Atlantic Sensible heat fluxes play a crucial role in both historical biases and future reductions in waviness, highlighting the key role of the ocean

The impacts of air‐sea interactions on the representation of tropical precipitation extremes are investigated using an atmosphere‐ocean‐mixed‐layer coupled model. The coupled model is compared to two atmosphere‐only simulations driven by the coupled‐model sea‐surface temperatures (SSTs): one with 31 day running means (31 d), the other with a repeating mean annual cycle. This allows separation of the effects of interannual SST variability from those of coupled feedbacks on shorter timescales. Crucially, all simulations have a consistent mean state with very small SST biases against present‐day climatology. 31d overestimates the frequency, intensity, and persistence of extreme tropical precipitation relative to the coupled model, likely due to excessive SST‐forced precipitation variability. This implies that atmosphere‐only attribution and time‐slice experiments may overestimate the strength and duration of precipitation extremes. In the coupled model, air‐sea feedbacks damp extreme precipitation, through negative local thermodynamic feedbacks between convection, surface fluxes, and SST. Key Points A new ocean‐mixed‐layer configuration of the MetUM is used to investigate air‐sea interactions and precipitation extremes Air‐sea coupled feedbacks reduce the frequency, intensity, and persistence of tropical precipitation extremes Existing atmosphere‐only experiments are likely to overestimate the intensity and persistence of tropical precipitation extremes

We perform quadrupled CO2 climate simulations with the Community Earth System Model version 1 (CESM1) to study how air‐sea coupling affects the response of tropical rainfall under global warming. We use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows directed perpendicular to the wind, and the near‐equator frictional flows directed in the same direction as the wind. We show that the Pacific Ocean's enhanced equatorial warming pattern (EEW) and equatorward ITCZ contraction observed in previous climate simulations emerge when the ocean model includes wind‐driven Ekman and frictional flows. Furthermore, the near‐equator frictional flow contributes more than half of the heat convergence in the equatorial Pacific Ocean. Finally, we show that although Ekman flow and near‐equator frictional flow can both result in EEW, their coupled interactions with the Hadley circulation lead to opposite feedbacks on EEW's strength. Plain Language Summary The ocean is important in modulating the atmospheric response to climate change. Here, we study how air‐sea coupling affects the response of tropical rainfall under global warming. To identify the importance of individual ocean processes, we use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows, and frictional flows. We show that including Ekman and frictional flows allows our simulation to produce the Pacific Ocean's enhanced equatorial warming pattern and equatorward ITCZ contraction noted in previous climate simulations. We also show that the frictional flow, which has yet to receive much attention, is as important as the Ekman flow in generating equatorial heat convergence. Key Points Air‐sea momentum coupling enhances equatorial sea surface temperature (SST) response The frictional ocean flow, parallel with the surface wind stress, contributes half of the ocean heat convergence response near the equator The SST and vertical wind response to greenhouse gas forcing are modulated by the ocean's temperature and vertical velocity distribution

The Kuroshio takes a large meander (LM) path since summer of 2017 for the first time since the 2004–2005 event and is the sixth LM event since 1965. It has been commonly recognized that a cool water pool is distributed broadly in the inshore region between the Kuroshio and southern coast of the Tokai district, Japan, during the LM periods. By using the recently-developed 1-km high-resolution sea surface temperature data, here we show marked coastal warming off the Tokai district during the LM periods, despite the Kuroshio not passing through the coastal area. The archived temperature-salinity profiles reveal that large positive anomalies off the Tokai district exist not only at the sea surface but also below 300 m and the water properties of which are those of the offshore Kuroshio water. The warm, salty waters are transported inshore by the westward Kuroshio which bifurcates at around 138° E, 34° N, during the LM path periods. We detect an increased upward heat release via turbulent heat fluxes along the coastal warming region from the new-generation atmosphere reanalysis data on a 25 km grid. These are common features to the past LMs and, furthermore, the region around the Kanto-Tokai districts becomes warmer than usual in warm seasons during the LM events. Our result reveals that the LM event can exert an influence upon the Japanese climate via the coastal air-sea interaction.