Submesoscale mixed layer eddies (MLE) and boundary layer turbulence (BLT) are two predominant physical processes in the ocean surface mixed layer that overlap in spatial and temporal scales. Their interaction may strongly modulate vertical mixing in the upper ocean, but has not yet been included in ocean general circulation models (GCM), in which their effects are parameterized separately. In addition, as MLEs are progressively resolved in global ocean GCMs with increasing computational resources, BLT will likely remain unresolved and require parameterizations in the foreseeable future. It is therefore essential to quantify the impact of parameterized BLT on the development and evolution of MLEs, in particular when BLT is actively driven by surface winds. Here we compare simulated MLEs at an idealized submesoscale front with down-front wind-driven BLT parameterized by KPP and k-". We found that a stronger parameterized BLT-induced vertical mixing results in intensified submesoscale vertical motions, enhanced vertical buoyancy transport, and increased MLE-induced restratification rates. This close coupling between MLE-induced restratification and BLT-induced destratification can be largely attributed to the increase of available potential energy by BLT-induced vertical mixing in frontal regions, which is extracted by MLEs through mixed layer instability. This suggests that uncertainties in BLT parameterizations can lead to uncertainties in the simulated MLEs. Therefore, their impact on vertical transport of heat and materials should be assessed together. In addition, our results also suggest that parameterizations of BLT and MLE in coarse-resolution GCMs should be implemented jointly rather than separately.

Langmuir turbulence in shallow-water coastal environments can reach the seafloor, developing into Langmuir supercells, which enhance size and mixing intensity. Two fundamental issues in coastal Langmuir turbulence remain unclear: (i) the energy cycle of the turbulence under different circumstances, and (ii) its effect on vertical mixing. We investigate these issues using large eddy simulations, considering aligned and opposing wind-wave and current directions. Results show that Langmuir supercells possess an intense full-column, narrow-band energetic mode, distinct from Langmuir turbulence in the energy spectrum. This mode occurs with aligned wind/wave and current directions but disappears when they oppose. In the latter case, only Langmuir and shear turbulence exist near surface and bottom boundaries; moreover, despite no stratification in simulations, their intensities are suppressed by a mid-layer barrier that limits surface-bottom interaction. When Langmuir supercells are present, the surface-bottom exchange of momentum is highly asymmetric between upwelling and downwelling limbs. Strong connections between surface and bottom turbulence, as indicated by the vortex-tube-connection events, can only be found in upwelling regions. As a result, the upwelling motions contribute considerably more to the momentum flux than the downwelling motions. All these results indicate that, despite the windrow pattern on the ocean surface from near-surface wind-wave interaction, whether full-column supercells can be activated or suppressed depends on different interactions between near-surface wind-wave forcing and near-bottom shear forcing. Once Langmuir supercells are activated, they differ significantly from Langmuir turbulence from the perspectives of energy and momentum transport; therefore, they cannot be simply treated as a “full column” version of Langmuir turbulence.

Langmuir turbulence (LT) plays an important role in enhancing vertical mixing in the ocean surface boundary layer (OSBL). Such enhanced mixing is strongly affected by the diurnally varying heat flux, especially in the early morning when there is a transition from cooling to heating. In this period, turbulence is weakened, yet the surface heat flux is changing rapidly, such that the deviation of transient turbulence from its equilibrium state is large. This may lead to biases in the parametrization of turbulent mixing due to LT in large-scale ocean circulation models, in which an equilibrium of the turbulence state with the surface forcing is often assumed. In this study, we investigate the transient response of LT to an abrupt onset of surface heating using idealized large eddy simulations, and compare it with the transient response of wind-driven shear turbulence (ST). Near the surface, the destabilizing Stokes shear force competes with the stabilizing surface heating, resulting in a gradual decay of the turbulence intensity, in contrast to ST whose intensity decreases rapidly at first and then partially recovers due to the formation of a stronger jet in the surface warm layer. Below the surface, the decay of coherent downwelling plumes of LT occurs faster than shear turbulence, resulting in a quicker response of LT than ST at depth. The vertical velocity variance of LT at depth decays at a rate initially following t-1 and later transitioning to t-2. We also examine the impact of details of the Stokes forcing on the transient response of LT. These results may help improve vertical mixing parametrizations in the OSBL.

Langmuir turbulence affects turbulent mixing in the ocean boundary layers and its effects require parameterizations in ocean circulation models. Most existing Langmuir turbulence parameterizations focus on the surface boundary layer in open oceans. In the shallow waters of coastal oceans, a surface boundary layer may interact and even merge with a bottom boundary layer. It is unclear how existing Langmuir turbulence parameterizations perform under such complex conditions. Here we assess the performance of two recent Langmuir turbulence parameterizations in an idealized case of merging boundary layers against turbulence-resolving large-eddy simulations (LES). In addition to assessing the solutions of free runs of single-column model (SCM) simulations, in which errors in the mean fields and turbulent fluxes are entangled, we also compare the simulated turbulent fluxes in SCM simulations with their mean fields nudged to those of the LES. In doing so, we focus on the parameterized turbulent fluxes in different parameterizations given the perfect mean fields. Our comparison highlights the tendency of parameterizations to deviate from the LES at each time instance, and thereby reveals the deficiencies of parameterizations in an instantaneous sense. It is shown that both parameterizations overestimate the near-bottom turbulent momentum flux when velocity shear is correct, resulting in too weak near-bottom shear in a free run. Consistent with previous studies, a down-Stokes drift shear momentum flux is necessary for capturing the momentum flux due to Langmuir turbulence but still misses the nonlocal momentum flux when coherent Langmuir supercells form.

Metre-scale boundary layer turbulence and kilometer-scale submesoscale mixed layer eddies play crucial roles in upper ocean stratification. It is well established that the former generates significant vertical fluxes that mix the upper ocean, while the latter acts to restratify it. However, the interaction between these two multi-scale processes is not well understood, particularly when atmospheric forces are non-negligible. MPAS-Ocean was firstly used to investigate the influence of boundary layer turbulence on submesoscale eddy-induced restratification under various initial conditions. Two parametrizations K-Profile Parametrization (KPP) and k-ε were used to represent different forms of turbulence-induced destratification under the same forcing conditions. Comparison analysis was carried out by comparing the resulting submesoscale eddy-induced restratification. Among all cases, KPP exhibited a larger magnitude of vertical buoyancy flux than k-, indicating stronger turbulence-induced destratification. This enhanced destratification can lead to more intense submesoscale eddy-induced restratification, which largely compensates the turbulence-induced destratification. Furthermore, the value of the mixed layer eddy-induced streamfunction strongly depends on the strength of boundary layer turbulence, suggesting that parameterizations of these two processes may need to consider their interactions. To further explore the bidirectional interactions between these two processes, we are currently employing large-eddy simulation to resolve both. A spatial filter is used to separate the flow into submesoscales and small-scale turbulence. Preliminary results of the large eddy simulations, aiming to elucidate the interactions between these two processes, will be discussed.

Processes at the air-sea interface govern the climate mean state and variability by determining the exchange of momentum, heat, and water between the atmosphere and ocean. Traditional climate models compute those exchanges across the air-sea interface by assuming an ocean surface with roughness determined by atmospheric wind and stability conditions, essentially assuming ocean surface waves are in equilibrium states. In reality, that is rarely the case. Such effects have been emphasized in numerical weather predictions for weather systems like tropical cyclones. An accurate representation of ocean surface waves requires a prognostic ocean surface wave model. The addition of WAVEWATCH III to the Community Earth System Model version 2 (CESM2) makes it possible to parameterize the impacts of ocean surface waves on the momentum and energy exchange. This study documents the implementation of a sea-state-dependent surface flux scheme in CESM2. It considers the effects of waves on ocean surface roughness and those of sea spray on sensible and latent heat. It is found that the new scheme significantly impacts mean atmospheric circulation and the upper ocean. The errors in mean atmospheric circulation and surface temperature patterns are reduced. The modified surface flux lowers the eddy-driven jet speed and weakens the Hadley circulation. Global sea surface temperature (SST) warm bias is reduced due to the cooling of the Southern Ocean and eastern boundary currents. In particular, some parts of eastern and central Pacific exhibit a weak cooling trend in the simulation for recent decades, reducing the existing SST trend bias in CESM2.

Advances in coastal modeling and computation provide the opportunity to examine non-hydrostatic and compressible fluid effects at very small scales, but the cost of these new capabilities and the accuracy of these models versus trusted non-hydrostatic codes has yet to be determined. Here the Coastal and Regional Ocean COmmunity model (CROCO, v1.2) and the National Center for Atmospheric Research large-eddy simulation (NCAR-LES) model are compared, with a focus on their simulation accuracy and computational efficiency. These models differ significantly in numerics and capabilities, so they are run on common classic problems of surface-forced, boundary-layer turbulence. In terms of accuracy, we compare turbulence statistics, including the effect of the explicit subgrid-scale (SGS) parameterization, the effect of the second (dilatational) viscosity, and the sensitivity to the speed of sound, which is used as part of the CROCO compressible turbulence formulation. To gauge how far CROCO is from the NCAR-LES, we first compare the NCAR-LES with two other non-hydrostatic Boussinesq approximation LES codes (PALM and Oceananigans), defining the notion and magnitude of accuracy for the LES and CROCO comparison. To judge efficiency of CROCO, strong and weak scaling simulation sets vary different problem sizes and workloads per processor, respectively. Additionally, the effects of 2D decomposition of CROCO and NCAR-LES and supercomputer settings are tested. In summary, the accuracy comparison between CROCO and the NCAR-LES is similar to the NCAR-LES compared to other LES codes. However, the additional capabilities of CROCO (e.g., nesting, non-uniform grid, and realism of ocean configuration in general) and its weakly compressible formulation come with roughly an order of magnitude of additional costs, despite efforts to reduce them by adjusting the second viscosity and speed of sound as far as accuracy allows. However, a new variant of the non-hydrostatic CROCO formulation is currently undergoing prototype testing and should enable faster simulations by releasing the stability constrain by the free surface. Overall, when the additional features of CROCO are needed (nesting, complex topography, etc.) additional costs are justified, while in idealized settings (a rectangular domain with periodic boundary conditions) the NCAR-LES is faster in arriving at nearly the same result.

While various parameterizations of vertical turbulent fluxes at different levels of complexity have been proposed, each has its own limitations. For example, simple first-order closure schemes such as the K-Profile Parameterization (KPP) lack energetic constraints; two-equation models like $k-\epsilon$ directly solve an equation for the turbulent kinetic energy but do not account for non-local fluxes, and high-order closures that include the non-local transport terms are computationally expensive. To address these, here we extend the Assumed-Distribution Higher-Order Closure (ADC) framework originally proposed for the atmospheric boundary layer and apply it to the ocean surface boundary layer (OSBL). By assuming a probability distribution function relationship between the vertical velocity and tracers, all second-order and higher-order moments are exactly constructed and turbulence closure is achieved in the ADC scheme. In addition, the ADC parameterization scheme has full energetic constraints. We have tested the ADC scheme against a combination of large eddy simulation (LES), KPP, and $k-\epsilon$ for surface buoyancy-driven convective mixing and found that the ADC scheme is robust with different vertical resolutions and compares well to the LES results.

This work evaluates the fidelity of various upper-ocean turbulence parameterizations subject to realistic monsoon forcing and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. The EV method emphasizes the dynamics of a turbulence closure multimodel ensemble and is applied to evaluate 10 different ocean surface boundary layer (OSBL) parameterizations within a single-column (SC) model against two boundary layer large-eddy simulations (LES). Both LES include realistic surface forcing, but one includes wind-driven shear turbulence only, while the other includes additional Stokes forcing through the wave-average equations that generate Langmuir turbulence. The finite-time EV framework focuses on what constitutes the local behavior of the mixed layer dynamical system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. Observations collected during the 2018 monsoon onset in the Bay of Bengal provide a case study to evaluate models under realistic and variable forcing conditions. The case study results highlight two regimes where models disagree 1) during wind-driven deepening of the mixed layer and 2) under strong diurnal forcing.

A process study using large-eddy simulations is carried out to explore the dominant 1-D processes that affect mixed layer (ML) properties during an event of summer Monsoon Intra-seasonal Oscillations (MISO) in the Bay of Bengal (BOB). These simulations use realistic air-sea fluxes and initial conditions that were collected during the summer 2018 MISO-BOB field experiment to explore the roles of thermal inversion layer (TIL) and Langmuir turbulence (LT) in modulating ML properties. The simulations span an active period with heavy rain and strong winds and a break period with strong solar heat flux and little rain. The mixed layer depth (MLD), sea surface temperature (SST) and sea surface salinity (SSS) are most affected by the presence of near-inertial oscillations, solar heating and precipitation, all of which occur at different timescales. The subsurface warming induced by the TIL reduces the SST variability at the MISO timescale when compared with the simulation without TIL. Comparison of simulations with and without LT indicates that LT enhances subsurface warming during the active phase and reduces diurnal SST modulation during the break phase. Simulations with 1-D mixing models show a wide disparity in the evolution of MLD, SST, and SSS.

There is increased concern about surface pollution associated with stratospheric ozone intrusion events. By using NASA's AQUA/AIRS, in combination with the European Center for Medium-Range Weather Forecasts (ECMWF) high resolution reanalysis data and the ground sites of excessive ozone, the ozone concentration, relative humidity and carbon monoxide (CO) concentration in the vertical profile data were obtained, and their spatial-temporal variations and stratospheric ozone intrusion were preliminarily investigated over China. Meanwhile, the intrusion of air downward towards the surface were analyzed by HYSPLIT model. The results indicated that three typical regions, Chaoyang, Shijiazhuang and Hangzhou, experienced stratospheric ozone intrusion events in the spring and summer of 2019. The ozone concentration has clear negative relation with the relative humidity and CO concentration, which their vertical profiles can be used as an auxiliary basis for judging stratospheric ozone intrusion. Further work needs to be done to evaluate the quantitative effects of stratospheric ozone intrusion on ground ozone concentration.

2 (E3SMv2) is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics.We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5°river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid-latitudes and 30 km at the equator and poles.The model performance is evaluated by means of a standard set of Coupled Model Intercomparison Project Phase 6 (CMIP6) Diagnosis, Evaluation, and Characterization of Klima (DECK) simulations augmented with historical simulations as well as simulations to evaluate impact of different forcing agents.The simulated climate is generally realistic, with notable improvements in clouds and precipitation compared to E3SMv1.E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K.In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Programme (WCRP) assessment.However, E3SMv2 significantly underestimates the global mean temperature in the second half of the historical record.An analysis of single-forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol-related forcing.

The ocean surface boundary layer links the atmosphere to the ocean. At the air-sea interface, ocean surface waves play an important role in momentum, energy and gas exchange. A new parameterization with wave-induced mixing is developed based on a set of Large Eddy Simulation experiments under different wind speeds and mixed layer depths. The new parameterization scheme is then incorporated into a one-dimensional turbulence model for verification. The inclusion of wave-induced mixing reduces the excessively high surface temperature simulated in summer and reduces the underestimation of the mixed layer depth in winter. Compared to the observation at Ocean Station Papa, the parameterization scheme with wave effects produces statistically more accurate results than the parameterization scheme without wave effects.

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Abstract. This paper documents the experimental setup and general features of the coupled historical and future climate simulations with the first version of the U.S. Department of Energy (DOE) Energy Exascale Earth System Model (E3SMv1.0). The future projected climate characteristics of E3SMv1.0 at the highest emission scenario (SSP5-8.5) designed in the Scenario Model Intercomparison Project (ScenarioMIP) and the SSP5-8.5 greenhouse gas (GHG) only forcing experiment are analyzed with a focus on regional responses of atmosphere, ocean, sea-ice, and land. Due to its high climate sensitivity, E3SMv1.0 is one of the CMIP6 models with the largest surface warming by the end of the 21st century under the high-emission SSP5-8.5 scenario. The global mean precipitation change is highly correlated to the global temperature change, while the spatial pattern of the change in runoff responds to the precipitation changes. The oceanic mixed layer generally shoals throughout the global ocean. The sea ice, especially in the Northern Hemisphere, rapidly decreases with large seasonal variability. The annual mean AMOC is overly weak with a slower change relative to other CMIP6 models. We detect a significant polar amplification in E3SMv1.0 from the atmosphere, ocean, and sea ice. Comparing the SSP5-8.5 all-forcing experiment with the GHG-only experiment, we find that the unmasking of the aerosol effects due to the decline of the aerosol loading in the future projection period causes accelerated warming in SSP5-8.5 all-forcing experiment. While the oceanic climate response is mainly controlled by the GHG forcing, the land runoff response is impacted primarily by forcings other than GHG over certain regions. However, the importance of the GHG forcing on the land runoff changes grows in the future climate projection period compared to the historical period.

Heinrich Stadial 1 (HS1) was the major climate event at the onset of the last deglaciation associated with rapid cooling in Greenland and lagged, slow warming in Antarctica. Although it is widely believed that temperature signals were triggered in the Northern Hemisphere and propagated southward associated with the Atlantic meridional overturning circulation (AMOC), understanding how these signals were able to cross the Antarctic Circumpolar Current (ACC) barrier and further warm up Antarctica has proven particularly challenging. In this study, we explore the physical processes that lead to the Antarctic warming during HS1 in a transient isotope-enabled deglacial simulation iTRACE, in which the interpolar phasing has been faithfully reproduced. We show that the increased meridional heat transport alone, first through the ocean and then through the atmosphere, can explain the Antarctic warming during the early stage of HS1 without notable changes in the strength and position of the Southern Hemisphere midlatitude westerlies. In particular, when a reduction of the AMOC causes ocean warming to the north of the ACC, increased southward ocean heat transport by mesoscale eddies is triggered by steeper isopycnals to warm up the ocean beyond the ACC, which further decreases the sea ice concentration and leads to more absorption of insolation. The increased atmospheric heat then releases to the Antarctic primarily by a strengthening zonal wavenumber-3 (ZW3) pattern. Sensitivity experiments further suggest that a ∼4°C warming caused by this mechanism superimposed on a comparable warming driven by the background atmospheric CO2 rise is able to explain the total simulated ∼8°C warming in the West Antarctica during HS1.

This work documents version two of the Department of Energy's Energy Exascale Earth System Model (E3SM). E3SMv2 is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics. We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5° river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid‐latitudes and 30 km at the equator and poles. The model performance is evaluated with Coupled Model Intercomparison Project Phase 6 Diagnosis, Evaluation, and Characterization of Klima simulations augmented with historical simulations as well as simulations to evaluate impacts of different forcing agents. The simulated climate has many realistic features of the climate system, with notable improvements in clouds and precipitation compared to E3SMv1. E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K. In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Program assessment. However, a number of important biases remain including a weak Atlantic Meridional Overturning Circulation, deficiencies in the characteristics and spectral distribution of tropical atmospheric variability, and a significant underestimation of the observed warming in the second half of the historical period. An analysis of single‐forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol‐related forcing.

Carbon dioxide and methane are the two most important greenhouse gases and are closely related to global warming and extreme weather events. To master their spatial and temporal variations, the CO2 and CH4 concentration data monitored by the GOSAT satellite in 2020 and 2021 were used to map and analyse the annual, seasonal and monthly changes in CO2 and CH4 concentrations in the world and major countries/regions. The results demonstrate that (1) in 2021, the average annual CO2 concentration over the global land area was 412.74 ppm, an increase in 0.64% compared with the same period last year, and there were spatial differences in the distribution of CO2 concentration, with high values mostly concentrated in the middle latitudes of the Northern Hemisphere; (2) compared with 2020, the CO2 concentration in China, the United States, India, the European Union and other countries/regions increased significantly; (3) in 2020 and 2021, the quarterly CO2 trend of the global and major countries/regions was the same, which was higher in the first (January, February, March) and second (April, May, June) quarters, significantly lower in the third (July, August, September) quarter, and gradually increased in the fourth (October, November, December) quarter. Further work on long time series and validation needs to be conducted.

Empirically generated indices are used to evaluate the skill of a global climate model in representing the monsoon intraseasonal oscillation (MISO). This work adapts the method of Suhas et al., an extended empirical orthogonal function (EEOF) analysis of daily rainfall data with the first orthogonal function indicating MISO strength and phase. This method is applied to observed rainfall and Community Earth System Model (CESM1.2) simulation results. Variants of the CESM1.2 including upper ocean parameterizations for Langmuir turbulence and submesoscale mixed layer eddy restratification are used together with the EEOF analysis to explore sensitivity of the MISO to global upper ocean process representations. The skill with which the model variants recreate the MISO strength and persistence is evaluated versus the observed MISO. While all model versions reproduce the northward rainfall propagation traditionally associated with the MISO, a version including both Langmuir turbulence and submesoscale restratification parameterizations provides the most accurate simulations of the time scale of MISO events.