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The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed-layer depth (MLD) in six pairs of non-eddying (1 ∘ grid spacing) and eddy-rich (up to 1/16 ∘) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model evaluation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low-resolution models exhibit large biases in the deep-water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode-water formation regions of the Northern Hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability in MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects. [ABSTRACT FROM AUTHOR]

The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed layer depth (MLD) in six pairs of non-eddying (1° resolution) and eddy-rich (up to 1/16°) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model validation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects. [ABSTRACT FROM AUTHOR]

Variations of the sea surface temperature (SST) and primary productivity in the northeast Pacific have far‐reaching implications. In addition to influencing the regional and global temperature and hydroclimate, these conditions also control marine ecosystems and their services, which subsequently impact regional economies. Yet, our understanding of the variability and controls of northeast Pacific SST and productivity on timescales exceeding observational records remains limited. Here, we use marine sediment records from seven locations, spanning 25.2°N–59.6°N, in the northeast Pacific to characterize the millennial‐scale variability of SST and productivity from 9,000 to 1,000 years BP. We explore the dynamics of their spatiotemporal evolution and compare these data with transient climate model outputs to identify potential drivers. Through a heat budget analysis and optimal fingerprinting analysis, we characterize the spatial pattern of forcings. We find that SST varied spatially in the northeast Pacific, with higher latitudes exhibiting greater magnitude changes than lower latitudes, which differs from previous work suggesting regional synchronicity and coherence during the Holocene. Our analysis did not find evidence for coherent variability of primary producer community nor carbon export, highlighting the difficulty of identifying the complex interactions between environmental conditions, producers, and carbon export. Model‐proxy disagreement demonstrates the need for higher resolution model frameworks, but shows nonetheless that observed variability in the proxy records can be explained by a combination of greenhouse gas and orbital forcing. We suggest that the complex SST variations and marine ecosystem responses to forced changes are important factors that can drive disagreements in model projections. Key Points: Holocene temperature and productivity changes in the northeast Pacific are spatially variableTemperature change in proxies is amplified poleward, and productivity is decoupled from temperatureComparison with TraCE21ka model simulations suggests that multiple forcings and dynamical processes are responsible for these changes [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As one kind of submesoscale instability, symmetric instability (SI) of the ocean surface mixed layer (SML) plays a significant role in modulating the SML energetics and material transport. The small spatial scales of SI, O(10 m–1 km), are not resolved by current climate ocean models and most regional models. This study describes comparisons in an idealized configuration of the SI parameterization scheme proposed by Bachman et al. (2017, https://doi.org/10.1016/j.ocemod.2016.12.003) (SI‐parameterized) versus the K‐Profile Parameterization scheme (SI‐neglected) as compared to a SI‐permitting model; all variants use the Coastal and Regional Ocean Community Model version of the Regional Ocean Modeling System and this study also serves to introduce the SI parameterization in that model. In both the SI‐parameterized and SI‐permitting models, the geostrophic shear production is enhanced and anticyclonic potential vorticity is reduced versus the SI‐neglected model. A comprehensive comparison of the energetics (geostrophic shear production, vertical buoyancy flux), mixed layer thickness, potential vorticity, and tracer redistribution indicate that all these variables in the SI‐parameterized case have structures closer to the SI‐permitting case in contrast to the SI‐neglected one, demonstrating that the SI scheme qualitatively improves representation of the impacts of SI. This work builds toward applying the SI scheme in a realistic regional or climate model. Plain Language Summary: Symmetric instability (SI) of the ocean surface mixed layer leads to small, fast features that affect energy and material transports. The coarse resolution of current climate and regional models cannot resolve SI, so accounting for its impacts requires parameterization. This work implements a SI parameterization scheme in the Coastal and Regional Ocean Community Model and the evaluation indicates that the scheme improves the simulation results. Key Points: A symmetric instability (SI) parameterization scheme is implemented in the Coastal and Regional Ocean Community ModelThe impacts of SI are evaluated based on three comparison simulations with different spatial resolutionsThe scheme qualitatively captures the impacts of SI and improves the model simulation results [ABSTRACT FROM AUTHOR]

The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, seaice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/seaice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, High- ResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations. [ABSTRACT FROM AUTHOR]

Air-sea fluxes from the Community Climate System Model version 4 (CCSM4) are compared with the Coordinated Ocean-Ice Reference Experiment (CORE) dataset to assess present-day mean biases, variability errors, and late twentieth-century trend differences. CCSM4 is improved over the previous version, CCSM3, in both air-sea heat and freshwater fluxes in some regions; however, a large increase in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. The authors provide a new baseline for assessment of flux variance at annual and interannual frequency bands in future model versions and contribute a new metric for assessing the coupling between the atmospheric and oceanic planetary boundary layer (PBL) schemes of any climate model. Maps of the ratio of CCSM4 variance to CORE reveal that variance on annual time scales has larger error than on interannual time scales and that different processes cause errors in mean, annual, and interannual frequency bands. Air temperature and specific humidity in the CCSM4 atmospheric boundary layer (ABL) follow the sea surface conditions much more closely than is found in CORE. Sensible and latent heat fluxes are less of a negative feedback to sea surface temperature warming in the CCSM4 than in the CORE data with the model's PBL allowing for more heating of the ocean's surface. [ABSTRACT FROM AUTHOR]

The influence of atmospheric CO2 concentration on the El Niño-Southern Oscillation (ENSO) is explored using 800-yr integrations of the NCAR Community Climate System Model, version 3.5 (CCSM3.5), with CO2 stabilized at the a.d. 1850, 1990, and 2050 levels. Model mean state changes with increased CO2 include preferential SST warming in the eastern equatorial Pacific, a weakening of the equatorial trade winds, increased vertical ocean stratification, and a reduction in the atmospheric Hadley and oceanic subtropical overturning circulations. The annual cycle of SST strengthens with CO2, likely related to unstable air-sea interactions triggered by an increased Northern Hemisphere land-sea temperature contrast. The mean trade wind structure changes asymmetrically about the equator, with increased convergence in the Northern Hemisphere and divergence in the Southern Hemisphere leading to corresponding deepening and shoaling of the thermocline. The proportion of eastern versus central Pacific-type El Niño events increases with CO2, but the significance of the changes is relatively low; ENSO amplitude also increases with CO2, although the change is insignificant at periods longer than 4 yr. The 2-4-yr ENSO response shows an enhancement in equatorial Kelvin wave variability, suggesting that stochastic triggering of El Niño events may be favored with higher CO2. However, the seasonal cycle-ENSO interaction is also modified by the asymmetric climatological changes, and forcing by the Southern Hemisphere becomes more important with higher CO2. Finally, higher-resolution CCSM4 control simulations show that ENSO weakens with CO2 given a sufficiently long integration time. The cause for the difference in ENSO climate sensitivity is not immediately obvious but may potentially be related to changes in westerly wind bursts or other sources of high-frequency wind stress variability. [ABSTRACT FROM AUTHOR]

Submesoscale symmetric instability (SI) in the ocean surface mixed layer (SML) plays a significant role in the forward energy cascade and vertical material transports. Due to its small spatial scales of typically less than 1 km, SI is not resolved by current climate ocean models and most regional models. This work investigates the SI effects in the Kuroshio Extension (KE) region by using a SI parameterization scheme implemented in the Coastal and Regional Ocean Community Model (CROCO). Compared to a SI-lacking model at the same spatial resolution of 500 m, a SI-parameterized model more closely resembles an ultra-high resolution (20 m) non-hydrostatic model that is intended to resolve SI directly on the basis of evaluating the energy source of SI: geostrophic shear production. As SI restratifies the SML, the SI-parameterized model simulates a shallower SML depth in the KE region compared to the SI-lacking one, which is closer to the observed state. The potential vorticity is also modulated and a reduction of negative PV is found due to the parameterized SI. The results here demonstrate the capability of the SI scheme in the representation of the SI impacts and an improvement in the simulation of the KE. [ABSTRACT FROM AUTHOR]