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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 ocean component of the Community Climate System Model version 4 (CCSM4) is described, and its solutions from the twentieth-century (20C) simulations are documented in comparison with observations and those of CCSM3. The improvements to the ocean model physical processes include new parameterizations to represent previously missing physics and modifications of existing parameterizations to incorporate recent new developments. In comparison with CCSM3, the new solutions show some significant improvements that can be attributed to these model changes. These include a better equatorial current structure, a sharper thermocline, and elimination of the cold bias of the equatorial cold tongue all in the Pacific Ocean; reduced sea surface temperature (SST) and salinity biases along the North Atlantic Current path; and much smaller potential temperature and salinity biases in the near-surface Pacific Ocean. Other improvements include a global-mean SST that is more consistent with the present-day observations due to a different spinup procedure from that used in CCSM3. Despite these improvements, many of the biases present in CCSM3 still exist in CCSM4. A major concern continues to be the substantial heat content loss in the ocean during the preindustrial control simulation from which the 20C cases start. This heat loss largely reflects the top of the atmospheric model heat loss rate in the coupled system, and it essentially determines the abyssal ocean potential temperature biases in the 20C simulations. There is also a deep salty bias in all basins. As a result of this latter bias in the deep North Atlantic, the parameterized overflow waters cannot penetrate much deeper than in CCSM3. [ABSTRACT FROM AUTHOR]

Numerous studies and observational analyses point to a connection between the annual cycle and tropical Atlantic variability, specifically the influence of the seasons. Although a previous study has shown that the annual cycle is not necessary for the generation of this variability, this study demonstrates that the annual cycle provides particular conditions that modulate this variability. Particular seasons are investigated through the use of a coupled ocean–atmosphere model using anomaly coupling as the coupling strategy in order to control the mean state of the system. To isolate the influence of each season, the model is integrated in perpetuated mean states that simulate perpetual boreal spring, summer, fall, and winter seasonal mean states. These are compared to a control simulation that contains an annual cycle. Evidence is shown that the annual cycle modulates tropical Atlantic variability in the following three ways: 1) the background mean state for some seasons provides favorable conditions for the growth of particular patterns through regional air–sea feedback mechanisms, 2) mechanisms that excite the variability are seasonally dependent, and 3) the progression through the annual cycle is important for certain variabilities to be excited and grow. [ABSTRACT FROM AUTHOR]

Many previous studies point to a connection between the annual cycle and interannual variability in the tropical Atlantic Ocean. To investigate the importance of the annual cycle in the generation of tropical Atlantic variability (TAV) as well as its associated coupled feedback mechanisms, a set of controlled experiments is conducted using a global coupled ocean–atmosphere general circulation model (GCM) in which the climatological annual cycle is modified. An anomaly coupling strategy was developed to improve the model-simulated annual cycle and mean sea surface temperature (SST), which is critical to the experiments. Experiments include a control simulation in which the annual cycle is present and a fixed annual cycle simulation in which the coupled model is forced to remain in a perpetual annual mean state. Results reveal that the patterns of TAV, defined as the leading three rotated EOFs, and their relationship to coupled feedback mechanisms are present even in the absence of the annual cycle, suggesting that the generation of TAV is not dependent on the annual cycle. Each pattern of variability arises from an alteration of the easterly trade winds. Results suggest that it is the presence of these winds in the mean state that is the determining factor for the structure of the coupled ocean–atmosphere variability. Additionally, the patterns of variability persist longer in the simulation with no annual cycle. Most remarkable is the doubling of the decay phase related to the north tropical Atlantic variability, which is attributed to the persistence of the local wind–evaporation–sea surface temperature (WES) feedback mechanism. The author concludes that the annual cycle acts to cut off or interrupt conditions favorable for feedback mechanisms to operate, therefore putting a limit on the length of the event life cycle. [ABSTRACT FROM AUTHOR]

Two high‐resolution versions of a Coupled Earth System Model (CESM1.3: 0.25° atmosphere, 1° ocean; CESM1.1: 0.25° atmosphere, 0.1° ocean) are compared to the standard resolution CESM1.1 and CESM1.3 (1° atmosphere, 1° ocean). The CESM1.3 versions are documented, and the consequences of model resolution, air‐sea coupling, and physics in the atmospheric models are studied with regard to storm tracks in the Southern Hemisphere as represented by 850‐hPa eddy kinetic energy. Increasing the resolution from 1° to 0.25° in the atmosphere (same physics) coupled to the 1° ocean intensifies the strength of the storm tracks closer to observations. The 0.25° atmosphere with the older CESM1.1 physics coupled to the 0.1° ocean has fewer low clouds, warmer Southern Ocean sea surface temperatures, a weaker meridional temperature gradient, and a degraded storm track simulation compared to the 0.25° atmosphere with CESM1.3 physics coupled to the 1° ocean. Therefore, deficient physics in the atmospheric model can negate the gains attained by higher resolution in atmosphere and ocean. Key Points: Progression from CESM1.1 to CESM1.3 is documented with improved physics and better simulation of low clouds in CESM1.3 compared to CESM1.1Southern Hemisphere storm tracks intensify in closer agreement with observations with higher resolution in the atmosphere except in the model version with older physicsDeficient physics in the atmospheric model can negate the gains attained by higher resolution in atmosphere and ocean Plain Language Summary: Southern Hemisphere storm tracks intensify in closer agreement with observations with higher resolution in the atmosphere except in the model version with older physics, such that deficient physics in the atmospheric model, which produce less low clouds, a reduced meridional sea surface temperature gradient, and weaker storms in the Southern Hemisphere, can negate the gains attained by higher resolution in atmosphere and ocean. [ABSTRACT FROM AUTHOR]

The Community Atmosphere Model version 6 (CAM6), released in 2018 as part of the Community Earth System Model version 2 (CESM2), is a major upgrade over the previous CAM5 that has been used in numerous global and regional climate studies. Since CESM2–CAM6 will participate in the upcoming Coupled Model Intercomparison Project phase 6 (CMIP6) and is likely to be adopted in many future studies, its simulation fidelity needs to be thoroughly examined. Here we evaluate the performance of a developmental version of the Community Atmosphere Model with parameterizations that will be used in version 6 (CAM6 α), with a default 1 ∘ horizontal resolution (0.9∘×1.25∘ , CAM6 α -1 ∘) and a high-resolution configuration (approximately 0.25 ∘ , CAM6 α -0.25 ∘), against various observational and reanalysis datasets of precipitation over Asia. CAM6 α performance is compared with CAM5 at default 1 ∘ horizontal resolution (CAM5-1 ∘) and a high-resolution configuration at 0.25 ∘ (CAM5-0.25 ∘). With the prognostic treatment of precipitation processes and the new microphysics module, CAM6 α is able to better simulate climatological mean and extreme precipitation over Asia, better capture the heaviest precipitation events, better reproduce the diurnal cycle of precipitation rates over most of Asia, and better simulate the probability density distributions of daily precipitation over Tibet, Korea, Japan and northern China. Higher horizontal resolution in CAM6 α improves the simulation of mean and extreme precipitation over northern China, but the performance degrades over the Maritime Continent. Moisture budget diagnosis suggests that the physical processes leading to model improvement are different over different regions. Both upgraded physical parameterizations and higher horizontal resolution affect the simulated precipitation response to the internal variability of the climate system (e.g., Asian monsoon variability, El Niño–Southern Oscillation – ENSO, Pacific Decadal Oscillation – PDO), but the effects vary across different regions. For example, higher horizontal resolution degrades the model performance in simulating precipitation variability over southern China associated with the East Asian summer monsoon. In contrast, precipitation variability associated with ENSO improves with upgraded physical parameterizations and higher horizontal resolution. CAM6 α -0.25 ∘ and CAM6 α -1 ∘ show an opposite response to the PDO over southern China. Basically, the response to increases in horizontal resolution is dependent on the CAM version. [ABSTRACT FROM AUTHOR]

The relationship between African dust and the climatology of tropical cyclones (TCs) in the North Atlantic is explored using the Community Atmosphere Model at a global horizontal resolution of 28 km. A simulation in which the aerosol model is modified to significantly reduce the amount of airborne dust is compared to a standard simulation. The simulation with reduced dust increases TC frequency globally, with the largest increase occurring in the North Atlantic. The increase in TC activity in the North Atlantic is consistent with an environment that is more conducive for the genesis and intensification of storms. TCs are more frequent (27%) and on average significantly longer lived (13%) in the low dust configuration but only slightly stronger (3%). This results in a 57% increase in accumulated cyclone energy per hurricane season on average. This work has implications for projections of future climate and resulting changes in TC activity. Plain Language Summary: Dust from the Sahara desert in Africa can impact hurricane formation and development in the North Atlantic. Carried by the wind over the North Atlantic and around the globe, the dust contributes to dry and stable air that can make it harder for thunderstorms over the ocean to develop into stronger tropical cyclones or hurricanes. In this study, we look at how this effect shows up in a climate model with sufficient resolution to represent hurricanes. After the control experiment (1980–2012), we reduced the amount of airborne dust in the model over the same historical period as a low dust experiment. In the low dust simulation, tropical cyclone frequency increases globally. In the North Atlantic where storms can be impacted directly by African dust, storms become more frequent, longer lived, and slightly stronger, increasing their destructive potential. Understanding how dust interacts with tropical cyclones in climate models will help us understand and prepare for the potential changes in hurricanes of the future. Key Points: Global and North Atlantic TC counts increase in a CAM5 global simulation with a large reduction in dustThe increase in North Atlantic TC genesis activity in the low dust simulation is consistent with the change in the genesis potential indexSimulated TCs are more intense and longer in duration in the low dust configuration [ABSTRACT FROM AUTHOR]

Numerical simulations are conducted across model platforms and resolutions with a focus on the North Atlantic. Barotropic vorticity diagnostics confirm that the subtropical gyre is characterized by an inviscid balance primarily between the applied wind stress curl and bottom pressure torque. In an area-integrated budget over the Gulf Stream, the northward return flow is balanced by bottom pressure torque. These integrated budgets are shown to be consistent across model platforms and resolution, suggesting that these balances are robust. Two of the simulations, at 100- and 10-km resolutions, produce a more northerly separating Gulf Stream but obtain the correct integrated vorticity balances. In these simulations, viscous torque is nonnegligible on smaller scales, indicating that the separation is linked to the details of the local dynamics. These results are shown to be consistent with a scale analysis argument that suggests that the biharmonic viscous torque in particular is upsetting the inviscid balance in simulations with a more northerly separation. In addition to providing evidence for locally controlled inviscid separation, these results provide motivation to revisit the formulation of subgrid-scale parameterizations in general circulation models. [ABSTRACT FROM AUTHOR]

This study analyzes important aspects of the tropical Atlantic Ocean from simulations of the fourth version of the Community Climate System Model (CCSM4): the mean sea surface temperature (SST) and wind stress, the Atlantic warm pools, the principal modes of SST variability, and the heat budget in the Benguela region. The main goal was to assess the similarities and differences between the CCSM4 simulations and observations. The results indicate that the tropical Atlantic overall is realistic in CCSM4. However, there are still significant biases in the CCSM4 Atlantic SSTs, with a colder tropical North Atlantic and a hotter tropical South Atlantic, that are related to biases in the wind stress. These are also reflected in the Atlantic warm pools in April and September, with its volume greater than in observations in April and smaller than in observations in September. The variability of SSTs in the tropical Atlantic is well represented in CCSM4. However, in the equatorial and tropical South Atlantic regions, CCSM4 has two distinct modes of variability, in contrast to observed behavior. A model heat budget analysis of the Benguela region indicates that the variability of the upper-ocean temperature is dominated by vertical advection, followed by meridional advection. [ABSTRACT FROM AUTHOR]