Matthew C. Long

The Community Earth System Model: A Framework for Collaborative Research

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad com...

Marine Ecosystem Dynamics and Biogeochemical Cycling in the Community Earth System Model [CESM1(BGC)]: Comparison of the 1990s with the 2090s under the RCP4.5 and RCP8.5 Scenarios

AbstractThe authors compare Community Earth System Model results to marine observations for the 1990s and examine climate change impacts on biogeochemistry at the end of the twenty-first century under two future scenarios (Representative Concentration Pathways RCP4.5 and RCP8.5). Late-twentieth-century seasonally varying mixed layer depths are generally within 10 m of observations, with a Southern Ocean shallow bias. Surface nutrient and chlorophyll concentrations exhibit positive biases at low latitudes and negative biases at high latitudes. The volume of the oxygen minimum zones is overestimated.The impacts of climate change on biogeochemistry have similar spatial patterns under RCP4.5 and RCP8.5, but perturbation magnitudes are larger under RCP8.5. Increasing stratification leads to weaker nutrient entrainment and decreased primary and export production (>30% over large areas). The global-scale decreases in primary and export production scale linearly with the increases in mean sea surface temperature....

Twentieth-century oceanic carbon uptake and storage in CESM1(BGC)

AbstractOcean carbon uptake and storage simulated by the Community Earth System Model, version 1–Biogeochemistry [CESM1(BGC)], is described and compared to observations. Fully coupled and ocean-ice configurations are examined; both capture many aspects of the spatial structure and seasonality of surface carbon fields. Nearly ubiquitous negative biases in surface alkalinity result from the prescribed carbonate dissolution profile. The modeled sea–air CO2 fluxes match observationally based estimates over much of the ocean; significant deviations appear in the Southern Ocean. Surface ocean pCO2 is biased high in the subantarctic and low in the sea ice zone. Formation of the water masses dominating anthropogenic CO2 (Cant) uptake in the Southern Hemisphere is weak in the model, leading to significant negative biases in Cant and chlorofluorocarbon (CFC) storage at intermediate depths. Column inventories of Cant appear too high, by contrast, in the North Atlantic. In spite of the positive bias, this marks an im...

Preindustrial-Control and Twentieth-Century Carbon Cycle Experiments with the Earth System Model CESM1(BGC)

Version1oftheCommunityEarth SystemModel, in theconfigurationwhereitsfullcarboncycleis enabled, is introduced and documented. In this configuration, the terrestrial biogeochemical model, which includes carbon‐ nitrogen dynamics and is present in earlier model versions, is coupled to an ocean biogeochemical model and atmospheric CO2 tracers. The authors provide a description of the model, detail how preindustrial-control and twentieth-century experimentswere initialized andforced, and examine thebehavior of the carbon cyclein those experiments. They examinehow sea- and land-to-air CO2fluxescontributetotheincreaseofatmosphericCO2in the twentieth century, analyze how atmospheric CO2 and its surface fluxes vary on interannual time scales, including how they respond to ENSO, and describe the seasonal cycle of atmospheric CO2 and its surfacefluxes. While the model broadly reproduces observed aspects of the carbon cycle, there are several notable biases, including having too large of an increase in atmospheric CO2 over the twentieth century and too small of a seasonal cycle of atmospheric CO2 in the Northern Hemisphere. The biases are related to a weak response of the carbon cycle to climatic variations on interannual and seasonal time scales and to twentieth-century anthropogenic forcings, including rising CO2, land-use change, and atmospheric deposition of nitrogen.

Finding forced trends in oceanic oxygen

Anthropogenically forced trends in oceanic dissolved oxygen are evaluated in Earth system models in the context of natural variability. A large ensemble of a single Earth system model is used to clearly identify the forced component of change in interior oxygen distributions and to evaluate the magnitude of this signal relative to noise generated by internal climate variability. The time of emergence of forced trends is quantified on the basis of anomalies in oxygen concentrations and trends. We find that the forced signal should already be evident in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins; widespread detection of forced deoxygenation is possible by 2030–2040. In addition to considering spatially discrete metrics of detection, we evaluate the similarity of the spatial structures associated with natural variability and the forced trend. Outside of the subtropics, these patterns are not wholly distinct on the isopycnal surfaces considered, and therefore, this approach does not provide significantly advanced detection. Our results clearly demonstrate the strong impact of natural climate variability on interior oxygen distributions, providing an important context for interpreting observations.

Timescales for detection of trends in the ocean carbon sink

The ocean has absorbed 41 per cent of all anthropogenic carbon emitted as a result of fossil fuel burning and cement manufacture. The magnitude and the large-scale distribution of the ocean carbon sink is well quantified for recent decades. In contrast, temporal changes in the oceanic carbon sink remain poorly understood. It has proved difficult to distinguish between air-to-sea carbon flux trends that are due to anthropogenic climate change and those due to internal climate variability. Here we use a modelling approach that allows for this separation, revealing how the ocean carbon sink may be expected to change throughout this century in different oceanic regions. Our findings suggest that, owing to large internal climate variability, it is unlikely that changes in the rate of anthropogenic carbon uptake can be directly observed in most oceanic regions at present, but that this may become possible between 2020 and 2050 in some regions.

Multi‐decadal trends in the advection and mixing of natural carbon in the Southern Ocean

[1] Multi-decadal trends in the advection, mixing, and airsea flux of natural carbon dioxide (CO2) in the Southern Ocean are investigated using output from a hindcast simulation of a non-eddy-resolving ocean model. Particular emphasis is placed on the model’s improved eddy-induced advection parameterization. From 1958 to 2007, the model predicts a significant increase in the outgassing of natural CO2 from the Southern Ocean, congruent with a positive trend in the wind speed over this period. The natural CO2 flux trend is largely driven by enhanced Eulerian-mean advection and diapycnal mixing of dissolved inorganic carbon (DIC) into the Southern Ocean surface. The natural CO2 flux trend would be larger, if not for an increase in the eddy-induced advection of DIC out of the Southern Ocean surface, caused by the multi-decadal increase in the model’s eddy-induced advection coefficient. Citation: Lovenduski, N. S., M. C. Long, P. R. Gent, and K. Lindsay (2013), Multi‐decadal trends in the advection and mixing of natural carbon in the Southern Ocean, Geophys. Res. Lett.,40,139–142, doi:10.1029/2012GL054483.

Southern Ocean Overturning Compensation in an Eddy-Resolving Climate Simulation

The Southern Ocean’s Antarctic Circumpolar Current (ACC) and meridional overturning circulation (MOC) response to increasing zonal wind stress is, for the first time, analyzed in a high-resolution (0.1° ocean and 0.25° atmosphere), fully coupled global climate simulation using the Community Earth System Model. Results from a 20-yr wind perturbation experiment, where the Southern Hemisphere zonal wind stress is increased by 50% south of 30°S, show only marginal changes in the mean ACC transport through Drake Passage—an increase of 6% [136–144 Sverdrups (Sv; 1 Sv ≡ 10^6 m^3 s^(−1))] in the perturbation experiment compared with the control. However, the upper and lower circulation cells of the MOC do change. The lower cell is more affected than the upper cell with a maximum increase of 64% versus 39%, respectively. Changes in the MOC are directly linked to changes in water mass transformation from shifting surface isopycnals and sea ice melt, giving rise to changes in surface buoyancy forcing. The increase in transport of the lower cell leads to upwelling of warm and salty Circumpolar Deep Water and subsequent melting of sea ice surrounding Antarctica. The MOC is commonly supposed to be the sum of two opposing components: a wind- and transient-eddy overturning cell. Here, the transient-eddy overturning is virtually unchanged and consistent with a large-scale cancellation of localized regions of both enhancement and suppression of eddy kinetic energy along the mean path of the ACC. However, decomposing the time-mean overturning into a time- and zonal-mean component and a standing-eddy component reveals partial compensation between wind-driven and standing-eddy components of the circulation.

Control of phytoplankton bloom inception in the Ross Sea, Antarctica, by Ekman restratification

[1] Observations from November 2006 in the southwestern Ross Sea indicate that stratification developed in a localized fashion, proximal to upper ocean fronts. These regions were hotspots for biological productivity, exhibiting greater drawdown of CO2 and accumulation of oxygen, indicative of enhanced photosynthesis and air-sea gas exchange. While the effect of stratification is clear, the reasons for its development was not; air temperatures were unseasonably cold, sea-ice melt and sea surface warming were not significant. By comparing a one-dimensional mixed layer model with two-dimensional numerical simulations that include horizontal density gradients characteristic of the region, it is shown that Ekman advection is critical to structuring early season stratification. Where fronts are forced by winds that oppose the surface frontal current, Ekman advection displaces lighter water over dense. As biological productivity is light limited in the Ross Sea, and thus sensitive to the depth of the mixed layer, Ekman restratification plays an important role in determining the spatial distribution and development of the annual phytoplankton bloom in the region. The presence of fronts is therefore of first-order importance to the restratification and bloom dynamics of the Ross Sea in the early spring. Citation: Long, M. C., L. N. Thomas, and R. B. Dunbar (2012), Control of phytoplankton bloom inception in the Ross Sea, Antarctica, by Ekman restratification, Global Biogeochem. Cycles, 26, GB1006, doi:10.1029/2010GB003982.

Partitioning uncertainty in ocean carbon uptake projections: Internal variability, emission scenario, and model structure

We quantify and isolate the sources of projection uncertainty in annual-mean sea-air CO2 flux over the period 2006–2080 on global and regional scales using output from two sets of ensembles with the Community Earth System Model (CESM) and models participating in the 5th Coupled Model Intercomparison Project (CMIP5). For annual-mean, globally-integrated sea-air CO2 flux, uncertainty grows with prediction lead time and is primarily attributed to uncertainty in emission scenario. At the regional scale of the California Current System, we observe relatively high uncertainty that is nearly constant for all prediction lead times, and is dominated by internal climate variability and model structure, respectively in the CESM and CMIP5 model suites. Analysis of CO2 flux projections over 17 biogeographical biomes reveals a spatially heterogenous pattern of projection uncertainty. On the biome scale, uncertainty is driven by a combination of internal climate variability and model structure, with emission scenario emerging as the dominant source for long projection lead times in both modeling suites.