Anthony P. Craig

Factors that affect the amplitude of El Nino in global coupled climate models

Abstract Historically, El Nino-like events simulated in global coupled climate models have had reduced amplitude compared to observations. Here, El Nino-like phenomena are compared in ten sensitivity experiments using two recent global coupled models. These models have various combinations of horizontal and vertical ocean resolution, ocean physics, and atmospheric model resolution. It is demonstrated that the lower the value of the ocean background vertical diffusivity, the greater the amplitude of El Nino variability which is related primarily to a sharper equatorial thermocline. Among models with low background vertical diffusivity, stronger equatorial zonal wind stress is associated with relatively higher amplitude El Nino variability along with more realistic east–west sea surface temperature (SST) gradient along the equator. The SST seasonal cycle in the eastern tropical Pacific has too much of a semiannual component with a double intertropical convergence zone (ITCZ) in all experiments, and thus does not affect, nor is it affected by, the amplitude of El Nino variability. Systematic errors affecting the spatial variability of El Nino in the experiments are characterized by the eastern equatorial Pacific cold tongue regime extending too far westward into the warm pool. The time scales of interannual variability (as represented by time series of Nino3 SSTs) show significant power in the 3–4 year ENSO band and 2–2.5 year tropospheric biennial oscillation (TBO) band in the model experiments. The TBO periods in the models agree well with the observations, while the ENSO periods are near the short end of the range of 3–6 years observed during the period 1950–94. The close association between interannual variability of equatorial eastern Pacific SSTs and large-scale SST patterns is represented by significant correlations between Nino3 time series and the PC time series of the first EOFs of near-global SSTs in the models and observations.

A new flexible coupler for earth system modeling developed for CCSM4 and CESM1

The Community Climate System Model (CCSM) has been developed over the last decade, and it is used to understand past, present, and future climates. The latest versions of the model, CCSM4 and CESM1, contain totally new coupling capabilities in the CPL7 coupler that permit additional flexibility and extensibility to address the challenges involved in earth system modeling. The CPL7 coupling architecture takes a completely new approach with respect to the high-level design of the system. CCSM4 now contains a top-level driver that calls model component initialize, run, and finalize methods through specified interfaces. The top-level driver allows the model components to be placed on relatively arbitrary hardware processor layouts and run sequentially, concurrently, or mixed. Improvements have been made to the memory and performance scaling of the coupler to support much higher resolution configurations. CCSM4 scales better to higher processor counts, and has the ability to handle global resolutions up to one-tenth of a degree.

CPL6: The New Extensible, High Performance Parallel Coupler for the Community Climate System Model

Coupled climate models are large, multiphysics applications designed to simulate the Earth’s climate and predict the response of the climate to any changes in the forcing or boundary conditions. The Community Climate System Model (CCSM) is a widely used state-of-the-art climate model that has released several versions to the climate community over the past ten years. Like many climate models, CCSM employs a coupler, a functional unit that coordinates the exchange of data between parts of the climate system such as the atmosphere and ocean. In this paper we describe the new coupler, cpl6, contained in the latest version of CCSM, CCSM3. Cpl6 introduces distributed-memory parallelism to the coupler, a class library for important coupler functions, and a standardized interface for component models. Cpl6 is implemented entirely in Fortran90 and uses the Model Coupling Toolkit as the base for most of its classes. Cpl6 gives improved performance over previous versions and scales well on multiple platforms.

Implementation and Initial Evaluation of the Glimmer Community Ice Sheet Model in the Community Earth System Model

AbstractThe Glimmer Community Ice Sheet Model (Glimmer-CISM) has been implemented in the Community Earth System Model (CESM). Glimmer-CISM is forced by a surface mass balance (SMB) computed in multiple elevation classes in the CESM land model and downscaled to the ice sheet grid. Ice sheet evolution is governed by the shallow-ice approximation with thermomechanical coupling and basal sliding. This paper describes and evaluates the initial model implementation for the Greenland Ice Sheet (GIS). The ice sheet model was spun up using the SMB from a coupled CESM simulation with preindustrial forcing. The models sensitivity to three key ice sheet parameters was explored by running an ensemble of 100 GIS simulations to quasi equilibrium and ranking each simulation based on multiple diagnostics. With reasonable parameter choices, the steady-state GIS geometry is broadly consistent with observations. The simulated ice sheet is too thick and extensive, however, in some marginal regions where the SMB is anomalousl...

Computational performance of ultra-high-resolution capability in the Community Earth System Model

With the fourth release of the Community Climate System Model, the ability to perform ultra-high-resolution climate simulations is now possible, enabling eddy-resolving ocean and sea-ice models to be coupled to a finite-volume atmosphere model for a range of atmospheric resolutions. This capability was made possible by enabling the model to use large scale parallelism, which required a significant refactoring of the software infrastructure. We describe the scalability of two ultra-high-resolution coupled configurations on leadership class computing platforms. We demonstrate the ability to utilize over 30,000 processor cores on a Cray XT5 system and over 60,000 cores on an IBM Blue Gene/P system to obtain climatologically relevant simulation rates for these configurations.

An application-level parallel I/O library for Earth system models

We describe the design and implementation of an application-level parallel I/O (PIO) library for the reading and writing of distributed arrays to several common scientific data formats. PIO provides the flexibility to control the number of I/O tasks through data rearrangement to an I/O friendly decomposition. This flexibility enables reductions in per task memory usage and improvements in disk I/O performance versus a serial I/O approach. We illustrate the impact various features within PIO have on memory usage and disk I/O bandwidth on a Cray XT5 system.

Performance of the community earth system model

The Community Earth System Model (CESM), released in June 2010, incorporates new physical process and new numerical algorithm options, significantly enhancing simulation capabilities over its predecessor, the June 2004 release of the Community Climate System Model. CESM also includes enhanced performance tuning options and performance portability capabilities. This paper describes performance and performance scaling on both the Cray XT5 and the IBM BG/P for four representative production simulations, varying both problem size and enabled physical processes. The paper also describes preliminary performance results for high resolution simulations using over 200,000 processor cores, indicating the promise of ongoing work in numerical algorithms and where further work is required.

Simulating transient ice-ocean Ekman transport in the Regional Arctic System Model and Community Earth System Model

Abstract This work evaluates the fidelity of the polar marine Ekman layer in the Regional Arctic System Model (RASM) and Community Earth System Model (CESM) using sea-ice inertial oscillations as a proxy for ice-ocean Ekman transport. A case study is presented that demonstrates that RASM replicates inertial oscillations in close agreement with motion derived using the GPS. This result is obtained from a year-long case study pre-dating the recent decline in perennial Arctic sea ice, using RASM with sub-hourly coupling between the atmosphere, sea-ice and ocean components. To place this work in context, the RASM coupling method is applied to CESM, increasing the frequency of oceanic flux exchange from once per day in the standard CESM configuration, to every 30 min. For a single year simulation, this change causes a considerable increase in the median inertial ice speed across large areas of the Southern Ocean and parts of the Arctic sea-ice zone. The result suggests that processes associated with the passage of storms over sea ice (e.g. oceanic mixing, sea-ice deformation and surface energy exchange) are underestimated in Earth System Models that do not resolve inertial frequencies in their marine coupling cycle.

A comparison of temperature, salinity, and chlorofluorocarbon observations with results from a 1° resolution three‐dimensional global ocean model

We describe the ability of a moderate-resolution global ocean model to simulate the general circulation and the ocean-atmosphere exchange and redistribution of chlorofluorocarbons (CFCs). The model was spun up from climatological initial conditions and has been integrated for decades representing 1930 to the present. Climatological monthly mean winds were imposed during the spin-up and first 50 years of the integration. From 1980, monthly mean European Centre for Medium-Range Weather Forecasts (ECMWF) wind stress fields were used. We compare model results to cruise data and to long-term mean observations and find good qualitative agreement in most areas. Overall, the model agrees reasonably well in regions where measurable CFCs have been observed, and the large-scale model ventilation pathways appear realistic. One of the most conspicuous shortcomings is the small volume of Antarctic Intermediate Water in the model results. This leads to errors in the general water mass structure and in CFC concentrations in the model in other regions. Notable CFC differences are found in regions where deep water masses are formed and in the upper subtropical gyre regions in which the Kuroshio extension exists. The model oceanic CFC sink represents <1% of all CFCs produced since 1930 and is small relative to the stratospheric sink for these compounds.

The DOE Parallel Climate Model (PCM): The Computational Highway and Backroads

The DOE Parallel Climate Model (PCM) is used to simulate the earths climate system and has been used to study the climate of the 20th century and to project possible climate changes into the 21st century and beyond. It was designed for use on distributed memory, highly parallel, architectures. The computational requirements and design of the model are discussed, as well as its performance and scalability characteristics. A method for port validation is demonstrated. The shortcomings of the current model are summarized and future design plans are presented.