Robert L. Jacob

Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming

Model Behavior The initial pulse of warming during the last deglaciation, which defined the start of an interval called the Bølling-Allerød, occurred abruptly about 14,500 years ago. To date, the most detailed simulations used models of intermediate complexity, not with more sophisticated Coupled Global Climate Models (CGCMs) that can synchronously couple both oceanic and the atmospheric components. Overcoming practical and technical challenges, Liu et al. (p. 310; see the Perspective by Timmermann and Menviel) performed such a simulation using CCSM3, a state-of-the-art ocean-atmosphere CGCM. In contrast to previous studies, which indicated that the Bølling-Allerød was triggered by a nonlinear bifurcation between modes of deep ocean circulation in the Atlantic, the results suggest that the event was a transient response caused by the cessation of meltwater input into the surface ocean in the North Atlantic region. A coupled atmosphere-ocean general circulation model simulates the warming of the last deglaciation. We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO2 forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.

The Model Coupling Toolkit: A New Fortran90 Toolkit for Building Multiphysics Parallel Coupled Models

Many problems in science and engineering are best simulated as a set of mutually interacting models, resulting in a coupled or multiphysics model. These models present challenges stemming from their interdisciplinary nature and from their computational and algorithmic complexities. The computational complexity of individual models, combined with the popularity of the distributed-memory parallel programming model used on commodity micro-processor-based clusters, results in a parallel coupling problem when building a coupled model. We define and elucidate this problem and how it results in a set of requirements for software capable of simplifying the construction of parallel coupled models. We describe the package, the Model Coupling Toolkit (MCT), which we have developed to meet these general requirements and the specific requirements of a parallel climate model. We present the MCT programming model with illustrative code examples. We present representative results that measure MCT’s scalability, performance portability, and a proxy for coupling overhead.

Pacific Decadal Variability: The Tropical Pacific Mode and the North Pacific Mode*

Pacific decadal variability is studied in a series of coupled global ocean‐atmosphere simulations aided by two ‘‘modeling surgery’’ strategies: partial coupling (PC) and partial blocking (PB). The PC experiments retain full ocean‐atmosphere coupling in selected regions, but constrain ocean‐atmosphere coupling elsewhere by prescribing the model climatological SST to force the atmospheric component of the coupled system. In PB experiments, sponge walls are inserted into the ocean component of the coupled model at specified latitudinal bands to block the extratropical‐tropical oceanic teleconnection. Both modeling and observational studies suggest that Pacific decadal variability is composed of two distinct modes: a decadal to bidecadal tropical Pacific mode (TPM) and a multidecadal North Pacific mode (NPM). The PC and PB experiments showed that the tropical Pacific mode originates predominantly from local coupled ocean‐atmosphere interaction within the tropical Pacific. Extratropical‐tropical teleconnections, although not a necessary precondition for the genesis of the tropical decadal variability, can enhance SST variations in the Tropics. The decadal memory in the Tropics seems to be associated with tropical higher baroclinic modes. The North Pacific mode originates from local atmospheric stochastic processes and coupled ocean‐atmosphere interaction. Atmospheric stochastic forcing can generate a weaker NPM-like pattern in both the atmosphere and ocean, but with no preferred timescales. In contrast, coupled ocean‐atmosphere feedback can enhance the variability substantially and generate a basin-scale multidecadal mode in the North Pacific. The multidecadal memory in the midlatitudes seems to be associated with the delayed response of the subtropical/subpolar gyre to wind stress variation in the central North Pacific and the slow growing/decaying of SST anomalies that propagate eastward in the Kuroshio Extension region. Oceanic dynamics, particularly the advection of the mean temperature by anomalous meridional surface Ekman flow and western boundary currents, plays an important role in generating the North Pacific mode.

Did the rifting of the Atlantic Ocean cause the Cretaceous thermal maximum

The Cretaceous thermal maximum was a major turning point in the history of Earth’s climate. This interval of peak warmth in the Turonian has been attributed to very high atmospheric pCO2 resulting from rapid outgassing rates, although crustal cycling rates peaked in the Aptian‐Albian interval. On the basis of coupled ocean-atmosphere model simulations of the middle Cretaceous, we hypothesize that the formation of an Atlantic gateway could have contributed to the Cretaceous thermal maximum. Differences between prerifting and postrifting climate experiments demonstrate substantial regional oceanographic changes in the North and South Atlantic Basins that are consistent with oxygen isotopic evidence used to infer a Cretaceous thermal maximum. The model results help reconcile the paleoclimate record inferred from foraminiferal d 18 O with our understanding of climate dynamics and Cretaceous tectonism.

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.

M X N Communication and Parallel Interpolation in Community Climate System Model Version 3 Using the Model Coupling Toolkit

The Model Coupling Toolkit (MCT) is a software library for constructing parallel coupled models from individual parallel models. MCT was created to address the challenges of creating a parallel coupler for the Community Climate System Model (CCSM). Each of the submodels that make up CCSM is a separate parallel application with its own domain decomposition, running on its own set of processors. This application contains multiple instances of the M × N problem, the problem of transferring data between two parallel programs running on disjoint sets of processors. CCSM also requires efficient data transfer to facilitate its interpolation algorithms. MCT was created as a generalized solution to handle these and other common functions in parallel coupled models. Here we describe MCT’s implementation of the data transfer infrastructure needed for a parallel coupled model. The performance of MCT scales satisfactorily as processors are added to the system. However, the types of decompositions used in the submodels can affect performance. MCT’s infrastructure provides a flexible and high-performing set of tools for enabling interoperability between parallel applications.

A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea breakup

Large fluctuations in continental configuration occur throughout the Mesozoic. While it has long been recognized that paleogeography may potentially influence atmospheric CO2 via the continental silicate weathering feedback, no numerical simulations have been done, because of the lack of a spatially resolved climate-carbon model. GEOCLIM, a coupled numerical model of the climate and global biogeochemical cycles, is used to investigate the consequences of the Pangea breakup. The climate module of the GEOCLIM model is the FOAM atmospheric general circulation model, allowing the calculation of the consumption of atmospheric CO2 through continental silicate weathering with a spatial resolution of 7.5°long × 4.5°lat. Seven time slices have been simulated. We show that the breakup of the Pangea supercontinent triggers an increase in continental runoff, resulting in enhanced atmospheric CO2 consumption through silicate weathering. As a result, atmospheric CO2 falls from values above 3000 ppmv during the Triassic down to rather low levels during the Cretaceous (around 400 ppmv), resulting in a decrease in global mean annual continental temperatures from about 20°C to 10°C. Silicate weathering feedback and paleogeography both act to force the Earth system toward a dry and hot world reaching its optimum over the last 260 Myr during the Middle-Late Triassic. In the super continent case, given the persistent aridity, the model generates high CO2 values to produce very warm continental temperatures. Conversely, in the fragmented case, the runoff becomes the most important contributor to the silicate weathering rate, hence producing a CO2 drawdown and a fall in continental temperatures. Finally, another unexpected outcome is the pronounced fluctuation in carbonate accumulation simulated by the model in response to the Pangea breakup. These fluctuations are driven by changes in continental carbonate weathering flux. Accounting for the fluctuations in area available for carbonate platforms, the simulated ratio of carbonate deposition between neritic and deep sea environments is in better agreement with available data.

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.

Seasonal and Long-Term Atmospheric Responses to Reemerging North Pacific Ocean Variability: A Combined Dynamical and Statistical Assessment*

Abstract The atmospheric response to a North Pacific subsurface oceanic temperature anomaly is studied in a coupled ocean–atmosphere general circulation model using a combined dynamical and statistical approach, with the focus on the evolution at seasonal and longer time scales. The atmospheric response is first assessed dynamically with an ensemble coupled experiment. The atmospheric response is found to exhibit a distinct seasonal evolution and a significant long-term response. The oceanic temperature anomaly reemerges each winter to force the atmosphere through an upward heat flux, forcing a clear seasonal atmospheric response locally over the Aleutian low and downstream over the North America/North Atlantic Ocean and the Arctic regions. The atmospheric response is dominated by the early winter response with a warm SST-equivalent barotropic ridge and a wave train downstream. Starting in later winter, the atmospheric response weakens significantly and remains weak throughout the summer. The seasonal res...

Computational Design and Performance of the Fast Ocean Atmosphere Model, Version One

The Fast Ocean Atmosphere Model (FOAM) is a climate system model intended for application to climate science questions that require long simulations. FOAM is a distributed-memory parallel climate model consisting of parallel general circulation models of the atmosphere and ocean with complete physics paramaterizations as well as sea-ice, land surface, and river transport models. FOAMs coupling strategy was chosen for high throughput (simulated years per day). A new coupler was written for FOAM and some modifications were required of the component models. Performance data for FOAM on the IBM SP3 and SGI Origin2000 demonstrates that it can simulate over thirty years per day on modest numbers of processors.