Susan C. Bates

The Community Earth System Model (CESM) large ensemble project: A community resource for studying climate change in the presence of internal climate variability

AbstractWhile internal climate variability is known to affect climate projections, its influence is often underappreciated and confused with model error. Why? In general, modeling centers contribute a small number of realizations to international climate model assessments [e.g., phase 5 of the Coupled Model Intercomparison Project (CMIP5)]. As a result, model error and internal climate variability are difficult, and at times impossible, to disentangle. In response, the Community Earth System Model (CESM) community designed the CESM Large Ensemble (CESM-LE) with the explicit goal of enabling assessment of climate change in the presence of internal climate variability. All CESM-LE simulations use a single CMIP5 model (CESM with the Community Atmosphere Model, version 5). The core simulations replay the twenty to twenty-first century (1920–2100) 30 times under historical and representative concentration pathway 8.5 external forcing with small initial condition differences. Two companion 1000+-yr-long preindu...

The CCSM4 Ocean Component

AbstractThe 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 observa...

Impact of the dynamical core on the direct simulation of tropical cyclones in a high-resolution global model

This paper examines the impact of the dynamical core on the simulation of tropical cyclone (TC) frequency, distribution, and intensity. The dynamical core, the central fluid flow component of any general circulation model (GCM), is often overlooked in the analysis of a models ability to simulate TCs compared to the impact of more commonly documented components (e.g., physical parameterizations). The Community Atmosphere Model version 5 is configured with multiple dynamics packages. This analysis demonstrates that the dynamical core has a significant impact on storm intensity and frequency, even in the presence of similar large-scale environments. In particular, the spectral element core produces stronger TCs and more hurricanes than the finite-volume core using very similar parameterization packages despite the latter having a slightly more favorable TC environment. The results suggest that more detailed investigations into the impact of the GCM dynamical core on TC climatology are needed to fully understand these uncertainties.

Mean and Variability of the Tropical Atlantic Ocean in the CCSM4

AbstractThis 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 Atlanti...

Mean Biases, Variability, and Trends in Air–Sea Fluxes and Sea Surface Temperature in the CCSM4

AbstractAir–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 temperatur...

Coupled Ocean–Atmosphere Interaction and Variability in the Tropical Atlantic Ocean with and without an Annual Cycle

Abstract 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...

The thermohaline circulation and tropical cyclones in past, present, and future climates

WHat: About 40 junior faculty discussed interactions between fast (tropical cyclones) and slow (oceanic overturning circulation) extreme events in the climate system and methods using both paleoclimate proxies and models that could improve our understanding of such events. WHen: 8–10 July 2008 WHere: Boulder, Colorado nderstanding the interactions between tropical cyclones and the thermohaline circulation (THC) and their contributions to climate and climate change is an intriguing, challenging, and important area of research. The oceanic overturning circulation is a slow process whereas tropical cyclones are a fast process, and both may be subject to abrupt or long-term changes. The Early Career Scientist Association’s Junior Faculty Forum explored ways to incorporate modeling, observations, and geologic reconstructions into understanding these interacting components of the climate system in past, present, and future climates. Following an extensive review of the current level of knowledge, including plenary talks by invited speakers, the bulk of the meeting was dedicated to two parallel discussion sessions focusing on the oceanic overturning circulation and tropical cyclones. Topic I of the forum focused on the observational needs as well as the modeling and decadal prediction of the THC [or meridional overturning circulation (MOC)]. The terminology used to describe THC has been much discussed and debated. The exact definition and proper use of the terms “THC” and “MOC” have been clarified because of the confusion in the recent literature (e.g., Wunsch 2002), and no ubiquitous metric exists to quantify either. Often, the general term MOC is used to refer specifically to the Atlantic MOC (AMOC), when in reality each ocean basin contains meridional overturning cells that are interconnected through the global circulation. It is agreed in Annex AFFILIATIONS: Done anD Hu—National Center for Atmospheric Research, Boulder, Colorado; Farmer—Hofstra University, Hempstead, New York; yin—The Florida State University, Tallahassee, Florida; bates—University of Washington, Seattle, Washington; Frappier—Boston College, Chestnut Hill, Massachusetts; HalkiDes—NASA JPL, California Institute of Technology, Pasadena, California; kilbourne—McDaniel College, Westminster, Maryland; sriver—The Pennsylvania State University, University Park, Pennsylvania; WooDruFF—MIT/ WHOI Joint Program, Woods Hole, Massachusetts CORRESPONDING AUTHOR: James M. Done, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000 E-mail: done@ucar@edu DOI:10.1175/2009BAMS2762.1

Seasonal Influences on Coupled Ocean–Atmosphere Variability in the Tropical Atlantic Ocean

Abstract 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 condit...

North Atlantic Barotropic Vorticity Balances in Numerical Models

AbstractNumerical 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...

Predicting Near-Term Changes in the Earth System: A Large Ensemble of Initialized Decadal Prediction Simulations Using the Community Earth System Model

CapsuleA new community data resource offers unprecedented statistical power for assessing hindcast accuracy and skill, quantifying the benefits associated with initialization, and exploring the probabilistic attributes of decadal predictions of climate and ocean biogeochemical fields.