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Bette L. Otto-Bliesner

Abstract

The purpose of this paper is to document the seasonal variation of the atmospheric energy cycle simulated by a low-resolution general circulation model in order to assess the model's strengths and weaknesses and better define its usefulness for future sensitivity studies. The numerical model is a global, spectral, primitive equation model with five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea surface temperature, snow cover, and solar declination angle; parameterizations of radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget.

The model simulates the observed vertically-integrated energetics of the troposphere reasonably well and the values are comparable to other general circulation models. At northern extratropical latitudes, the very long waves, wavenumbers 1 through 3, which are associated with thermal and orographic zonal asymmetries, dominate the model energy cycle. The annual cycle prevails, with maximum values approximately one month after the winter solstice. These model aspects are similar to those documented observationally.

Analysis of the time variation of the model energetics details the presence of asymmetric and steplike variations superimposed on the annual cycles at northern extratropical latitudes. The. model statistics also reveal important latitudinal variations. In particular, more sinusoidal seasonal cycles and a predominance of energy at wavenumbers 4 through 7 are found at southern extratropical latitudes, and the thermally direct east-west circulations, associated with land-sea and precipitation zonal asymmetries, are important in the tropical energy cycle.

The major discrepancies in the model simulation of the atmospheric energy cycle are the overestimation of zonal and eddy available potential energy and the conversion between them, an exaggerated semiannual component in the seasonal cycle of eddy available potential energy, a deficiency of eddy kinetic energy at the jet core level, and an underestimation of the transients. The model limitations leading to these discrepancies are discussed, as well as improvements made in subsequent versions of the model.

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Bette L. Otto-Bliesner and Donald R. Johnson

Abstract

A diagnostic approach to infer three-dimensional distribution of the thermally-forced, time-averaged horizontal mass and energy transport (Johnson and Townsend, 1981), which was previously applied in the Southern Hemisphere (Zillman, 1972), is used to determine the corresponding Northern Hemisphere circulations. The method is based on the steady form of the time-averaged isentropic continuity equation and allows calculation of the irrotational components of the mass circulations which are consistent with modeled diabatic heating fields determined from climatic information. The three-dimensional distributions of atmospheric heating in the Northern Hemisphere are estimated from summer and winter climatic data of precipitation, turbulent exchange of sensible heat and radiative fluxes.

The results highlight the large-scale coupling of the Northern Hemisphere heat source and heat sink regions by Hadley-type and Walker-type circulations. The zonally-averaged mass circulation exhibits a thermally direct, meridional cell spanning the entire hemisphere in winter. This circulation shifts northward and weakens in summer with the corresponding Southern Hemisphere winter Hadley-type circulation now extending to northern latitudes. The composite three-dimensional mass circulations also reveal prominent mass transports associated with the Asian monsoon in summer and winter. Zonal asymmetries in the heating lead to longitudinal variations of the meridional circulation and pronounced east-west overturnings. A complementary study (Johnson and Townsend, 1981) in which similar mass circulations were derived from the FGGE observational global data set validates the large-scale patterns established in this study.

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Bette L. Otto-Bliesner and Esther C. Brady

Abstract

A 300-yr simulation with the NCAR Climate System Model (CSM), version 1, captured only ∼60% of the observed ENSO signal and exaggerated the interannual variability of SST in the western tropical Pacific. Here, a simulation with a new version of the CSM, which significantly improves the spatial and temporal patterns of tropical Pacific variability, is described. Maximum SST variability is shifted to the central and eastern Pacific. A better simulation of the equatorial Pacific thermocline structure results in Niño-3 and Niño-4 statistics comparable to the observed estimates for the last century. The evolution of SST and subsurface temperature anomalies is in excellent agreement with observed events. The majority of events evolve as a standing mode with weak SST anomalies occurring in the northern spring in the eastern tropical Pacific and maximum anomalies covering the eastern tropical Pacific Ocean to the date line by the following northern winter. At the same time, subsurface temperature anomalies spread eastward and upward along the tropical thermocline. The “delayed oscillator” and Wyrtki's “buildup” hypothesis are consistent with aspects of the CSM simulation. On the equator, a westerly wind stress anomaly in the central Pacific forces off-equatorial upwelling anomalies, which propagate westward, reaching the western boundary about one-half year later. This upwelling signal then propagates eastward along the equator, arriving 2 months before cooling in the eastern Pacific basin. The tropical Pacific atmospheric response to warm oceanic events also agrees with observational analyses with a negative Southern Oscillation pattern in sea level pressure, wind stress anomalies, and low-level convergence to the west of the maximum SST anomalies and enhanced deep convection and precipitation in the central and eastern tropical Pacific.

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Eero Holopainen and Bette L. Otto-Bliesner

Abstract

Two long-term simulations of a low-resolution spectral general circulation model (Otto-Bliesner et al., 1982) have been made. The single difference between these two runs is that in one (CONTROL) the horizontal flux of momentum by unresolved synoptic eddies is represented by a second-degree, Fickian-type diffusion scheme (down-gradient), whereas in the other (DIFFEX), this flux is zero. The amount of eddy kinetic energy in DIFFEX is found to be significantly larger and more realistic than in CONTROL. Also, the spectral distributions of both eddy kinetic energy and eddy available potential energy in DIFFEX are more realistic than in CONTROL. However, the long-term mean circulations are not statistically different in the two runs.

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Yun-Qi Ni, Bette L. Otto-Bliesner, and David D. Houghton

Abstract

An analysis is made of the effect of orography on the atmospheric energetics in a low-resolution general circulation model to determine the temporal and scale dependency of these effects. The numerical model is a global, spectral, primitive equation model of the atmosphere with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. A one-year seasonal simulation of the general circulation without mountains is compared to the results from a five-year seasonal simulation of the general circulation with mountains. The statistical significance of the topographic effects is evaluated by comparing them to magnitudes of model interannual variability determined from the five-year control simulation.

A small, but important, portion of the changes due to topography are significant. At northern extratropical latitudes, the increases of eddy activities and baroclinic instability in summer resulting from incorporation of the effect of the mountains give rise to significantly increased eddy components of atmospheric energetics and the conversion from eddy available potential energy to eddy kinetic energy. These increases are generally present and significant at each wavenumber and for the overall stationary component. In winter, topography significantly increases the zonal kinetic energy and dissipation. Examination of the individual zonal spectral components for winter reveals that topography increases long-wave energies and their transfer, but with proportional decreases at medium waves, resulting in little change in the total eddy components. A similar compensation occurs between the stationary and transient components of the heat transport. Less pronounced topographic features at tropical and southern extratropical latitudes result in fewer significant changes due to topography.

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Robert G. Gallimore, Bette L. Otto-Bliesner, and John E. Kutzbach

Abstract

The sensitivity of a low resolution, spectral general circulation model (GCM) to specification of physical processes is examined using a new version of the model with refined parameterizations. Specific refinements in parameterization include: 1) smoothing the original orography to greatly diminish undesirable topographic “ripples” occurring near high mountain ranges; 2) adding snowcover on the Tibetan plateau and representing winter snowcover in middle latitudes more realistically; 3) decreasing the land ground wetness and adjusting the drag coefficient and parameters governing condensation-moist convective adjustment.

Results of comparative 5-year integrations show that better parameterization in the low resolution model produces significant improvement in simulation without resorting to the use of higher horizontal or vertical resolution. The combined changes in ground wetness, drag coefficient and condensation-moist convective parameters produce more realistic zonal banding of precipitation belts and a better representation of continental precipitation relative to the ocean. In addition, mass is more nearly conserved and mean sea level pressure and temperature patterns are in better agreement with observations than in the previous model. Major deficiencies in simulation that are not improved include zonal jet and stratospheric temperature structures. Overall, the improvements in simulation suggest a wider applicability of the low resolution model for use in climate sensitivity studies.

Analysis of sensitivity experiments assessing specific effects of parameterization indicate that decreased ground

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Bette L. Otto-Bliesner, Grant W. Branstator, and David D. Houghton

Abstract

A global, spectral, primitive equation model is developed to study the seasonal climatology of the large-scale features of the atmosphere. The model resolution is five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea-surface temperatures, snowcover, and solar declination angle; parameterizations for radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget. The external seasonal forcing of the model atmosphere is composed of sinusoidal time variations in the incoming solar radiation and latitude of the snowline and more complicated variations in the albedo of the snow and the sea-surface temperatures. A five-year seasonal simulation has been analyzed. The model reasonably reproduces the general features of the observed atmospheric circulation, seasonal cycles, interannual variations and hemispheric differences. The success of this low-resolution model in simulating the large-scale features of the atmospheric seasonal cycle illustrates the usefulness of such models for climate studies in conjunction with high-resolution general circulation model simulations.

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Samantha Stevenson, Bette Otto-Bliesner, John Fasullo, and Esther Brady

Abstract

The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to the hydrologic cycle and on teleconnections with the El Niño–Southern Oscillation (ENSO), complicating the interpretation of offsets between proxy reconstructions and model output. Here, these effects are separated, using the Community Earth System Model Last Millennium Ensemble (CESM-LME), by examination of ensemble realizations with distinct posteruption ENSO responses. Hydroclimate anomalies in monsoon Asia and the western United States resemble the El Niño teleconnection pattern after “Tropical” and “Northern” eruptions, even when ENSO-neutral conditions are present. This pattern results from Northern Hemisphere (NH) surface cooling, which shifts the intertropical convergence zone equatorward, intensifies the NH subtropical jet, and suppresses the Southeast Asian monsoon. El Niño events following an eruption can then intensify the ENSO-neutral hydroclimate signature, and El Niño probability is enhanced two boreal winters following all eruption types. Additionally, the eruption-year ENSO response to eruptions is hemispherically dependent: the winter following a Northern eruption tends toward El Niño, while Southern volcanoes enhance the probability of La Niña events and Tropical eruptions have a very slight cooling effect. Overall, eruption-year hydroclimate anomalies in CESM disagree with the proxy record in both Southeast Asia and North America, suggesting that model monsoon representation cannot be solely responsible. Possible explanations include issues with the model ENSO response, the spatial or temporal structure of volcanic aerosol distribution, or data uncertainties.

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Bette L. Otto-Bliesner, Esther C. Brady, Gabriel Clauzet, Robert Tomas, Samuel Levis, and Zav Kothavala

Abstract

The climate sensitivity of the Community Climate System Model version 3 (CCSM3) is studied for two past climate forcings, the Last Glacial Maximum (LGM) and the mid-Holocene. The LGM, approximately 21 000 yr ago, is a glacial period with large changes in the greenhouse gases, sea level, and ice sheets. The mid-Holocene, approximately 6000 yr ago, occurred during the current interglacial with primary changes in the seasonal solar irradiance.

The LGM CCSM3 simulation has a global cooling of 4.5°C compared to preindustrial (PI) conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at LGM. Tropical sea surface temperature (SST) cools by 1.7°C and tropical land temperature cools by 2.6°C on average. Simulations with the CCSM3 slab ocean model suggest that about half of the global cooling is explained by the reduced LGM concentration of atmospheric CO2 (∼50% of present-day concentrations). There is an increase in the Antarctic Circumpolar Current and Antarctic Bottom Water formation, and with increased ocean stratification, somewhat weaker and much shallower North Atlantic Deep Water. The mid-Holocene CCSM3 simulation has a global, annual cooling of less than 0.1°C compared to the PI simulation. Much larger and significant changes occur regionally and seasonally, including a more intense northern African summer monsoon, reduced Arctic sea ice in all months, and weaker ENSO variability.

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Samantha Stevenson, Andrew T. Wittenberg, John Fasullo, Sloan Coats, and Bette Otto-Bliesner

Abstract

The majority of future projections in the Coupled Model Intercomparison Project (CMIP5) show more frequent exceedances of the 5 mm day−1 rainfall threshold in the eastern equatorial Pacific rainfall during El Niño, previously described in the literature as an increase in “extreme El Niño events”; however, these exceedance frequencies vary widely across models, and in some projections actually decrease. Here we combine single-model large ensemble simulations with phase 5 of the Coupled Model Intercomparison Project (CMIP5) to diagnose the mechanisms for these differences. The sensitivity of precipitation to local SST anomalies increases consistently across CMIP-class models, tending to amplify extreme El Niño occurrence; however, changes to the magnitude of ENSO-related SST variability can drastically influence the results, indicating that understanding changes to SST variability remains imperative. Future El Niño rainfall intensifies most in models with 1) larger historical cold SST biases in the central equatorial Pacific, which inhibit future increases in local convective cloud shading, enabling more local warming; and 2) smaller historical warm SST biases in the far eastern equatorial Pacific, which enhance future reductions in stratus cloud, enabling more local warming. These competing mechanisms complicate efforts to determine whether CMIP5 models under- or overestimate the future impacts of climate change on El Niño rainfall and its global impacts. However, the relation between future projections and historical biases suggests the possibility of using observable metrics as “emergent constraints” on future extreme El Niño, and a proof of concept using SSTA variance, precipitation sensitivity to SST, and regional SST trends is presented.

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