Esther C. Brady

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.

Last Glacial Maximum and Holocene Climate in CCSM3

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 an...

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.

Diagnostic Model of the Three-Dimensional Circulation in the Upper Equatorial Pacific Ocean

Abstract To investigate the processes that maintain the large-scale, annual-average thermal structure of the equatorial Pacific, the three-dimensional ocean circulation for a large area is determined from a diagnostic model applied to repeated, meridional hydrographic sections along 150°W and 11O°W from 5°N to 5°S. Geostrophic balances are used to determine velocity profiles from 0 to 500 db across the boundaries of the region: zonal velocities across 150°W and 110°W at approximately 1° -lattitude intervals from 5°N to 5°S and meridional velocities across 5°N and 5°S averaged over the zonal distance between 150°W and 110°W. Poleward wind-driven flows across 5°N and 5°S based on climatological zonal wind stress are added to the geostrophic velocities in the mixed layers. To achieve overall mass conservation, the reference dynamic height field at 500 db is adjusted at four of the 21 stations by about 1 dyn cm. Horizontal nondivergence is used to determine meridional velocities between 0.75° and 5° latitudes...

Last glacial maximum ocean thermohaline circulation: PMIP2 model intercomparisons and data constraints

The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere-ocean simulations with four climate models that are also being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing.

Modeling El Niño and its tropical teleconnections during the last glacial‐interglacial cycle

(1) Simulations with the NCAR Climate System Model (CSM), a global, coupled ocean-atmosphere-sea ice model, for the last glacial-interglacial cycle reproduce recent estimates, based on alkenones and Mg/Ca ratios, of sea surface temperature (SST) changes and gradients in the tropical Pacific and predict weaker El Ninos/La Ninas compared to present for the Holocene and stronger El Ninos/La Ninas for the Last Glacial Maximum (LGM). Changes for the LGM (Holocene) are traced to a weakening (strengthening) of the tropical Pacific zonal SST gradient, wind stresses, and upwelling and a sharpening (weakening) of the tropical thermocline. Results suggest that proxy evidence of weaker precipitation variability in New Guinea and Ecuador are explained not only by changes in El Nino/ La Nina but also changes in the atmospheric circulation and hydrologic cycle. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4267 Oceanography: General: Paleoceanography; 4522 Oceanography: Physical: El Nino; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 9604 Information Related to Geologic Time: Cenozoic. Citation: Otto-Bliesner, B. L., E. C. Brady, S.-I. Shin, Z. Liu, and C. Shields, Modeling El Nino and its tropical teleconnections during the last glacial-interglacial cycle, Geophys. Res. Lett., 30(23), 2198, doi:10.1029/2003GL018553, 2003.

Climate Variability and Change since 850 CE: An Ensemble Approach with the Community Earth System Model

AbstractThe climate of the past millennium provides a baseline for understanding the background of natural climate variability upon which current anthropogenic changes are superimposed. As this period also contains high data density from proxy sources (e.g., ice cores, stalagmites, corals, tree rings, and sediments), it provides a unique opportunity for understanding both global and regional-scale climate responses to natural forcing. Toward that end, an ensemble of simulations with the Community Earth System Model (CESM) for the period 850–2005 (the CESM Last Millennium Ensemble, or CESM-LME) is now available to the community. This ensemble includes simulations forced with the transient evolution of solar intensity, volcanic emissions, greenhouse gases, aerosols, land-use conditions, and orbital parameters, both together and individually. The CESM-LME thus allows for evaluation of the relative contributions of external forcing and internal variability to changes evident in the paleoclimate data record, a...

Sensitivity to glacial forcing in the CCSM4

AbstractResults are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO2 sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO2) and to a quadrupling of the preindustrial concentration (4×CO2), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of −6.2 W m−2. The radiative forcing includes the effects of the reduced LGM greenhouse gases...

Climate Sensitivity of Moderate and Low Resolution Versions of CCSM3 to Preindustrial Forcings

Preindustrial (PI) simulations of the Community Climate System Model version 3 (CCSM3) at two resolutions, a moderate and a low resolution, are described and compared to the standard controls for present-day (PD) simulations. Because of computational efficiency, the moderate- and low-resolution versions of CCSM3 may be appropriate for climate change studies requiring simulations of the order of hundreds to thousands of years. The PI simulations provide the basis for comparison for proxy records that represent average late Holocene conditions. When forced with PI trace gases, aerosols, and solar irradiance estimates, both resolutions have a global cooling of 1.2°–1.3°C, increased sea ice in both hemispheres, and less precipitation near the equator and at midlatitudes as compared to simulations using PD forcing. The response to PI forcings differs in the two resolutions for North Atlantic meridional overturning circulation (MOC), the Antarctic Circumpolar Current (ACC), and ENSO. The moderate-resolution CCSM3 has enhanced ACC, North Atlantic MOC, and tropical Pacific ENSO variability for PI forcings as compared to PD. The low-resolution CCSM3 with more extensive sea ice and colder climate at high northern latitudes in the PD simulation shows less sensitivity of the North Atlantic MOC to PI forcing. ENSO variability and the strength of the ACC do not increase with PI forcing in the low-resolution CCSM3.

How warm was the last interglacial? New model-data comparisons.

A Community Climate System Model, Version 3 (CCSM3) simulation for 125 ka during the Last Interglacial (LIG) is compared to two recent proxy reconstructions to evaluate surface temperature changes from modern times. The dominant forcing change from modern, the orbital forcing, modified the incoming solar insolation at the top of the atmosphere, resulting in large positive anomalies in boreal summer. Greenhouse gas concentrations are similar to those of the pre-industrial (PI) Holocene. CCSM3 simulates an enhanced seasonal cycle over the Northern Hemisphere continents with warming most developed during boreal summer. In addition, year-round warming over the North Atlantic is associated with a seasonal memory of sea ice retreat in CCSM3, which extends the effects of positive summer insolation anomalies on the high-latitude oceans to winter months. The simulated Arctic terrestrial annual warming, though, is much less than the observational evidence, suggesting either missing feedbacks in the simulation and/or interpretation of the proxies. Over Antarctica, CCSM3 cannot reproduce the large LIG warming recorded by the Antarctic ice cores, even with simulations designed to consider observed evidence of early LIG warmth in Southern Ocean and Antarctica records and the possible disintegration of the West Antarctic Ice Sheet. Comparisons with a HadCM3 simulation indicate that sea ice is important for understanding model polar responses. Overall, the models simulate little global annual surface temperature change, while the proxy reconstructions suggest a global annual warming at LIG (as compared to the PI Holocene) of approximately 1°C, though with possible spatial sampling biases. The CCSM3 SRES B1 (low scenario) future projections suggest high-latitude warmth similar to that reconstructed for the LIG may be exceeded before the end of this century.