Atmospheric Research Climate System Model Research Articles

Long‐term variations of North Atlantic tropical cyclone activity downscaled from a coupled model simulation of the last millennium

AbstractThe observed historical record of North Atlantic tropical cyclones (TCs) is relatively short and is subject to potential biases owing to a lack of observation platforms such as aircraft reconnaissance and satellite imagery in earlier decades. Therefore, studies of long‐term variability in TC activity are hindered by the limitations and uncertainty within the historical data. An alternative approach is to study long‐term Atlantic TC variability within the framework of a coupled ocean‐atmosphere climate model simulation. We have taken such an approach using a simulation of the National Center for Atmospheric Research Climate System Model 1.4 forced with estimated natural and anthropogenic forcing over the past millennium. Atmospheric variables from the long‐term model simulation are used to drive a recently developed downscaling relationship that simulates TC genesis and tracking over the course of the 1150 year model simulation. This downscaling process generates a long‐term synthetic TC track data set, free of observational biases, though subject to limitations in the model climatology. The synthetic TC data are used to perform an analysis of long‐term variability in Atlantic TCs, specifically focusing on TC landfalls, within the context of the coupled model simulation. Ultimately, analysis of various TC time series reveals that counts of landfalling TCs and even landfalling hurricanes track relatively well with the total basin‐wide TC activity on multidecadal and longer timescales.

Evolution of carbon sinks in a changing climate

Climate change is expected to influence the capacities of the land and oceans to act as repositories for anthropogenic CO2 and hence provide a feedback to climate change. A series of experiments with the National Center for Atmospheric Research-Climate System Model 1 coupled carbon-climate model shows that carbon sink strengths vary with the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster CO2 emissions. Furthermore, there is a positive feedback between the carbon and climate systems, so that climate warming acts to increase the airborne fraction of anthropogenic CO2 and amplify the climate change itself. Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses. Analysis of our results in the context of comparable models suggests that destabilization of the tropical land sink is qualitatively robust, although its degree is uncertain.

Evaluating the National Center for Atmospheric Research climate system model over West Africa: Present‐day and the 21st century A1 scenario

We present an analysis of observed and simulated June–July–August (JJA) West African climate during the last 2 decades of the 20th and 21st centuries. The National Center for Atmospheric Research coupled atmosphere‐ocean climate system model (CSM) simulation is compared to long‐term observations and National Centers for Environmental Prediction reanalysis with an emphasis on the wet season during the late 20th century. While there are significant improvements in the simulation of the African Easterly Jet (AEJ) and African easterly waves relative to the uncoupled Community Climate Model, version 3, biases still exist. These biases are related to a poor simulation of the Azores high, which extends into eastern Europe, allowing for cold air advection into North Africa. There is also little improvement in the upper troposphere Tropical Easterly Jet in the CSM, which is too weak and does not extend westward over the Atlantic Ocean. In the late 21st century, JJA summer temperatures in West Africa have warmed 1.5°–2.5°C relative to the 1951–1980 period. Moreover, the Sahelian region is wetter in the late 21st century in association with an increase in atmospheric moisture, stronger meridional winds from the Gulf of Guinea, and a slightly stronger AEJ.

Modeling evidence for enhanced El Niño–Southern Oscillation amplitude during the Last Glacial Maximum

We present a numerical eigenmode analysis of an intermediate El Niño–Southern Oscillation (ENSO) model which is driven by present‐day observed background conditions as well as by simulated background conditions for the Last Glacial Maximum (LGM) about 21,000 years ago. The background conditions are obtained from two LGM simulations which were performed with the National Center for Atmospheric Research climate system model (CSM1.4) and an Earth system model of intermediate complexity (ECBilt‐CLIO). Our analysis clearly shows that the leading present‐day unstable recharge‐discharge mode changes its stability as well as its frequency during LGM conditions. Simulated LGM background conditions were favorable to support large‐amplitude self‐sustained interannual ENSO variations in the tropical Pacific. Our analysis indicates that off‐equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the LGM ENSO mode.

Climate model sensitivity to atmospheric CO 2 levels in the Early–Middle Paleogene

This study examines the sensitivity of the slab ocean version of the National Center for Atmospheric Research Climate System Model with revised Eocene geography, orography, and vegetation to changing carbon dioxide (CO 2) levels. We compare model results with temperature proxies from the geologic record for the Early–Middle Paleogene. We ran three modeling experiments with CO 2 levels at 500, 1000, and 2000 ppm, and all with atmospheric methane levels of 3.5 ppm. Surface temperatures in the two higher CO 2 scenarios are warmer than those of the 500 ppm scenario. The largest warming with increasing CO 2 occurred in the high latitudes, particularly in the Northern Hemisphere, during the wintertime. Compared to the 500 ppm case, Arctic wintertime temperatures increased by ∼10°C for the 1000 ppm scenario, and ∼20°C for the 2000 ppm scenario. The 1000 and 2000 ppm scenarios produced mean annual and cold month mean temperatures in mid- and high latitudes that are much more compatible with the climate interpretations from Eocene flora, especially for data from the Southern Hemisphere. Tropical sea surface temperatures (SSTs) in the 2000 ppm scenario, however, are still ∼4°C higher than the warmest temperatures inferred from proxy data. The better match between temperatures in the high CO 2 modeling scenario and high latitude climate interpretations is consistent with the idea that the CO 2 levels during the Eocene were high, at least 3–4 times the pre-industrial value of 280 ppm, but the discrepancies in the tropics suggest that SST estimates from proxies are too low or that the models lack some tropical cooling mechanism that was important at this time.

Late Cretaceous ocean: Coupled simulations with the National Center for Atmospheric Research Climate System Model

Deep‐ocean circulation may be a significant factor in determining climate. Here, we describe two long, fully coupled atmosphere‐ocean simulations with the National Center for Atmospheric Research Climate System Model for the Late Cretaceous (80 Ma). Our results suggest that higher levels of atmospheric CO2 and the altered paleogeography of the Late Cretaceous resulted in a surface ocean state, temperature, salinity, and circulation, significantly different than at present. This, in turn, resulted in deepwater features that, although formed by mechanisms similar to the present, were quite different from the present. The simulations exhibit large overturning cells in both hemispheres extending from the surface to the ocean bottom and with intensity comparable to the present‐day North Atlantic simulated overturning. In the Northern Hemisphere the sinking takes place in the Pacific due to cooling of the much warmer and saltier waters compared to the present day. In the Southern Hemisphere the sinking occurs primarily in the southern Atlantic and Indian Oceans. For a simulation with atmospheric CO2reduced from 6 times to 4 times preindustrial concentrations, the southern branch is reduced by 35%percnt; due to less poleward transport of salty waters in the South Atlantic Ocean. Warm waters inferred from proxy data in deep‐sea cores can be explained by the high‐latitude sites of overturning. These results contradict the traditional hypothesis that warm Cretaceous ocean bottom waters must have formed by sinking in shallow low‐latitude seas.

Effects of Enhanced Shortwave Absorption on Coupled Simulations of the Tropical Climate System

Abstract The effects of enhanced shortwave absorption on coupled simulations of the tropical climate have been tested using the National Center for Atmospheric Research Climate System Model. The enhancement in cloudy-sky shortwave absorption is consistent with several recent intercomparisons of models and observations. The primary reasons for introducing enhanced absorption are to improve the fidelity of the modeled shortwave fluxes in comparison with observations, to improve the simulation of sea surface temperature in the tropical Pacific, and to reduce or eliminate transient behavior in the model associated with compensating errors between latent heat fluxes and surface insolation. The simulations incorporate a new method to increase shortwave absorption in cloudy atmospheres. In previous studies, adjustments to cloud optical properties or modifications to the shortwave radiative transfer were used to increase shortwave cloud absorption. In the new “generic” implementation, the in-cloud shortwave flux ...

Annual cycle of the Brazil‐Malvinas confluence region in the National Center for Atmospheric Research Climate System Model

The objective of this study is to compare the mean and seasonal variability of the circulation in the southwest Atlantic with observations. The results used in the comparison are from the last 200 years of a 300 year control integration of the Climate System Model (CSM). The area of study includes the confluence region between the subtropical and subpolar waters represented by the Brazil and Malvinas Currents. The seasonal variation of transport and its relationship to changes in the wind stress forcing and in the sea surface temperature are examined and compared to available oceanographic observations. This study shows that a coarse resolution climate model, such as the CSM, can successfully reproduce major characteristics of the Brazil‐Malvinas confluence seasonally, although the mesoscale features involving recirculation and meander dynamics are not resolved. The CSM transport values in the region of 38°S are consistent with hydrographically derived values. The transport of the CSM Brazil Current is higher during austral summer and smaller during austral winter. Conversely, the Malvinas Current transport is weaker during austral summer and stronger during austral winter. This is also consistent with observations. The CSM seasonal cycle in transport associated with both the Brazil and Malvinas Currents and its meridional displacement is closely linked to the seasonal variations in the local wind stress curl. However, the displacement is much smaller in the model than in observations. The CSM results show that the latitudinal displacement of the 24°C and 17°C at the South American coast beween austral summer and winter is 20° and 12°, respectively. This is very similar to the displacement seen in observations.

Oceanic wave dynamics and interdecadal variability in a climate system model

Western boundary disturbances detected in the first multicentury integration of the National Center for Atmospheric Research Climate System Model are analyzed, and the role they play in the model interdecadal variability is investigated. The boundary signals propagate southwestward along the coast of North America. At depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary. Their southwestward propagation is associated with the evolution of the western boundary current system (the Gulf Stream system in the upper part of the water column and the Deep Western Boundary Current at depth) from weaker than average to stronger than average conditions. Thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales. They show similarities with the boundary “waves” found in ocean‐only models exhibiting interdecadal variability, and their role in the model's response to changes in the rate of sinking appears to be in agreement with the theory of the spin‐up of the deep ocean circulation. The main result of this study is that the similarities with ocean‐only results and theories support the idea that interdecadal timescales of variability originate from the ocean dynamics.

Bayesian Climate Change Assessment

Abstract A Bayesian fingerprinting methodology for assessing anthropogenic impacts on climate was developed. This analysis considers the effect of increased CO2 on near-surface temperatures. A spatial CO2 fingerprint based on control and forced model output from the National Center for Atmospheric Research Climate System Model was developed. The Bayesian approach is distinguished by several new facets. First, the prior model for the amplitude of the fingerprint is a mixture of two distributions: one reflects prior uncertainty in the anticipated value of the amplitude under the hypothesis of “no climate change.” The second reflects behavior assuming“climate change forced by CO2.” Second, within the Bayesian framework, a new formulation of detection and attribution analyses based on practical significance of impacts rather than traditional statistical significance was presented. Third, since Bayesian analyses can be very sensitive to prior inputs, a robust Bayesian approach, which investigates the ranges of...

Ocean general circulation model sensitivity to forcing from scatterometer winds

Enhanced global surface wind fields are constructed from a blend of NASA scatterometer (NSCAT) and ERS 2 scatterometer data and National Centers for Environmental Prediction (NCEP) analyses, at 6‐hour intervals, for a repeatable annual cycle from August 1996 through July 1997. Wind field properties (wind speed, zonal and meridional wind stresses, wind stress curl, and kinetic energy input) for the enhanced winds are compared with the NCEP analyses for the same time period. Large‐scale, zonal patterns dominate annual average difference maps for wind speed and both components of wind stress. Wind stress curl differences are largest in the subpolar North Atlantic and in the midlatitudes of the southern hemisphere. The importance of wind field differences are measured by their impacts on the response of an ocean general circulation model (OGCM). Twin experiments are performed using the National Center for Atmospheric Research Climate System Model ocean component in stand‐alone mode. The annual mean OGCM responses to the enhanced winds and to surface winds from the NCEP analyses are compared for barotropic stream function, surface velocities, upper ocean upwelling, sea surface temperature (SST), surface heat flux, meridional overturning stream function, and total northward heat transport. Differences in the annual mean responses are attributable to differences in the mean forcing and not to the mesoscale signal that is present in the enhanced winds but not resolved by the OGCM. Differences in SST and surface heat flux are partitioned between local thermodynamic balances and balances involving ocean dynamics as well. Northward heat transport differences of order −0.2 PW in the southern hemisphere midlatitudes are consistent with weaker eastward and southward stresses in the westerlies and a 27% reduction in kinetic energy input from enhanced winds. This difference in northward heat transport is compensated in the southern hemisphere tropical Pacific where surface stresses and the OGCM surface response to enhanced winds are more westward and more divergent, resulting in 27% greater kinetic energy input. An extensive appendix details the realistic high‐wavenumber character of the enhanced winds.

On the Wind-Driven Circulation of the Uncoupled and Coupled NCAR Climate System Ocean Model*

Abstract The time-mean wind-driven circulation of the uncoupled and coupled National Center for Atmospheric Research Climate System Model ocean model is investigated. Although the coupled surface wind stress and wind stress curl magnitudes are, in general, larger than the uncoupled distributions, the coupled wind stresses are realistic, given the substantial uncertainties in the observational estimates. The Sverdrup balance, which represents a simple dynamical relation, can indeed describe the model depth-integrated transports to a large degree and shows that the increase in the coupled ocean barotropic transports is mainly due to the larger wind stress curl of the coupled system. Another simple dynamical tool, the Ekman transport analysis, shows that the coupled ocean upwelling and downwelling velocities are, in general, larger than in uncoupled ocean, consistent with the larger wind stress curl. Both models have similar upper-ocean upwelling magnitudes in the equatorial Atlantic. In the equatorial Pacif...

The Project for Intercomparison of Land-surface Parametrization Schemes (PILPS): 1992 to 1995

The World Climate Research Programme Project for Intercomparison of Land-surface Parametrization Schemes (PILPS) is an on-going international intercomparison of land surface schemes designed for use in climate modelling and weather prediction. The five phases of PILPS are described in this work with an indication of the status of each. Phase 0 documented the status of land surface schemes. Phase 1 performed a series of off-line tests using synthetic atmospheric forcing. Phase 2 exploited observational data in off-line tests. Phase 3 was comprised of coupled tests within the Atmospheric Model Intercomparison Project (AMIP) project and finally Phase 4 will consider the performance of land-surface schemes when coupled to their host climate models in fully coupled evaluations. Results from Phase 1 indicate that there is a wide range among models. Phase 2 indicates that while some models are consistent with observations, there remains a large range among models and that many diverge greatly from observations. PILPS phases 2(a) and 2(b) results suggest that individual land-surface schemes capture specific aspects of the complex system with reasonable accuracy but no one scheme captures the whole system satisfactorily and consistently. In Phase 3 the intercomparison of PILPS schemes as a component of global atmospheric circulation models is being conducted jointly with the AMIP as diagnostic subproject number 12. Preliminary results suggest that results differ by about the same range as in the offline experiments in Phases 1 and 2. Phase 4 will couple selected land-surface schemes to the USA's National Center for Atmospheric Research climate system model and to the Australian Bureau of Meteorology limited area model.