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Gerald A. Meehl
,
Grant W. Branstator
, and
Warren M. Washington

Abstract

In this paper, an attempt is made to estimate possible sensitivities of El Niño-Southern Oscillation (ENSO)-related effects in a climate with increased carbon dioxide (CO2). To illustrate this sensitivity, results are shown from two different interactive ocean-atmosphere model configurations and an atmospheric model with prescribed heating anomalies. In the first, an atmospheric general circulation model (GCM) is coupled to a global coarse-grid dynamical ocean GCM (coupled model). In the second, the same atmospheric model is coupled to a simple nondynamic slab-ocean mixed-layer model (mixed-layer model). In the third, an atmospheric model is run in perpetual January mode with observed sea surface temperatures (SSTs) and prescribed tropical tropospheric heating anomalies (prescribed-heating model). Results from the coupled model show that interannual SST variability (with warm and cold events relative to the mean SST) continues to occur in the tropics with a doubling of CO2. This variability is superimposed on mean SSTs in the tropical eastern Pacific that are higher by about 1°. The pattern of precipitation and soil-moisture anomalies in the tropics is similar in model warm events with present amounts of CO2 (1 × CO2) and in warm events with instantaneously doubled CO2 (2 × CO2). When a warm-event SST anomaly is superimposed, the rise in mean SST in the tropical eastern Pacific from the doubling of CO2 leads to increased evaporation and low-level moisture convergence, greater precipitation over the SST anomaly, and an intensification of atmospheric anomalies in the tropics involved with the anomalous large-scale east-west (Walker) circulation. Consequently, differences of precipitation and soil moisture between 1 × CO2 and 2 × CO2 warm events show that most anomalously dry areas become drier (implying risk of increased drought in those regions in 2 × CO2 Warm events) and anomalously wet areas wetter in the coupled model. In the extratropics, the increased CO2 causes a large change in the midlatitude atmospheric circulation. This is associated with an alteration of extratropical teleconnections in 2 × CO warm events compared to 1 × CO2 warm events in a relative sense, with more zonally symmetric anomalies in sea level pressure and 200- mb height. Similar results in the tropics and extratropics are obtained for the mixed-layer model with warm-event SST anomalies in the tropical Pacific prescribed for 1 × CO2 and 2 × CO2 mean climates, and from the prescribed-heating model with anomalous heat sources in the tropical troposphere analogous to those in 1 × CO2 and 2 × CO2 warm events. This study is a precursor to future higher-resolution model studies that could also address possible changes in ENSO but with better representation of coupled mechanisms thought to contribute to ENSO.

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Haiyan Teng
,
Grant Branstator
,
Ahmed B. Tawfik
, and
Patrick Callaghan

Abstract

A series of idealized prescribed soil moisture experiments is performed with the atmosphere/land stand-alone configuration of the Community Earth System Model, version 1, in an effort to find sources of predictability for high-impact stationary wave anomalies observed in recent boreal summers. We arbitrarily prescribe soil water to have a zero value at selected domains in the continental United States and run 100-member ensembles to examine the monthly and seasonal mean response. Contrary to the lack of a substantial response in the boreal winter, the summertime circulation response is robust, consistent, and circumglobal. While the stationary wave response over the North America and North Atlantic sectors can be well explained by the reaction of a linear dynamical system to heating anomalies caused by the imposed dry land surface, nonlinear processes involving synoptic eddies play a crucial role in forming the remote response in Eurasia and the North Pacific Ocean. A number of other possible factors contributing to the circulation responses are also discussed. Overall, the experiments suggest that, in the boreal summer, soil moisture may contribute to the predictability of high-impact stationary wave events, which can impact regions that are great distances from these source regions.

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Siegfried D. Schubert
,
Max J. Suarez
,
Yehui Chang
, and
Grant Branstator

Abstract

This study examines the variability in forecasts of the January–February–March (JFM) mean extratropical circulation and how that variability is modulated by the El Niño–Southern Oscillation. The analysis is based on ensembles of seasonal simulations made with an atmospheric general circulation model (AGCM) forced with sea surface temperatures observed during the 1983 El Niño and 1989 La Niña events. The AGCM produces pronounced interannual differences in the magnitude of the extratropical seasonal mean noise (intraensemble JFM variability). The North Pacific, in particular, shows extensive regions in which the 1989 seasonal mean noise kinetic energy (SKE), which is dominated by a “Pacific–North American (PNA)–like” spatial structure, is more than 2 times that of the 1983 forecasts. The larger SKE in 1989 is associated with a larger-than-normal barotropic conversion of kinetic energy from the mean Pacific jet to the seasonal mean noise. The generation of SKE by submonthly transients also shows substantial interannual differences, though these are much smaller than the differences in the mean flow conversions. An analysis of the generation of monthly mean noise kinetic energy and its variability suggests that the seasonal mean noise is predominantly a statistical residue of variability resulting from dynamical processes operating on monthly and shorter timescales.

A stochastically forced barotropic model (linearized about the AGCM's 1983 and 1989 seasonal and ensemble mean states) is used to further assess the role of the basic state, submonthly transients, and tropical forcing in modulating the uncertainties in the seasonal AGCM forecasts. When forced globally with spatially white noise, the linear model generates much larger variance for the 1989 basic state, consistent with the AGCM results. The extratropical variability for the 1989 basic state is dominated by a single eigenmode and is strongly coupled with forcing over the tropical western Pacific and the Indian Ocean. Linear calculations that include forcing from the AGCM variance of the tropical forcing and submonthly transients show a small impact on the variability over the PNA region as compared with that of the basic-state differences.

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Thomas W. Schlatter
,
Grant W. Branstator
, and
Linda G. Thiel

Abstract

A multivariate statistical analysis procedure has been developed for estimating geopotential height h and wind (u, v) on a global latitude-longitude grid. Estimates are obtained by modifying the “first guess” from a prediction model by a linear combination of forecast errors deduced from observed data. Because the scheme is multivariate, the regression coefficients (weights) are matrices, which depend upon covariance among forecast errors in h, u and v. These covariances are modeled mathematically with geostrophic constraints. In the tropics, however, only the wind field is analyzed, covariances are modeled under the constraint of nondivergence, and heights are obtained from a balance equation. At high latitudes, analyses are performed in polar stereographic coordinates.

The objective analysis scheme fits observed data as well as the “Cressman scheme” that was used operationally at the National Meteorological Center until recently and also as well as a skilled analyst. In data-rich areas, the analyses are insensitive to the type of fist guess. Realistic ageostrophic and divergent components are present in the analyzed winds, and the kinetic energy spectrum at 40°N is reasonable at zonal wavenumbers less than 20. When both wind and height observations are plentiful, two univariate schemes (one for height, one for wind) fit the data as well as the multivariate scheme, but forecasts based upon the latter are consistently better. Experiments suggest that for a fixed amount of initial data, small gains in forecast accuracy can be made by improving the analysis procedure.

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Thomas W. Schlatter
,
Grant W. Branstator
, and
Linda G. Thiel

Abstract

No abstract available.

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Maurice L. Blackmon
,
Grant W. Branstator
,
Gary T. Bates
, and
John E. Geisler

Abstract

Perpetual January experiments have been performed using versions of the NCAR Community Climate Model (CCM) with and without mountains. Features of the mean simulations of the “no mountains” experiment are compared with those of the standard CCM, the “mountains” experiment. The stationary waves in the “no mountains” experiment have smaller amplitudes than those in the “mountains” case, especially for zonal wavenumbers 2 and 3. The mean zonal wind in the “no mountains” case also has weaker horizontal gradients than in the “mountains” case.

The response of these two versions of the CCM to equatorial Pacific sea surface temperature (SST) anomalies is investigated. Two anomalies are considered for each model configuration. Differences in the responses of the two models to the same anomalous forcing are discussed. The Northern Hemisphere midlatitude response of the “no mountains” model has nearly the same spatial scale as that of the “mountains” model, but details of the shape of the response pattern are different. The amplitude of the response in the Northern Hemisphere is weaker in the “no mountains” case than in the “mountains” case by about a factor of 2. On the other hand, the response in the Southern Hemisphere is stronger in the “no mountains” case than in the “mountains” case. It is shown that this is consistent with the interpretation that the Pacific/North American (PNA) teleconnection pattern extracts energy from the mean zonal flow by barotropic conversion. The importance of barotropic conversion in the Southern Hemisphere is also demonstrated.

A linear barotropic vorticity equation model is used to compare the response to localized tropical forcing in each of the two basic states, for “mountains” and “no mountains,” produced by the CCM. When forced in the vicinity of the SST anomaly, the linear model shows a sensitivity to the state about which it is linearized that is similar to the sensitivity shown by the CCM to its climatic state. This sensitivity is shown to be influenced by barotropic conversion processes, which in turn are influenced by the basic state configuration. Furthermore, calculations indicate that forcing in virtually any region of the tropics tends to produce a stronger (weaker) Northern (Southern) Hemisphere response for the “mountains” basic state than the “no mountains” basic state. It is also shown that anomalous upper troposphere convergence around Indonesia may be contributing to the CCM response to the eastern Pacific SST anomalies being considered in this study.

We conclude that the stationary waves in each CCM simulation affect the midlatitude response of that model to tropical forcing anomalies.

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Kettyah C. Chhak
,
Andrew M. Moore
,
Ralph F. Milliff
,
Grant Branstator
,
William R. Holland
, and
Michael Fisher

Abstract

At midlatitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that associated with the seasonal cycle. Stochastic forcing is therefore likely to have a significant influence on the ocean circulation. In this work, the influence of the stochastic component of the wind stress forcing on the large-scale, wind-driven circulation of the North Atlantic Ocean is examined. To this end, a quasigeostrophic model of the North Atlantic was forced with estimates of the stochastic component of wind stress curl obtained from the NCAR Community Climate Model. Analysis reveals that much of the stochastically induced variability in the ocean circulation occurs in the vicinity of the western boundary and some major bathymetric features. Thus, the response is localized even though the stochastic forcing occurs over most of the ocean basin. Using the ideas of generalized stability theory, the stochastically induced response in the ocean circulation can be interpreted as a linear interference of the nonorthogonal eigenmodes of the system. This linear interference process yields transient growth of stochastically induced perturbations. By examining the model pseudospectra, it is seen that the nonnormal nature of the system enhances the transient growth of perturbation enstrophy and therefore elevates and maintains the variance of the stochastically induced circulations in the aforementioned regions. The primary causes of nonnormality in the enstrophy norm are bathymetry and the western boundary current circulation.

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Kettyah C. Chhak
,
Andrew M. Moore
,
Ralph F. Milliff
,
Grant Branstator
,
William R. Holland
, and
Michael Fisher

Abstract

As discussed in Part I of this study, the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation. By forcing a quasigeostrophic model of the North Atlantic Ocean circulation with stochastic wind stress curl data from the NCAR CCM3, it was found in Part I that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide. This can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes. Using generalized stability theory (GST), it was found in Part I that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance. In the present study, the GST analysis of Part I is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically induced perturbations, and it is this process that maintains the variance of the stochastically induced circulations. Analysis of the large-scale circulation also reveals that the system possesses a large number of degrees of freedom, which has significant implications for ocean prediction. Sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration.

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Grant Branstator
,
Haiyan Teng
,
Gerald A. Meehl
,
Masahide Kimoto
,
Jeff R. Knight
,
Mojib Latif
, and
A. Rosati

Abstract

Initial-value predictability measures the degree to which the initial state can influence predictions. In this paper, the initial-value predictability of six atmosphere–ocean general circulation models in the North Pacific and North Atlantic is quantified and contrasted by analyzing long control integrations with time invariant external conditions. Through the application of analog and multivariate linear regression methodologies, average predictability properties are estimated for forecasts initiated from every state on the control trajectories. For basinwide measures of predictability, the influence of the initial state tends to last for roughly a decade in both basins, but this limit varies widely among the models, especially in the North Atlantic. Within each basin, predictability varies regionally by as much as a factor of 10 for a given model, and the locations of highest predictability are different for each model. Model-to-model variations in predictability are also seen in the behavior of prominent intrinsic basin modes. Predictability is primarily determined by the mean of forecast distributions rather than the spread about the mean. Horizontal propagation plays a large role in the evolution of these signals and is therefore a key factor in differentiating the predictability of the various models.

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Michael Alexander
,
Jeffrey Yin
,
Grant Branstator
,
Antonietta Capotondi
,
Christophe Cassou
,
Richard Cullather
,
Young-oh Kwon
,
Joel Norris
,
James Scott
, and
Ilana Wainer

Abstract

Extratropical atmosphere–ocean variability over the Northern Hemisphere of the Community Climate System Model version 3 (CCSM3) is examined and compared to observations. Results are presented for an extended control integration with a horizontal resolution of T85 (1.4°) for the atmosphere and land and ∼1° for the ocean and sea ice.

Several atmospheric phenomena are investigated including storms, clouds, and patterns of variability, and their relationship to both tropical and extratropical SST anomalies. The mean storm track, the leading modes of storm track variability, and the relationship of the latter to tropical and midlatitude sea surface temperature (SST) anomalies are fairly well simulated in CCSM3. The positive correlations between extratropical SST and low-cloud anomalies in summer are reproduced by the model, but there are clear biases in the relationship between clouds and the near-surface meridional wind. The model accurately represents the circulation anomalies associated with the jet stream waveguide, the Pacific–North American (PNA) pattern, and fluctuations associated with the Aleutian low, including how the latter two features are influenced by the El Niño–Southern Oscillation (ENSO). CCSM3 has a reasonable depiction of the Pacific decadal oscillation (PDO), but it is not strongly connected to tropical Pacific SSTs as found in nature. There are biases in the position of the North Atlantic Oscillation (NAO) and other Atlantic regimes, as the mean Icelandic low in CCSM3 is stronger and displaced southeastward relative to observations.

Extratropical ocean processes in CCSM3, including upper-ocean mixing, thermocline variability, and extratropical to tropical flow within the thermocline, also influence climate variability. As in observations, the model includes the “reemergence mechanism” where seasonal variability in mixed layer depth (MLD) allows SST anomalies to recur in consecutive winters without persisting through the intervening summer. Remote wind stress curl anomalies drive thermocline variability in the Kuroshio–Oyashio Extension region, which influences SST, surface heat flux anomalies, and the local wind field. The interior ocean pathways connecting the subtropics to the equator in both the Pacific and Atlantic are less pronounced in CCSM3 than in nature or in ocean-only simulations forced by observed atmospheric conditions, and the flow from the subtropical North Atlantic does not appear to reach the equator through either the western boundary or interior pathways.

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