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Anthony J. Broccoli
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Anthony J. Broccoli

Abstract

The sensitivity of tropical temperature to glacial forcing is examined by using an atmosphere–mixed layer ocean (A–MLO) model to simulate the climate of the last glacial maximum (LGM) following specifications established by the Paleoclimate Modeling Intercomparison Project. Changes in continental ice, orbital parameters, atmospheric CO2, and sea level constitute a global mean radiative forcing of −4.20 W m−2, with the vast majority of this forcing coming, in nearly equal portions, from the changes in continental ice and CO2. In response to this forcing, the global mean surface air temperature decreases by 4.0 K, with the largest cooling in the extratropical Northern Hemisphere. In the Tropics, a more modest cooling of 2.0 K (averaged from 30°N to 30°S) is simulated, but with considerable spatial variability resulting from the interhemispheric asymmetry in radiative forcing, contrast between oceanic and continental response, advective effects, and changes in soil moisture. Analysis of the tropical energy balance reveals that the decrease in top-of-atmosphere longwave emission associated with the tropical cooling is balanced primarily by the combination of increased reflection of shortwave radiation by clouds and increased atmospheric heat transport to the extratropics.

Comparisons with a variety of paleodata indicate that the overall tropical cooling is comparable to paleoceanographic reconstructions based on alkenones and species abundances of planktonic microorganisms, but smaller than the cooling inferred from noble gases in aquifers, pollen, snow line depression, and the isotopic composition of corals. The differences in the magnitude of tropical cooling reconstructed from the different proxies preclude a definitive evaluation of the realism of the tropical sensitivity of the model. Nonetheless, the comparisons with paleodata suggest that it is unlikely that the A–MLO model exaggerates the actual climate sensitivity. The similarity between the sensitivity coefficients (i.e., the ratio of the change in global mean surface air temperature to the change in global mean radiative forcing) for the LGM simulation and a simulation of CO2 doubling suggests that similar climate feedbacks are involved in the responses to these two perturbations. More comprehensive simulation of the tropical temperature sensitivity to glacial forcing will require the use of coupled models, for which a number of technical obstacles remain.

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Amy McGovern and Anthony J. Broccoli
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Craig Lindberg and Anthony J. Broccoli

Abstract

Spectral climate models are distinguished by their representation of variables as finite sums of spherical harmonics, with coefficients computed by an orthogonal projection of the variables onto the spherical harmonics. Representing the surface elevation in this manner results in its contamination by Gibbs-like truncation artifacts, which appear as spurious valleys and mountain chains in the topography. These “Gibbs ripples” are present in the surface topographies of spectral climate models from a number of research institutions. Integrations of the Geophysical Fluid Dynamics Laboratory (GFDL) climate model over a range of horizontal resolutions indicate that the Gibbs ripples lead to spurious, small-scale extrema in the spatial distribution of precipitation. This “cellular precipitation pathology” becomes more pronounced with increasing horizontal resolution, causing a deterioration in the fidelity of simulated precipitation in higher resolution models.

A method is described for reducing the Gibbs ripples that occur when making an incomplete spherical harmonic expansion of the topography. The new spherical harmonic representations of topography are formed by fitting a nonuniform spherical smoothing spline to geodetic data and found by solving a fixed-point problem. This regularization technique results in less distortion of features such as mountain height and continental boundaries than previous smoothing methods. These new expansions of the topography, when used as a lower boundary surface in the GFDL climate model, substantially diminish the cellular precipitation pathology and produce markedly more realistic simulations of precipitation. These developments make the prospect of using higher resolution spectral models for studies of regional hydrologic climate more attractive.

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Masakazu Yoshimori and Anthony J. Broccoli

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The equilibrium response to various forcing agents, including CO2, solar irradiance, tropospheric ozone, black carbon, organic carbon, sulfate, and volcanic aerosols, is investigated using an atmospheric general circulation model coupled to a mixed layer ocean model. The experiments are carried out by altering each forcing agent separately. Realistic spatial patterns of forcing constituents are applied but the magnitude of the forcing is adjusted so that each forcing constituent yields approximately the same strength of radiative forcing. It is demonstrated that the global mean temperature response depends on the types of forcing agents and the efficacy with respect to CO2 forcing ranges from 58% to 100%. The smallest efficacy is seen in one of the black carbon experiments and is associated with negative cloud feedback. The sign of the cloud feedback is shown to be sensitive to the vertical distribution of black carbon. The feedback analysis suggests that the small efficacy in tropospheric ozone is due to a large negative lapse rate feedback. Global mean precipitation increases when the earth warms except for the case of black carbon in which precipitation decreases. In all experiments, the global mean convective mass flux decreases when the earth’s surface warms. When the applied radiative forcing resulting from a particular forcing agent is stronger in one hemisphere, anomalous heat exchange between the hemispheres results in conjunction with changes in the Hadley circulation. The magnitude of interhemispheric heat transport is little sensitive to the details of the forcing, but is determined primarily by the interhemispheric contrast in forcing. The change in the Hadley circulation strongly impacts the precipitation changes in low latitudes.

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Arielle J. Catalano and Anthony J. Broccoli

Abstract

Extratropical cyclones (ETCs) are responsible for most of the large storm-surge events in the northeastern United States. This study uses the ECMWF atmospheric reanalysis of the twentieth century (ERA-20C) and NOAA tide gauge data to examine the local, regional, and large-scale atmospheric circulation accompanying the 100 largest ETC-driven surge events at three locations along the northeastern coast of the United States: Sewells Point (Norfolk), Virginia; the Battery (New York City), New York; and Boston, Massachusetts. Results from a k-means cluster analysis indicate that the largest surges are generated when slowly propagating ETCs encounter a strong anticyclone, which produces a tighter pressure gradient and longer duration of onshore winds. The strength of the anticyclone is evident in the middle and upper troposphere where there are positive 500-hPa geopotential height anomalies overlying the surface anticyclone for the majority of clusters and nearly all of the five biggest surge events. Multiple clusters feature a slower-than-average storm and a strong anticyclone, indicating that various circulation scenarios can produce a large storm surge. This favorable environment for large surge events is influenced by well-known modes of climate variability including El Niño, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Pacific–North American (PNA) pattern. ETCs are more likely to produce a large surge during El Niño conditions, which have been shown to enhance the East Coast storm track. At Boston and the Battery, maximum surge occurs preferentially during the positive phase of PNA and the negative phases of AO/NAO.

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Anthony J. Broccoli and Robert P. Harnack

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Statistical models were developed to specify and predict the mean monthly sea level pressure (SLP) distribution over the central and eastern North Pacific Ocean from the mean monthly sea surface temperature (SST) distribution for the same area. These models were derived from data for the period 1947–71, with data from two additional periods (1933–41 and 1972–76) retained for independent testing.

The earlier period SST data is taken from a new data set compiled at the National Climatic Center and processed for use in examining large-scale air-sea interactions. This procedure is described.

Empirical orthogonal function (EOF) analysis was used to represent each field (SST and SLP) by a small number of composite variables. Regression analysis. was then used in which SST EOF amplitudes were the predictors and SLP EOF amplitudes were the predictands. The analyses were stratified by month, with lags from 0–3 months considered. Of the 84 models developed, 18 were statistically significant at the 10% level. The number of significant relationships was found to decrease with increasing tag, being greatest for SST contemporaneous with SLP. All statistically significant models involved SST's from the period June–January.

Each of the significant models was tested on the independent samples, using the reduction of error (RE) statistic as the measure of skill. An adjustment was made to the 1933–41 SST data to remove a systematic bias, and the RE scores recomputed. Using the adjusted data, RE scores for the 1933–41 period improved, with 10 of 18 models demonstrating skill overall. Most of the skill, especially at longer lags, was associated with models using late autumn and early winter SST as predictors.

Possible reasons for the seasonal distribution of the skillful relationships are discussed.

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Paul C. Loikith and Anthony J. Broccoli

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Motivated by a desire to understand the physical mechanisms involved in future anthropogenic changes in extreme temperature events, the key atmospheric circulation patterns associated with extreme daily temperatures over North America in the current climate are identified. The findings show that warm extremes at most locations are associated with positive 500-hPa geopotential height and sea level pressure anomalies just downstream with negative anomalies farther upstream. The orientation, physical characteristics, and spatial scale of these circulation patterns vary based on latitude, season, and proximity to important geographic features (i.e., mountains, coastlines). The anomaly patterns associated with extreme cold events tend to be similar to, but opposite in sign of, those associated with extreme warm events, especially within the westerlies, and tend to scale with temperature in the same locations. Circulation patterns aloft are more coherent across the continent than those at the surface where local surface features influence the occurrence of and patterns associated with extreme temperature days. Temperature extremes may be more sensitive to small shifts in circulation at locations where temperature is strongly influenced by mountains or large water bodies, or at the margins of important large-scale circulation patterns making such locations more susceptible to nonlinear responses to future climate change. The identification of these patterns and processes will allow for a thorough evaluation of the ability of climate models to realistically simulate extreme temperatures and their future trends.

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Paul C. Loikith and Anthony J. Broccoli

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The influence of the Pacific–North American (PNA) pattern, the northern annular mode (NAM), and the El Niño–Southern Oscillation (ENSO) on extreme temperature days and months over North America is examined. Associations between extreme temperature days and months are strongest with the PNA and NAM and weaker for ENSO. In general, the association with extremes tends to be stronger on monthly than daily time scales and for winter as compared to summer. Extreme temperatures are associated with the PNA and NAM in the vicinity of the centers of action of these circulation patterns; however, many extremes also occur on days when the amplitude and polarity of these patterns do not favor their occurrence. In winter, synoptic-scale, transient weather disturbances are important drivers of extreme temperature days; however, many of these smaller-scale events are concurrent with amplified PNA or NAM patterns. Associations are weaker in summer when other physical mechanisms affecting the surface energy balance, such as anomalous soil moisture content, also influence the occurrence of extreme temperatures.

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Paul C. Loikith and Anthony J. Broccoli

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Circulation patterns associated with extreme temperature days over North America, as simulated by a suite of climate models, are compared with those obtained from observations. The authors analyze 17 coupled atmosphere–ocean general circulation models contributing to the fifth phase of the Coupled Model Intercomparison Project. Circulation patterns are defined as composites of anomalies in sea level pressure and 500-hPa geopotential height concurrent with days in the tails of temperature distribution. Several metrics used to systematically describe circulation patterns associated with extreme temperature days are applied to both the observed and model-simulated data. Additionally, self-organizing maps are employed as a means of comparing observed and model-simulated circulation patterns across the North American domain. In general, the multimodel ensemble resembles the observed patterns well, especially in areas removed from complex geographic features (e.g., mountains and coastlines). Individual model results vary; however, the majority of models capture the major features observed. The multimodel ensemble captures several key features, including regional variations in the strength and orientation of atmospheric circulation patterns associated with extreme temperatures, both near the surface and aloft, as well as variations with latitude and season. The results from this work suggest that these models can be used to comprehensively examine the role that changes in atmospheric circulation will play in projected changes in temperature extremes because of future anthropogenic climate warming.

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