Brian V. Smoliak

Projecting North American Climate over the Next 50 Years: Uncertainty due to Internal Variability*

AbstractThis study highlights the relative importance of internally generated versus externally forced climate trends over the next 50 yr (2010–60) at local and regional scales over North America in two global coupled model ensembles. Both ensembles contain large numbers of integrations (17 and 40): each of which is subject to identical anthropogenic radiative forcing (e.g., greenhouse gas increase) but begins from a slightly different initial atmospheric state. Thus, the diversity of projected climate trends within each model ensemble is due solely to intrinsic, unpredictable variability of the climate system. Both model ensembles show that natural climate variability superimposed upon forced climate change will result in a range of possible future trends for surface air temperature and precipitation over the next 50 yr. Precipitation trends are particularly subject to uncertainty as a result of internal variability, with signal-to-noise ratios less than 2. Intrinsic atmospheric circulation variability i...

Simulated versus observed patterns of warming over the extratropical Northern Hemisphere continents during the cold season

A suite of the historical simulations run with the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models forced by greenhouse gases, aerosols, stratospheric ozone depletion, and volcanic eruptions and a second suite of simulations forced by increasing CO2 concentrations alone are compared with observations for the reference interval 1965–2000. Surface air temperature trends are disaggregated by boreal cold (November-April) versus warm (May-October) seasons and by high latitude northern (N: 40°–90 °N) versus southern (S: 60 °S–40 °N) domains. A dynamical adjustment is applied to remove the component of the cold-season surface air temperature trends (over land areas poleward of 40 °N) that are attributable to changing atmospheric circulation patterns. The model simulations do not simulate the full extent of the wintertime warming over the high-latitude Northern Hemisphere continents during the later 20th century, much of which was dynamically induced. Expressed as fractions of the concurrent trend in global-mean sea surface temperature, the relative magnitude of the dynamically induced wintertime warming over domain N in the observations, the simulations with multiple forcings, and the runs forced by the buildup of greenhouse gases only is 7∶2∶1, and roughly comparable to the relative magnitude of the concurrent sea-level pressure trends. These results support the notion that the enhanced wintertime warming over high northern latitudes from 1965 to 2000 was mainly a reflection of unforced variability of the coupled climate system. Some of the simulations exhibit an enhancement of the warming along the Arctic coast, suggestive of exaggerated feedbacks.

A prominent pattern of year-to-year variability in Indian Summer Monsoon Rainfall

The dominant patterns of Indian Summer Monsoon Rainfall (ISMR) and their relationships with the sea surface temperature and 850-hPa wind fields are examined using gridded datasets from 1900 on. The two leading empirical orthogonal functions (EOFs) of ISMR over India are used as basis functions for elucidating these relationships. EOF1 is highly correlated with all India rainfall and El Niño–Southern Oscillation indices. EOF2 involves rainfall anomalies of opposing polarity over the Gangetic Plain and peninsular India. The spatial pattern of the trends in ISMR from 1950 on shows drying over the Gangetic Plain projects onto EOF2, with an expansion coefficient that exhibits a pronounced trend during this period. EOF2 is coupled with the dominant pattern of sea surface temperature variability over the Indian Ocean sector, which involves in-phase fluctuations over the Arabian Sea, the Bay of Bengal, and the South China Sea, and it is correlated with the previous winter’s El Niño–Southern Oscillation indices. The circulation anomalies observed in association with fluctuations in the time-varying indices of EOF1 and EOF2 both involve distortions of the low-level monsoon flow. EOF1 in its positive polarity represents a southward deflection of moist, westerly monsoon flow from the Arabian Sea across India, resulting in a smaller flux of moisture to the Himalayas. EOF2 in its positive polarity represents a weakening of the monsoon trough over northeastern India and the westerly monsoon flow across southern India, reminiscent of the circulation anomalies observed during break periods within the monsoon season.

Dynamical Adjustment of the Northern Hemisphere Surface Air Temperature Field: Methodology and Application to Observations*

AbstractThe influence of atmospheric circulation changes reflected in spontaneously occurring sea level pressure (SLP) anomalies upon surface air temperature (SAT) variability and trends is investigated using partial least squares (PLS) regression, a statistical method that seeks to maximally explain covariance between a predictand time series or field and a predictor field. Applying PLS regression in any one of the three variants described in this study (pointwise, PC-wise, and fieldwise), the method yields a dynamical adjustment to the observed NH SAT field that accounts for approximately 50% of the variance in monthly mean, cold season data. It is shown that PLS regression provides a more parsimonious and statistically robust dynamical adjustment than an adjustment method based on the leading principal components of the extratropical SLP field. The usefulness of dynamical adjustment is demonstrated by applying it to the attribution of cold season SAT trends in two reference intervals: 1965–2000 and 192...

Attribution of Climate Change in the Presence of Internal Variability

Spontaneous, internally-generated variability of the climate system is pervasive. On the multidecadal time scale it dominates the variability of surface air temperature averaged over extratropical land areas as large as the contiguous United States, and it modulates the rate of rise of global mean temperature in response to the buildup of greenhouse gases. Unforced variability is one of the factors that imposes limitations on the degree of confidence that can be attached to assessments and predictions of human-induced climate change. This chapter summarizes results of some recent studies based on the analysis of large ensembles of numerical integrations run with a suite of different atmospheric initial conditions but with the same prescribed external forcing scenario. The future trajectory of the real climate system is, in some sense, like the trajectory of an individual member of such an ensemble. The diversity of the trends among the different ensemble members is a part of the irreducible uncertainty inherent in projections of future climate change. It is shown how statistical methods can be used to diagnose the causes of this diversity, most of which is in response to member-to-member diversity in the atmospheric circulation trends, as reflected in the associated patterns of the sea-level pressure trends. Interactions between the atmosphere, oceans, and land also contribute to the variability of surface air temperature trends on the multidecadal time scale, as discussed in Chapters XX and XX. It is argued that in the face of such large uncertainties in the attribution of climate change in the extratropics, more attention should be focused on climate change in the tropics, where the greenhouse warming signal stands out more clearly, and on the broader suite of environmental issues that impact food security and the viability of ecosystems.

On the Leading Patterns of Northern Hemisphere Sea Level Pressure Variability

AbstractThe leading patterns of variability of the monthly mean Northern Hemisphere (NH) sea level pressure (SLP) field, as derived from empirical orthogonal teleconnection (EOT) analysis of a 93-yr (1920–2012) record of NOAA–CIRES 20th Century Reanalyses, are presented and discussed, with emphasis on wintertime patterns. The analysis yields nine or more highly reproducible wintertime hemispheric EOTs, the first six of which closely resemble EOF1 or EOF2 in their respective sectors of the hemisphere. Collectively, the first nine wintertime patterns account for 70% of the variance of NH SLP, 40% of the variance of NH surface air temperature (SAT), and 52% of the variance of the time series of NH-mean SAT poleward of 20°N. Wintertime EOT1 corresponds to the NH annular mode (NAM) and EOT2 corresponds to the SLP expression of the Pacific–North America pattern. The remaining wintertime EOT patterns are monopoles arranged like the links of a chain wrapped around the primary center of action of the annular mode....

Choosing Carbon Mitigation Strategies Using Ethical Deliberation

Anthropogenic greenhouse gas emissions change earth’s climate by altering the planet’s radiative balance. An important first step in mitigation of climate change is to reduce annual increases in these emissions. However, the many suggested means of limiting emissions rates have led to few actual changes in policy or behavior. This disconnection can be attributed in part to the difficulty of convening groups of stakeholders with diverse values, the polarizing nature of current political systems, poor communication across disciplines, and a lack of clear, usable information about emission mitigation strategies. Here, electronically facilitated ethical deliberation, a method of determining courses of action on common goals by collaborative discussion, is used to evaluate Pacala and Socolow’s climate change stabilization strategies based on economic, technological, social, and ecological impacts across a wide range of spatial and temporal scales. Few previous analyses of climate mitigation strategies include all of these factors; rather, short-term technological feasibility studies and economic cost‐benefit analyses predominate. After accounting for tradeoffs among disparate criteria, strategies involving end-user efficiency (e.g., efficient buildings and vehicles), wind, and solar power rank highest, while carbon capture and storage, hydrogen fuel cells, and biofuels options rank lowest. This electronically facilitated deliberation method offers an alternative to oppositional debate or cost‐benefit analysis for assessing strategies where both quantitative and qualitative factors are important, information from disparate disciplines is relevant, and stakeholders are geographically dispersed.