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CGD Publications: Archived Abstracts
November 2006 Abstracts
Effects of black carbon aerosols on the Indian monsoon
Figure. A six member ensemble of 20th century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model is analyzed to study the effects of black carbon (BC) aerosols on the Indian monsoon. The BC aerosols act to increase lower tropospheric heating over south Asia and reduce the amount of solar radiation reaching the surface during the dry season as noted in previous studies. The increased meridional tropospheric temperature gradient in the pre-monsoon months of March-April-May, particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months. With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau. Precipitation over China generally decreases due to the BC aerosol effects. There is a weakened latitudinal SST gradient due to BC aerosols as seen in observations, and is present in the multiple forcings experiments with CCSM3 that include natural and anthropogenic forcings (including BC aerosols). The BC aerosols and consequent weakened latitudinal SST gradient in those experiments are associated with increased precipitation during MAM in northern India and over the Tibetan Plateau, with some decreased precipitation over southwest India, the Bay of Bengal, Burma, Thailand, and Malaysia as seen in observations. In the summer monsoon season, the BC aerosols appear to have contributed to observed decreasing precipitation trends over parts of India, with increasing trends in the CCSM3 multiple-forcing experiments and observations over Pakistan and the Tibetan Plateau, and decreasing trends over Bangladesh, Burma, and Thailand.
Authored by Gerald A. Meehl , Julie M. Arblaster, and William D. Collins
National Center for Atmospheric Research (1), PO Box 3000, Boulder, CO 80307
Submitted to Journal of Climate, November 9, 2006
A coupled air-sea response mechanism to solar forcing in the Pacific region
Figure. The 11 year solar cycle (Decadal Solar Oscillation, or DSO) at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of 20th century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols) and natural (volcanoes and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11 year ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Due mainly to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the SE-trades thereby strengthening the ITCZ and SPCZ. Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Nina events) occurs. There is a strengthening of the SE-trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.
Authored by Gerald A. Meehl* , Julie M. Arblaster, Grant Branstator
National Center for Atmospheric Research (1), PO Box 3000, Boulder, CO 80307
Authored by Harry van Loon
CORA/NWRA, 3380 Mitchell Lane, Boulder, CO 80301
And National Center for Atmospheric Research (1), PO Box 3000, Boulder CO 80307
Submitted to Journal of Climate, November 8, 2006
A Strategy for Climate Change Stabilization Experiments with AOGCMs and ESMs
Figure. Climate models used for climate change projections are on the threshold of including much greater biological and chemical detail. Today, standard climate models (referred to generically as atmosphere-ocean general circulation models, or AOGCMs) include components that simulate the coupled atmosphere, ocean, land and sea ice. Some modeling centers are now incorporating carbon cycle models into AOGCMs in a move towards an Earth System Model (ESM) capability. Additional candidate components for ESMs include aerosols, chemistry, and dynamic vegetation (e.g., Cox et al. 2000, Friedlingstein et al. 2006).
Modeling groups are making decisions this year (2006) on what form their next generation climate models will take with an eye to running new climate change experiments that may be evaluated in the next IPCC assessment. Additionally, new emission scenarios have been developed by the integrated assessment community reflecting recommendations of the 25th IPCC Session.
Thus there has been a confluence of activities in model and scenario development that must be communicated and coordinated across various groups and scientific communities. To this end, a session of the Aspen Global Change Institute was convened from July 30-August 5, 2006. Participants represented communities from the WCRP, IGBP, Task Group on New Emissions Scenarios (TGNES), and IPCC Working Groups (WG) 1, 2 and 3 to address four objectives:
1. Identify new components in preparation for inclusion in AOGCMs.
2. Establish communication through WCRP, IGBP, IPCC, and Integrated Assessment (IA) modeling teams to coordinate activities for climate change simulations that will be performed with next-generation ESMs.
3. Propose an experimental design for 21st century climate change experiments.
4. Specify the requirements for time series of constituents from new stabilization scenarios (particularly with regard to impacts, mitigation, and adaptation).
Authored by Kathy A. Hibbard, Gerald A. Meehl (and session participants)
Submitted to EOS, November 13, 2006
The Large-scale Energy Budget of the Arctic
Figure. This paper synthesizes a variety of atmospheric and oceanic data to examine the large-scale energy budget of the Arctic. Assessment of the atmospheric budget relies primarily on the ERA-40 reanalysis. The seasonal cycles of vertically-integrated atmospheric energy storage and the convergence of energy transport from ERA-40, as evaluated for the polar cap (defined by the 70oN latitude circle), in general compare well with realizations from the NCEP/NCAR reanalysis over the period 1979-2001. However, shortcomings in top of atmosphere radiation, as compared with satellite data, and the net surface flux, contribute to large energy budget residuals in ERA-40. The seasonal cycle of atmospheric energy storage is strongly modulated by the net surface flux, which is also the primary driver of seasonal changes in heat storage within the Arctic Ocean. Averaged for an Arctic Ocean domain, the July net surface flux from ERA-40 of -100 W m-2 (i.e., into the ocean), associated with sea ice melt and oceanic sensible heat gain, exceeds the atmospheric energy transport convergence of 91 Wm-2. During winter, oceanic sensible heat loss and sea ice growth yield an upward surface flux of 50-60 W m-2, complemented with an atmospheric energy convergence of 80-90 W m-2 to provide a net radiation loss to space of 175-180 W m-2.
Mark C. Serreze, Andrew P. Barrett, Andrew J. Slater
Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado
Michael Steele, Jinlun Zhang
Applied Physics Laboratory, Polar Science Center, University of Washington, Seattle, WA
Kevin E. Trenberth1
National Center for Atmospheric Research, Boulder, Colorado
Submitted to Journal of Geophysical Research, Atmospheres, November 2006
1The National Center for Atmospheric Research is sponsored by the National Science Foundation
Hurricanes and Warming
Figure. The summer of 2004 was a major wake up call as an unprecedented four hurricanes hit Florida while across the Pacific 10 typhoons made landfall in Japan, four more than the previous high. What was the cause of this increase in tropical cyclones hitting land and causing major damage? Conflicting explanations were put forward by the scientific community, especially with regards to the role of global warming. Then just as a flurry of new studies designed to throw light on these issues were coming out in 2005, Mother Nature weighed in with a record breaking 2005 hurricane season in the North Atlantic that included Katrina (see Box). Subsequently, in 2006, after insurance rates had soared in the southeast United States as insurance companies responded to the previous two seasons and forecasts of another vigorous hurricane season, the number of North Atlantic storms was a lot fewer than predicted by hurricane forecasters and no tropical storms of note made landfall. Is there an explanation for all this? If global warming is playing a role, why was the 2006 season was so quiet?
This article outlines what is thought to be happening with regard to hurricanes and, specifically the role of global warming, as well as the outstanding issues and ways forward. It is essential to place the recent events in historical context, but how confident are we in the observational record? What are the physical factors involved that affect hurricanes, and how are they changing? How good are climate models and our ability to simulate tropical cyclones? And hence, how good is our understanding and thus our basis for making projections into the future, using models and other tools? In the following, a brief review is given of the observed hurricane record and the factors that influence hurricanes and thus govern changes over time. This is followed by the results of some prominent recent studies, an outline of factors that have played into the recent increase in hurricane activity, including global warming, and why 2006 appears to be at odds with this trend.
Authored by Kevin Trenberth
National Center for Atmospheric Research, Boulder, Colorado
Submitted to Journal of Scientific American, November 17, 2006
1The National Center for Atmospheric Research is sponsored by the National Science Foundation
North Atlantic Climate Variability
Figure. Marine ecosystems are undergoing rapid change at local and global scales. To understand these changes, including the relative roles of natural variability and anthropogenic effects, and to predict the future state of marine ecosystems requires quantitative understanding of the physics, biogeochemistry and ecology of oceanic systems at mechanistic levels. Central to this understanding is the role played by dominant patterns or “modes” of atmospheric and oceanic variability, which orchestrate coherent variations in climate over large regions with profound impacts on ecosystems. We review the spatial structure of extratropical climate variability over the Northern Hemisphere and, specifically, focus on modes of climate variability over the extratropical North Atlantic. A leading pattern of weather and climate variability over the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in surface air temperature, winds, storminess and precipitation over the Atlantic as well as the adjacent continents. The NAO also affects the ocean through changes in heat content, gyre circulations, mixed layer depth, salinity, high latitude deep water formation and sea ice cover. Thus, indices of the NAO have become widely used to document and understand how this mode of variability alters the structure and functioning of marine ecosystems. There is no unique way, however, to define the NAO. Several approaches are discussed, including both linear (e.g., principal component analysis) and nonlinear (e.g., cluster analysis) techniques. The former, which have been most widely used, assume preferred atmospheric circulation states come in pairs, in which anomalies of opposite polarity have the same spatial structure. In contrast, nonlinear techniques search for recurrent patterns of a specific amplitude and sign. They reveal, for instance, spatial asymmetries between different phases of the NAO that are likely important for ecological studies. It also follows that there is no universally accepted index to describe the temporal evolution of the NAO. Several of the most common measures are presented and compared. All reveal that there is no preferred time scale of variability for the NAO: large changes occur from one winter to the next and from one decade to the next. There is also a large amount of within-season variability in the patterns of atmospheric circulation of the North Atlantic, so that most winters cannot be characterized solely by a canonical NAO structure. A better understanding of how the NAO responds to external forcing, including sea surface temperature changes in the tropics, stratospheric influences, and increasing greenhouse gas concentrations, is crucial to the current debate on climate variability and change.
Authored by James W. Hurrell and Clara Deser
1National Center for Atmospheric Research, Boulder, Colorado
Submitted to Journal of Marine Systems, November 2006
1The National Center for Atmospheric Research is sponsored by the National Science Foundation
Detection and interpretation of tropical thermocline cooling in the Indian and Pacific Oceans during recent decades
Abstract: Figure. A warming trend has been detected in the world's oceans in recent decades. The basin-averaged warming, however, shows a complex vertical structure in the Indian and Pacific Oceans. A significant warming near the surface is accompanied by strong cooling in the upper thermocline and weaker warming in the lower part of the thermocline. Analysis of observed data and model solutions from this study reveals that the complex structure is confined mainly to the tropics. While increased greenhouse gases act to warm up the surface mixed layer, anomalous winds in the tropics enhance the upward Ekman pumping velocity and shoal the thermocline, resulting in the upper-thermocline cooling. This cooling process is well demonstrated by a simple model based on the ventilated thermocline theory. The study has important implications for climate change and fishery.
Weiqing Han
Department of Atmospheric and Oceanic Sciences, University of Colorado,UCB 311, Boulder, Colorado 80309
Email: whan@enso.colorado.edu, Tel: 303-735-3079
G. Meehl and A. Hu
National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80305
Submitted to Geophysical Research Letters, November 2006
Multi-model changes in El Nino teleconnections over North America in a future warmer climate
Abstract: Previous studies with single models have suggested that El Nino teleconnections over North American could be different in a future warmer climate due to factors involving changes of El Nino event amplitude and/or changes in the midlatitude base state circulation. Here we analyze a six member multi-model ensemble, three models with increasing future El Nino amplitude, and three models with decreasing future El Nino amplitude, to determine characteristics and possible changes to El Nino teleconnections during northern winter over the North Pacific and North America in a future warmer climate. Compared to observed El Nino events, all the models qualitatively produce general features of the observed teleconnection pattern over the North Pacific and North America, with an anomalously deepened Aleutian Low, a ridge over western North America, and anomalous low pressure over the southeastern U.S. However, associated with systematic errors in the location of SST and convective heating anomalies in the central and western equatorial Pacific (the models’ anomaly patterns are shifted to the west), the anomalous low pressure center in the North Pacific is weaker and shifted somewhat south compared to the observations. For future El Nino events, two different stabilization experiments are analyzed, one with CO2 held constant at year 2100 concentrations in the SRES A1B scenario (roughly doubled CO2), and another with CO2 concentrations held constant at 4XCO2. Consistent with the earlier single model results, the future El Nino teleconnections are changed in the models, with a weakened and eastward-shifted anomalous low in the North Pacific, weakened anomalous warming over northern North America, strengthened cooling over southern North America, and precipitation increases in the Pacific Northwest compared to present-day El Nino teleconnections. These changes are consistent with the altered base state upper tropospheric circulation with a wave-5 pattern noted in previous studies in the models. The future teleconnection changes are most consistent with this anomalous wave-5 pattern in the models with future increases of El Nino amplitude, but less so for the models with future decreases of El Nino amplitude. Thus El Nino amplitude and mean base state atmospheric circulation are both important for understanding the possible changes of El Nino teleconnections over the North Pacific and North America in a future warmer climate.
Figure. a) differences of El Nino event teleconnections for SLP (hPa), stabilized A1B minus 20th century stabilization experiment, for the three member composite for models with projected decreased El Nino amplitude; b) same as (a) except for 4XCO2 minus present-day control events; c) same as (a) except for the three models with projected increases of El Nino amplitude; d) same as (b) except for the three models with projected increases of El Nino amplitude; e) same as (a) except for the 6 member multi-model ensemble; f) same as (b) except for the 6 member multi-model ensemble. Shading indicates differences in El Nino teleconnections significant at the 10% level.
Authored by 1Gerald A. Meehl and Haiyan Teng
National Center for Atmospheric Research (1), PO Box 3000, Boulder, CO 80307
Corresponding author: meehl@ncar.ucar.edu 303-497-1331
Submitted to Climate Dynamics, November 17, 2006