Chris Hewitt

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

Ensembles-based predictions of climate changes and their impacts

Predictions of natural climate variability and the human impact on climate are inherently probabilistic, due to uncertainties in the initial conditions of forecasts, the representation of key processes within models, and climatic forcing factors. Hence, reliable estimates of climatic risk can be made only through ensemble integrations of Earth system models in which these uncertainties are explicitly incorporated. The ENSEMBLES project, funded through a 5-year contract with the European Commission, aims to provide probabilistic estimates of climatic risk through ensemble integrations of Earth system models in which the uncertainties noted here are explicitly incorporated.

Last glacial maximum ocean thermohaline circulation: PMIP2 model intercomparisons and data constraints

The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere-ocean simulations with four climate models that are also being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing.

Intercomparison of Simulated Global Vegetation Distributions in Response to 6 kyr BP Orbital Forcing

Abstract The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated sh...

A fully coupled GCM simulation of the climate of the mid‐Holocene

The response of a fully coupled ocean-atmosphere model to mid-Holocene insolation forcing is compared with results from an atmospheric model with prescribed present day sea surface temperatures and mid-Holocene insolation. The ocean response increases the amplitude of the seasonal temperature response over land, and increases its lag behind the solar forcing. The tropical oceans in the coupled model are coldest, relative to the control, in May at 6ka leading to a relative reduction in evaporation, precipitation and ascending motion over the ocean, and relative ascent and increased precipitation over equatorial Africa. By September, the tropical oceans are warmer than in the fixed SST experiment which leads to increased moisture flux from the oceans into the southeast monsoon, and a larger increase in monsoon precipitation than in the fixed SST experiment. In addition, the enhanced warming of the northern hemisphere SSTs relative to the southern subtropics leads to a northward shift of the Intertropical Convergence Zone. As a result, increased wetness of the tropical land surface in the mid Holocene experiments is more pronounced and extends further northwards when the sea surface temperatures are allowed to respond. The coupled model results are more in accord with lake level data, especially over North Africa.

A coupled model study of the Last Glacial Maximum: Was part of the North Atlantic relatively warm?

A coupled ocean-atmosphere general circulation model is used to simulate the climates of today and the last glacial maximum (LGM). The model, which does not require artificial flux adjustments, produces a pattern of cooling at the LGM that is broadly consistent with the findings from simpler models and palaeoclimatic data. However, changes to the ocean circulation produce anomalously warm LGM surface conditions over parts of the North Atlantic, seemingly at odds with palaeoceanographic data. The thermohaline circulation is intensified for several centuries, as is the northward heat transport in the Atlantic equatorward of 55°N, but this may be a transient result. Mechanisms that lead to this response are discussed.

Northern Hemisphere storm-tracks in present day and last glacial maximum climate simulations: a comparison of the European PMIP models

Abstract Extratropical weather systems are an essential feature of the midlatitude climate and global circulation. At the last glacial maximum (LGM), the formation of regions of high transient activity, referred to as “storm tracks,” is strongly affected by the presence of large ice sheets over northern America and Scandinavia and by differences in sea surface temperature (SST) distributions. In the framework of the Palaeoclimate Modelling Intercomparison Project, simulations of the LGM climate have been run with a wide range of atmospheric general circulation models (AGCMs) using the same set of boundary conditions, allowing a valuable comparison between simulations of a climate very different from the present one. In this study, the authors focus on the storm track representation in the models and its relationship with the surface temperatures, the mean flow, and the precipitation. Storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three-dimensional ...

Estimating shortwave radiative forcing and response in climate models

Abstract Feedback analysis in climate models commonly involves decomposing any change in the system’s energy balance into radiative forcing terms due to prescribed changes, and response terms due to the radiative effects of changes in model variables such as temperature, water vapor, clouds, sea ice, and snow. The established “partial radiative perturbation” (PRP) method allows an accurate separation of these terms, but requires processing large volumes of model output with an offline version of the model’s radiation code. Here, we propose an “approximate PRP” (APRP) method for the shortwave that provides an accurate estimate of the radiative perturbation, but derived from a quite modest amount of monthly mean model output. The APRP method is based on a simplified shortwave radiative model of the atmosphere, where surface absorption and atmospheric scattering and absorption are represented by means of three parameters that are diagnosed for overcast and clear-sky portions of each model grid cell. The accu...

Using Climate Predictions to Better Serve Society's Needs

Societies have always faced challenges and opportunities arising from variations in climate and have often flourished or collapsed, depending on their ability to adapt to such changes. In light of these challenges, there is a growing and urgent need to improve societys resilience to climate-related hazards and better manage the risks and opportunities from climate variability and climate change. This situation was recognized by governments, scientists, and decision makers at the World Climate Conference-3 in 2009, subsequently leading to the creation of the Global Framework for Climate Services (GFCS) under the leadership of several United Nations agencies.

New Study For Climate Modeling, Analyses, and Scenarios

The European Commission is funding the ENSEMBLES project, which aims to provide policy makers with information from the latest climate modeling, analyses, and scenarios. Currently, the most comprehensive estimates of climate change are made using general circulation models (GCMs) and Earth system models, but these have been used mostly to simulate futures that do not factor in climate mitigation policy. The results of these simulations typically show global average warming greatly exceeding the European Union (EU) climate policy target of 2°C above preindustrial levels, with associated large impacts on human and natural systems. To date, simple climate models typically have been used to assess the emissions trajectories that are required for meeting this target. The ENSEMBLES project is the first international multiclimate model intercomparison using a politically relevant aggressive mitigation scenario, referred to as E1 (Figures 1a and 1b). This scenario leads to a peak in the carbon dioxide (CO2) equivalent concentration in the atmosphere at around 535 parts per million (ppm) in 2045 before eventually stabilizing at around 450 ppm during the 22nd century. The climate models used are generally improved or extended versions of models contributing to the Intergovernmental Panel on Climate Changes (IPCC) Fourth Assessment Report.