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This project will investigate the role that land ice and, in particular the Greenland ice sheet, and sea ice play on modulating the present day and future thermohaline circulation in the Atlantic under a warming climate. This will be achieved by coupling a suite of sub-models that define the mass balance and behaviour of land and sea ice in the Arctic into a fast Earth model of Intermediate Complexity and a medium resolution fully coupled Atmosphere-Ocean GCM based on HadCM3. The coupled ice-ocean-atmosphere models will be used to investigate, in detail, the interaction of the cryosphere with the rest of the climate system, with particular emphasis on the thermohaline circulation.
Output from state-of-the-art coupled climate models will be analysed in conjunction with very long instrumental climate data and an extensive archive of annual- and selected decadal-resolution palaeoclimate data to study climate changes during the past millennium. Actual and model-derived synthetic networks of palaeoclimate data will be used to estimate the extent to which (i) variations in Atlantic meridional overturning circulation strength; (ii) variations in the North Atlantic Oscillation; and (iii) the sensitivity of climate to external forcing changes can be reconstructed from different networks of palaeoclimate data, making assumptions about coverage, seasonality of response and reliability of expressed climate signal.
We propose to assess the probability of rapid climate change under future climate scenarios, in particular different greenhouse gas concentration profiles. We will use Bayesian statistical methods to synthesize all sources of uncertainty using expert knowledge, ocean and climate data, and informative runs of coupled ocean/atmosphere models, applying recently developed methodology for the analysis of large slow computer simulators. We will start with an intermediate-complexity model, C-GOLDSTEIN, using this as a stepping-stone to analysing a full climate GCM such as HADCM3.
Salinity is a major contributor to ocean circulation, stability and variability. Its structure depends on the surface freshwater flux whose distribution is likely to change significantly under global warming, giving rise to the potential for abrupt changes in climate. This project will investigate the mechanisms and feedback loops which govern the salinity distribution in coupled ocean-atmosphere models. Carefully designed experiments where feedbacks are removed will provide a major contribution to understand the role of salinity in the climate response to greenhouse gas forcing. Innovative diagnostics will be developed to assess the salinity/fresh water cycle performance of climate forecast models.
The aims of the proposal are to compare high-resolution isotope records from terrestrial archives in NW Europe with model simulations of isotopes in precipitation in order to investigate the role of different forcing factors in rapid climate change during the late glacial and Holocene and to undertake model validation. The proposal constitutes a UK contribution to the PAGES ISOMAP initiative. A water isotope model will be developed for the UK Hadley centre model HadCM3. Comparisons will be made between simulations of the isotopic composition of precipitation during periods of rapid climatic change and reconstructions from well-dated and well-calibrated palaeo-archives (lake sediments, peat and speleothem) generated in this study and obtained from the literature, in order to investigate the causes and nature of abrupt climatic events.
The aim of this research is to increase the understanding of the processes that control the distribution of evaporation and precipitation and their difference in the Atlantic region and in particular that the atmosphere gives a net export of water vapour from that basin. This export is vital to the existence and stability of the oceanic thermohaline circulation in the Atlantic. Contrast will be made with the Pacific Ocean region in which the atmosphere imports water. The research will be pursued through detailed analysis of the new ERA-40 data set, fundamental experimentation with an atmospheric GCM and analysis of data sets from climate change experiments.
The main aims of this proposal are to determine the role that surface forcing variability plays in causing rapid changes in the ocean circulation and to examine the effect of such changes on climate. We will address these issues through a combined analysis of coupled model output and observational datasets. The focus of the analysis will be the North Atlantic thermohaline circulation (THC) although the results will be interpreted in the broader context of the global climate system. Variations in the air-sea fluxes of surface heat and freshwater have the potential to cause rapid changes in the ocean circulation eg through their influence on deep convection. However, the relationship between surface forcing variability and rapid changes in the ocean remains to be properly determined; our goal is to significantly improve understanding of this area.
The objective is to use a combination of palaeoclimate reconstruction from annually-banded corals and the fully coupled HadCM3 atmosphere-ocean general circulation model to develop an understanding of the controls on variability in the strength and frequency of ENSO, and to improve our ability to predict the likelihood of future rapid changes in this important element of the climate system. To achieve this, we target three periods:0-2.5 ka: Representative of near-modern climate forcing; will reveal the internal variability in the system.6-9 ka: a period of weak or absent ENSO, and different orbital forcing; a test of the model's ability to capture externally-forced change in ENSO.200-2100 AD: by using the palaeo periods to test and optimise model parameterisation, we will produce a new, improved, prediction of ENSO variability in a warming world.See also project results page
We will investigate two aspects of the Nordic Seas circulation of importance to the North Atlantic meridional overturning circulation (MOC): (1) Sources of water in the Greenland-Scotland overflows: recent tracer release and transient tracer observations will be used to constrain inverse models of the sources of Denmark Straits and Faroe-Bank channel overflow waters. (2) The initiation of convection and its relation to submesoscale hydrodynamics: very high-resolution non-hydrostatic models for the Central Greenland Sea will be used to model recent observations, which show convection to be intimately related to local sub-mesoscale structure.: The objective will be to develop improved descriptions of convection for use in OGCMs, to more accurately describe how the sinking branch of the MOC will be affected by changes in forcing.
The role of sloping topography in controlling the overturning of the North Atlantic will be examined using a hierarchy of isopycnic model experiments with realistic topography for idealised and realistic forcing. The study will focus on how sloping topography affects where water masses are formed, the communication of overturning signals via wave propagation, as well as the transport, recirculation and evolution of dense, water masses. The study will provide a context to interpret monitoring signals from RAPID identifying how overturning signals are communicated from high to low latitudes along sloping western boundaries. A tied studentship will examine how the circulation of North Atlantic Deep Water alters in a glacial environment, the separate effect of forcing and sea level changes, and the large-scale consequences for atmospheric CO2 uptake.
The Barents Sea is an important site for the production of dense intermediate water. Up to one half of this intermediate water flows into the North Atlantic over the Scotland-Greenland Ridge, constituting an important branch of the global thermohaline circulation. The presence of numerous coastal polynyas and the relatively low river input into the Barents Sea explain why this region is a significant site for water for water mass transformation. Parameterisations for dense water production in polynyas for application in non-polynya resolving ocean circulation models, will be developed and tested in a coupled sea ice-shelf sea model of the Barents Sea. The latter will be used to study present day water mass transformation processes and to predict how they will change in a warmer climate.
Records of changing atmospheric radiocarbon concentration (d14C), reported as per mil deviations from pre-industrial values after correction for decay and fractionation) from the last deglaciation suggest that an anomaly during the Younger Dryas cold phase is the largest of the last 15,000 years. However, the cause of the Younger Dryas d14C changes are debated, and are either attributed to changes in the production rate of 14C due to changes in solar activity or the Earth's magnetic field and/or changes in the carbon cycle. The latter is strongly influenced by carbon exchange between the atmosphere and other reservoirs, such as the deep ocean. Since reorganization of the North Atlantic's thermohaline circulation is widely held to drive abrupt climate change a corresponding response in surface-ocean and atmospheric Dryas d14C is expected. We demonstrate a direct correlation between surface ocean ventilation (expressed here as a marine radiocarbon reservoir age) and changing atmospheric Dryas d14C reconstructed from the Cariaco Basin, in the southern Caribbean Sea. We are motivated to generate these high quality Dryas 14C data for the surface waters of the NE Atlantic during the last deglaciation because they provide a unique opportunity to study processes central to our understanding of long-term variability in ocean overturning rate. By focussing on a time interval (Younger Dryas) when climate transitions were both rapid and large-scale, we will utilize the quantitative data generated to critically test scenarios of overturning rate through a collaborative modelling study
Both the scientific press and the broadcast media have become interested in the conjecture that increased melting of polar ice in a globally-warming climate could cause severe regional cooling, effectively by 'capping' the cooling and sinking of warm waters at high latitudes. Projections by climate forecasters of future climate show that the 'Conveyor Belt', the northwards movement of warm waters in the Atlantic balanced by the southwards movement of this cooled, deep water, could slow down or stop, with drastic consequences particularly for north-west Europe, including the UK. Now much of the southgoing water comes from Arctic, and it flows over a shallow submarine ridge between Greenland, Iceland and Scotland before sinking to great depths as it travels south. It is a surprising fact that no-one really knows how this water is 'made' - meaning where and how the warm surface waters get converted to the cold southgoing deep waters. It is a very difficult question to answer with measurements because you really seem to need to know everything, everywhere, at all times, and we just cannot manage that. Therefore our project will use a very high-resolution numerical model (like a weather forecast model in the ocean) to solve the problem, and we will make sure that our model does the best possible job by comparing it with some special high-quality measurements of ocean circulation and sea ice. This will let us tell climate forecasters how to make these cooled waters correctly, so that they will do a better job with their forecasts
To make the best use of the historical research ship records as well as new observations from autonomous ocean profiling floats and special observing programs such as Rapid climate change, it is proposed to assimilate all of the available data from the past 40 years into a high quality ocean circulation model that can represent complete fields of ocean properties. In this way derived quantities such as the north-south mass and heat transports which are vital to understanding the oceans role in controlling climate, can also be determined. The project will also put into context the various timeseries of observations that have been compiled from local regions which suggest that important changes in ocean circulation and transports have been ongoing in the past decades. These timeseries will thus be put into a basin scale and global scale context of ongoing change. The program will determine the relationship between hydrographic signals in different parts of the ocean basins (particularly the N Atlantic). The program will also provide a method for assimilating data from the thermohaline monitoring arrays into an ocean model that could then be used as part of a coupled climate model for multi-annual climate prediction.
The major impact of climate change in the European region is likely to be through changes in the storms coming through from the North Atlantic storm-track and in the blocking high-pressure systems that can occur there. Changes in the latitude, frequency or intensity of storms would have implications in terms of flooding and wind damage as well as average precipitation. Blocking highs bring settled spells with little precipitation and temperatures that can be much above average in summer and below average in winter, sometimes with snow. Again changes in their position, frequency or intensity would have important impacts. Reduction in the strength of the thermohaline circulation in the North Atlantic could induce rapid climate change through its impact on the storm-track and blocking. Even smoothly increasing greenhouse gases could lead to rapid changes in the storm-track and blocking either through a reduced thermohaline circulation or a non-linear response. At present there is little confidence in the climate models' abilities to project such changes. In this project, new high-resolution atmospheric models, new analyses of the atmosphere since 1958 and new diagnostic techniques will be used to give such projections and an assessment of the confidence that can be had in them.
The proposal aims to identify observational signals associated with changes in the overturning circulation and heat transport in the North Atlantic, exploiting the development of the monitoring system put in place by international programmes and the UK RAPID monitoring initiative. Our study aims to analyse both existing historical data obtained over the last 50 years and the latest emerging observations. Our approach is to apply a forward circulation model with an accompanying adjoint for the North Atlantic in order to identify how signals in the data are formed by external forcing and ocean physics. The adjoint provides a highly effective method of identifying the sensitivity of model signals and optimising the model towards selected data signals, which will be used together to identify how signals in the data are controlled. The study will focus on several themes: identifying how data signals along the western boundary and ocean interior are controlled using the adjoint approach; identifying the role of geostrophic eddies in limiting the adjoint approach; mapping hydrographic changes along the margins of the Atlantic from historical data and identifying the propagation of coastal wave modes from altimetric data.
Forecasts of the future behaviour of the Atlantic Thermohaline Circulation (THC) are needed to inform policy on climate change. Such forecasts must be probabilistic taking into account the principal sources of uncertainty. It is not possible to sample exhaustively all sources of uncertainty because the number of degrees of freedom is too great. Consequently a future forecasting system will be reliant on strategies to identify those dimensions of uncertainty that are most important. This project will develop an objective methodology to identify the dominant sources of uncertainty in General Circulation Model predictions of the THC. Perturbations to oceanic initial conditions and climate model parameters that generate the most rapid change in the THC and related aspects of climate will be identified. These perturbations will be used to produce an early probabilistic forecast for the behaviour of the THC up to 2100. The results will also feed directly into the next generation of ensemble climate predictions being developed at the UK Hadley Centre.
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