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J. M. Murphy
and
J. F. B. Mitchell

Abstract

A high-resolution (2.75° lat × ° 3.75° long) coupled ocean-atmosphere model has been used to simulate the transient response of climate to a gradual increase in atmospheric carbon dioxide concentrations. Although the radiative forcing increases linearly, there is a delay of about 30 yr before the ocean warms appreciably. This “cold start” is, at least partly, an artifact of the experimental design. At the time of doubling (after 70 yr), the patterns of change are similar to those found in comparable studies of the equilibrium response, except in the high latitudes of the Southern Ocean and the North Atlantic, where the warming is considerably reduced. The mechanisms leading to this reduction are discussed. After two to three decades, the pattern of warming is well established. The warming over land is substantially larger than that over the sea, with a consequent lowering of surface pressure over the northern continents in summer. The patterns of changes in precipitation and soil moisture take longer to establish themselves, although locally there are consistent changes after the third decade.

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J. F. B. Mitchell
and
W. J. Ingram

Abstract

Changes in cloud distribution may provide a major feedback on climate change. General circulation model simulations show an upward shift of high cloud and a general reduction of free-tropospheric cloud when climate warms. The shift of high cloud seems due to an upward shift of the tropopause. It is argued that the reduction in relative humidity and cloud cover below can be attributed to the increased depth of vertical motions in the warmer climate, which in turn follows from the upward shift of atmospheric radiative cooling as specific humidifies increase. A diagnostic study of the response of a general circulation model is consistent with this mechanism.

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C. D. Hewitt
and
J. F. B. Mitchell

Abstract

A simulation of the climate for 6 kyr BP, using the Hadley Centre's atmospheric GCM with prescribed SSTs is described. The control simulation successfully reproduces the large-scale features of the present-day climate and has realistic atmospheric interannual variability. The anomaly simulation for 6 kyr BP produces a climate with an enhanced Northern Hemisphere seasonal cycle, and, in particular, a strengthened African-Asian summer monsoon. Integrated over the full annual cycle, the land surface of the southern Tropics dries while the northern Tropics get wetter, and the high northern latitudes also dry. The model simulates large regional interdecadal differences in the response at 6 kyr BP highlighting the need to allow for and account for variability on long, that is, at least decadal, timescales. The authors describe the consequences of part of the experimental design employed, whereby the SSTs for the 6 kyr BP simulation are the same as in the control as recommended by the Paleoclimate Modelling Intercomparison Project, in particular, the potential importance of ocean and sea ice feedbacks.

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C. A. Senior
and
J. F. B. Mitchell

Abstract

The importance of the representation of cloud in a general circulation model is investigated by utilizing four different parameterization schemes for layer cloud in a low-resolution version of the general circulation model at the Hadley Centre for Climate Prediction and Research at the United Kingdom Meteorological Office. The performance of each version of the model in terms of cloud and radiation is assessed in relation to satellite data from the Earth Radiation Budget Experiment (ERBE). Schemes that include a prognostic cloud water variable show some improvement on those with relative humidity-dependent cloud, but all still show marked differences from the ERBE data. The sensitivity of each of the versions of the model to a doubling of atmospheric C02 is investigated. Midlevel and lower-level clouds decrease when cloud is dependent on relative humidity, and this constitutes a strong positive feedback. When interactive cloud water is included, however, this effect is almost entirely compensated for by a negative feedback from the change of phase of cloud water from ice to water. Additional negative feedbacks are found when interactive radiative properties of cloud are included and these lead to an overall negative cloud feedback. The global warming produced with the four models then ranges from 5.4° with a relative humidity scheme to 1.9°C with interactive cloud water and radiative properties. Improving the treatment of ice cloud based on observations increases the model's sensitivity slightly to 2.1°C. Using an energy balance model, it is estimated that the climate sensitivity using the relative humidity scheme along with the negative feedback from cloud radiative properties would be 2.8°C. Thus, 2.8°–2.1°C appears to be a better estimate of the range of equilibrium response to a doubling Of C02.

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J. F. B. Mitchell
and
T. C. Johns

Abstract

There is increasing evidence that the response of climate to increasing greenhouse gases may be modified by accompanying increases in sulfate aerosols. In this study, the patterns of response in the surface climatology of a coupled ocean–atmosphere general circulation model forced by increases in carbon dioxide alone is compared with those obtained by increasing carbon dioxide and aerosol forcing. The simulations are run from early industrial times using the estimated historical forcing and continued to the end of the twenty-first century assuming a nonintervention emissions scenario for greenhouse gases and aerosols. The comparison is made for the period 2030–2050 when the aerosol forcing is a maximum. In winter, the cooling due to aerosols merely tends to reduce the response to carbon dioxide, whereas in summer, it weakens the monsoon circulations and reverses some of the changes in the hydrological cycle on increasing carbon dioxide. This response is in some respects similar to that found in simulations with changed orbital parameters, as between today and the middle Holocene. The hydrological response in the palaeosimulations is supported by palaeoclimatic reconstructions. The results of changes in aerosol concentrations of the magnitude projected in the scenarios would have a major effect on regional climate, especially over Europe and Southeast Asia.

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J. M. Gregory
,
J. F. B. Mitchell
, and
A. J. Brady

Abstract

A time-dependent climate-change experiment with a coupled ocean–atmosphere general circulation model has been used to study changes in the occurrence of drought in summer in southern Europe and central North America. In both regions, precipitation and soil moisture are reduced in a climate of greater atmospheric carbon dioxide. A detailed investigation of the hydrology of the model shows that the drying of the soil comes about through an increase in evaporation in winter and spring, caused by higher temperatures and reduced snow cover, and a decrease in the net input of water in summer. Evaporation is reduced in summer because of the drier soil, but the reduction in precipitation is larger. Three extreme statistics are used to define drought, namely the frequency of low summer precipitation, the occurrence of long dry spells, and the probability of dry soil. The last of these is arguably of the greatest practical importance, but since it is based on soil moisture, of which there are very few observations, the authors’ simulation of it has the least confidence. Furthermore, long time series for daily observed precipitation are not readily available from a sufficient number of stations to enable a thorough evaluation of the model simulation, especially for the frequency of long dry spells, and this increases the systematic uncertainty of the model predictions. All three drought statistics show marked increases owing to the sensitivity of extreme statistics to changes in their distributions. However, the greater likelihood of long dry spells is caused by a tendency in the character of daily rainfall toward fewer events, rather than by the reduction in mean precipitation. The results should not be taken as firm predictions because extreme statistics for small regions cannot be calculated reliably from the output of the current generation of GCMs, but they point to the possibility of large increases in the severity of drought conditions as a consequence of climate change caused by increased CO2.

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Hong Xing Cao
,
J. F. B. Mitchell
, and
J. R. Lavery

Abstract

The variability of surface temperature simulated by a global climate model with a simple mixed-layer ocean is analyzed. The simulated diurnal and seasonal ranges of temperature are compared with observation, as is the day-to-day and interannual variability of temperature. The qualitative changes in these quantities due to doubling atmospheric carbon dioxide concentration are also presented.

The simulation of the seasonal cycle of surface temperature has a cold bias in much of the extratropics, including central Europe, even allowing for the difficulties in comparing grid-box surface temperatures with station temperature at screen height. The simulated diurnal range of temperature for present-day climate is similar to that observed, though the diurnal cycle in the model in midlatitudes is generally less than observed. On doubling C02, the diurnal range over land decrease by 0.3°C whereas mean temperatures increase by 6.3°C (global average over land). In CO2-doubling experiments with a one-dimensional radiative-convective model, atmospheric absorption by carbon dioxide and water vapor increases, reducing the solar heating at the surface, and surface evaporation increases faster with temperature than the transfer of sensible heat (due to the Clausius-Clapyeron relation), both of which tend to reduce the diurnal cycle. However, in the three-dimensional model, the diurnal cycle increases substantially where the snow line recedes, where the land surface becomes drier, or where there are substantial decreases in cloud cover. The diurnal cycle of surface temperature decreases where sea ice is replaced by open water because of the increase in thermal inertia of the surface.

The simulated patterns of interannual standard deviation of surface temperature are in general aqreement with observations, except in high latitudes in winter, where the model values are larger than the observed, and over the tropical oceans where model values are smaller then observed. The changes on doubling CO2 are generally small and not statistically significant. There are, however, reductions along the sea-ice margins in winter and increases in some regions of northern midlatitudes in summer. On doubling C02, the general patterns of diurnal ranges and daily standard deviations of surface temperature change little, even though the changes in mean temperature am substantial and significant.

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K. D. Williams
,
C. A. Senior
, and
J. F. B. Mitchell

Abstract

A comparison of the response to increasing greenhouse gas concentrations of two versions of the Met Office's (Hadley Centre) coupled atmosphere–ocean model reveals differences that result in large local variations in the modeled impact of climate change. With the aim of understanding the important processes and feedbacks associated with climate change, and ultimately reducing uncertainty in predictions, a series of sensitivity experiments were performed using a coupled atmosphere–mixed layer ocean model. The primary differences in the atmospheric response of the coupled models studied are found to be due to changes made to the physical representation of the atmosphere rather than to the ocean. In particular, many of the different patterns of response can be explained through changes made to the boundary layer scheme combining in a nonlinear way with changes to the cloud scheme to alter the tropical temperature and precipitation response in the model. A new land surface exchange scheme largely accounts for the different Northern Hemisphere continental surface temperature response.

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J. F. B. Mitchell
,
R. A. Davis
,
W. J. Ingram
, and
C. A. Senior

Abstract

The effect of changes in atmospheric carbon dioxide concentrations and sulphate aerosols on near-surface temperature is investigated using a version of the Hadley Centre atmospheric model coupled to a mixed layer ocean. The scattering of sunlight by sulphate aerosols is represented by appropriately enhancing the surface albede.

On doubling atmospheric carbon dioxide concentrations, the global mean temperature increases by 5.2 K. An integration with a 39% increase in CO2, giving the estimated change in radiative heating due to increases in greenhouse gases since 1900, produced an equilibrium warming of 2.3 K, which, even allowing for oceanic inertia, is significantly higher than the observed warming over the same period. Furthermore, the simulation suggests a substantial warming everywhere, whereas the observations indicate isolated regions of cooling including parts of the northern midlatitude continents. The addition of an estimate of the effect of scattering by current industrial aerosols (uncertain by a factor of at least 3) leads to improved agreement with the observed pattern of changes over the northern continents and reduces the global mean warming by about 30%. Doubling the aerosol forcing produces patterns that are still compatible with the observations, but further increase leads to unrealistically extensive cooling in the midlatitudes.

The diurnal range of surface temperature decreases over most of the northern extratropics on increasing CO2, in agreement with recent observations. The addition of the current industrial aerosol had little detectable effect on the diurnal range in the model because the direct effect of reduced solar heating at the surface is approximately balanced by the indirect effects of cooling. Thus, the ratio of the reduction in diurnal range to the mean warming is increased, in closer agreement with observations.

Results from further sensitivity experiments with larger increases in aerosol and CO2 are presented. Although the aerosol forcing is a strong maximum in the northern midlatitudes in summer, the response is fairly even throughout the year because sea ice feedbacks amplify the cooling in winter. Increasing the aerosol loading produces a consistent increase in the globally averaged diurnal temperature range, associated with the mean reduction in temperature, though the diurnal range decreases slightly where the aerosol loading is greatest. The response to increased CO2 is compared with that in other models.

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C. D. Hewitt
,
A. J. Broccoli
,
M. Crucifix
,
J. M. Gregory
,
J. F. B. Mitchell
, and
R. J. Stouffer

Abstract

The commonly held view of the conditions in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the “strength of the overturning circulation” is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation.

Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors’ model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project’s reconstruction of glacial sea surface temperature and sea ice extent.

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