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Piers Mde F. Forster
and
Jonathan M. Gregory

Abstract

One of the major uncertainties in the ability to predict future climate change, and hence its impacts, is the lack of knowledge of the earth’s climate sensitivity. Here, data are combined from the 1985–96 Earth Radiation Budget Experiment (ERBE) with surface temperature change information and estimates of radiative forcing to diagnose the climate sensitivity. Importantly, the estimate is completely independent of climate model results. A climate feedback parameter of 2.3 ± 1.4 W m−2 K−1 is found. This corresponds to a 1.0–4.1-K range for the equilibrium warming due to a doubling of carbon dioxide (assuming Gaussian errors in observable parameters, which is approximately equivalent to a uniform “prior” in feedback parameter). The uncertainty range is due to a combination of the short time period for the analysis as well as uncertainties in the surface temperature time series and radiative forcing time series, mostly the former. Radiative forcings may not all be fully accounted for; however, an argument is presented that the estimate of climate sensitivity is still likely to be representative of longer-term climate change. The methodology can be used to 1) retrieve shortwave and longwave components of climate feedback and 2) suggest clear-sky and cloud feedback terms. There is preliminary evidence of a neutral or even negative longwave feedback in the observations, suggesting that current climate models may not be representing some processes correctly if they give a net positive longwave feedback.

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Timothy Andrews
,
Piers M. Forster
, and
Jonathan M. Gregory

Abstract

A surface forcing response framework is developed that enables an understanding of time-dependent climate change from a surface energy perspective. The framework allows the separation of fast responses that are unassociated with global-mean surface air temperature change (ΔT), which is included in the forcing, and slow feedbacks that scale with ΔT. The framework is illustrated primarily using 2 × CO2 climate model experiments and is robust across the models. For CO2 increases, the positive downward radiative component of forcing is smaller at the surface than at the tropopause, and so a rapid reduction in the upward surface latent heat (LH) flux is induced to conserve the tropospheric heat budget; this reduces the precipitation rate. Analysis of the time-dependent surface energy balance over sea and land separately reveals that land areas rapidly regain energy balance, and significant land surface warming occurs before global sea temperatures respond. The 2 × CO2 results are compared to a solar increase experiment and show that some fast responses are forcing dependent. In particular, a significant forcing from the fast hydrological response found in the CO2 experiments is much smaller in the solar experiment. The different fast response explains why previous equilibrium studies found differences in the hydrological sensitivity between these two forcings. On longer time scales, as ΔT increases, the net surface longwave and LH fluxes provide positive and negative surface feedbacks, respectively, while the net surface shortwave and sensible heat fluxes change little. It is found that in contrast to their fast responses, the longer-term response of both surface energy fluxes and the global hydrological cycle are similar for the different forcing agents.

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Buwen Dong
,
Jonathan M. Gregory
, and
Rowan T. Sutton

Abstract

Climate model simulations consistently show that surface temperature over land increases more rapidly than over sea in response to greenhouse gas forcing. The enhanced warming over land is not simply a transient effect caused by the land–sea contrast in heat capacities, since it is also present in equilibrium conditions. This paper elucidates the transient adjustment processes over time scales of days to weeks of the surface and tropospheric climate in response to a doubling of CO2 and to changes in sea surface temperature (SST), imposed separately and together, using ensembles of experiments with an atmospheric general circulation model. These adjustment processes can be grouped into three stages: immediate response of the troposphere and surface processes (day 1), fast adjustment of surface processes (days 2–5), and adjustment of the whole troposphere (days 6–20).

Some land surface warming in response to doubled CO2 (with unchanged SSTs) occurs immediately because of increased downward longwave radiation. Increased CO2 also leads to reduced plant stomatal resistance and hence restricted evaporation, which increases land surface warming in the first day. Rapid reductions in cloud amount lead in the next few days to increased downward shortwave radiation and further warming, which spreads upward from the surface, and by day 5 the surface and tropospheric response is statistically consistent with the equilibrium value. Land surface warming in response to imposed SST change (with unchanged CO2) is slower. Tropospheric warming is advected inland from the sea, and over land it occurs at all levels together rather than spreading upward from the surface. The atmospheric response to prescribed SST change in about 20 days is statistically consistent with the equilibrium value, and the warming is largest in the upper troposphere over both land and sea. The land surface warming involves reduction of cloud cover and increased downward shortwave radiation, as in the experiment with CO2 change, but in this case it is due to the restriction of moisture supply to the land (indicated by reduced soil moisture), whereas in the CO2 forcing experiment it is due to restricted evaporation despite increased moisture supply (indicated by increased soil moisture). The warming over land in response to SST change is greater than over the sea and is the dominant contribution to the land–sea warming contrast under enhanced CO2 forcing.

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Paulo Ceppi
,
Giuseppe Zappa
,
Theodore G. Shepherd
, and
Jonathan M. Gregory

Abstract

Poleward shifts of the extratropical atmospheric circulation are a common response to CO2 forcing in global climate models (GCMs), but little is known about the time dependence of this response. Here it is shown that in coupled climate models, the long-term evolution of sea surface temperatures (SSTs) induces two distinct time scales of circulation response to steplike CO2 forcing. In most GCMs from phase 5 of the Coupled Model Intercomparison Project as well as in the multimodel mean, all of the poleward shift of the midlatitude jets and Hadley cell edge occurs in a fast response within 5–10 years of the forcing, during which less than half of the expected equilibrium warming is realized. Compared with this fast response, the slow response over subsequent decades to centuries features stronger polar amplification (especially in the Antarctic), enhanced warming in the Southern Ocean, an El Niño–like pattern of tropical Pacific warming, and weaker land–sea contrast. Atmosphere-only GCM experiments demonstrate that the SST evolution drives the difference between the fast and slow circulation responses, although the direct radiative effect of CO2 also contributes to the fast response. It is further shown that the fast and slow responses determine the long-term evolution of the circulation response to warming in the representative concentration pathway 4.5 (RCP4.5) scenario. The results imply that shifts in midlatitude circulation generally scale with the radiative forcing, rather than with global-mean temperature change. A corollary is that time slices taken from a transient simulation at a given level of warming will considerably overestimate the extratropical circulation response in a stabilized climate.

Open access
Timothy Andrews
,
Jonathan M. Gregory
, and
Mark J. Webb

Abstract

Experiments with CO2 instantaneously quadrupled and then held constant are used to show that the relationship between the global-mean net heat input to the climate system and the global-mean surface air temperature change is nonlinear in phase 5 of the Coupled Model Intercomparison Project (CMIP5) atmosphere–ocean general circulation models (AOGCMs). The nonlinearity is shown to arise from a change in strength of climate feedbacks driven by an evolving pattern of surface warming. In 23 out of the 27 AOGCMs examined, the climate feedback parameter becomes significantly (95% confidence) less negative (i.e., the effective climate sensitivity increases) as time passes. Cloud feedback parameters show the largest changes. In the AOGCM mean, approximately 60% of the change in feedback parameter comes from the tropics (30°N–30°S). An important region involved is the tropical Pacific, where the surface warming intensifies in the east after a few decades. The dependence of climate feedbacks on an evolving pattern of surface warming is confirmed using the HadGEM2 and HadCM3 atmosphere GCMs (AGCMs). With monthly evolving sea surface temperatures and sea ice prescribed from its AOGCM counterpart, each AGCM reproduces the time-varying feedbacks, but when a fixed pattern of warming is prescribed the radiative response is linear with global temperature change or nearly so. It is also demonstrated that the regression and fixed-SST methods for evaluating effective radiative forcing are in principle different, because rapid SST adjustment when CO2 is changed can produce a pattern of surface temperature change with zero global mean but nonzero change in net radiation at the top of the atmosphere (~−0.5 W m−2 in HadCM3).

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Oleg A. Saenko
,
Duo Yang
, and
Jonathan M. Gregory

Abstract

Using a set of experiments with an eddy-parameterizing ocean model, it is found that the strength of the Atlantic meridional overturning circulation (AMOC) intensifies with the decrease of the density-dependent mesoscale eddy transfer. However, the intensification is weaker than that suggested by simple scaling relationships previously applied. Perturbing the model control sea surface temperature (SST) to mimic its change in response to doubling of CO2, it is shown that the associated ocean heat uptake (OHU) increases and penetrates deeper with the decrease of the mesoscale eddy transfer. It is shown that the OHU correlates with the AMOC strength, and both these quantities are affected by the mesoscale eddy transfer. Passive tracer experiments in the ocean model provide a possible explanation for the finding in coupled-model climate simulations that the ocean heat uptake efficiency (OHUE) increases with the AMOC strength and decreases with the eddy energy generated from the mean state. It is also found that the OHU in the SST-perturbation experiments scales with the net downward advection of heat. The contribution of the AMOC to the downward heat flux is illustrated using a streamfunction in depth–temperature space.

Open access
Eleftheria Exarchou
,
Till Kuhlbrodt
,
Jonathan M. Gregory
, and
Robin S. Smith

Abstract

The quasi-equilibrium heat balances, as well as the responses to 4 × CO2 perturbation, are compared among three global climate models with the aim to identify and explain intermodel differences in ocean heat uptake (OHU) processes. It is found that, in quasi equilibrium, convective and mixed layer processes, as well as eddy-related processes, cause cooling of the subsurface ocean. The cooling is balanced by warming caused by advective and diapycnally diffusive processes. It is also found that in the CO2-perturbed climates the largest contribution to OHU comes from changes in vertical mixing processes and the mean circulation, particularly in the extratropics, caused both by changes in wind forcing and by changes in high-latitude buoyancy forcing. There is a substantial warming in the tropics: a significant part of which occurs because of changes in horizontal advection in extratropics. Diapycnal diffusion makes only a weak contribution to the OHU, mainly in the tropics, because of increased stratification. There are important qualitative differences in the contribution of eddy-induced advection and isopycnal diffusion to the OHU among the models. The former is related to the different values of the coefficients used in the corresponding scheme. The latter is related to the different tapering formulations of the isopycnal diffusion scheme. These differences affect the OHU in the deep ocean, which is substantial in two of the models, with the dominant region of deep warming being the Southern Ocean. However, most of the OHU takes place above 2000 m, and the three models are quantitatively similar in their global OHU efficiency and its breakdown among processes and as a function of latitude.

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Willem P. Sijp
,
Matthew H. England
, and
Jonathan M. Gregory

Abstract

This study examines criteria for the existence of two stable states of the Atlantic Meridional Overturning Circulation (AMOC) using a combination of theory and simulations from a numerical coupled atmosphere–ocean climate model. By formulating a simple collection of state parameters and their relationships, the authors reconstruct the North Atlantic Deep Water (NADW) OFF state behavior under a varying external salt-flux forcing. This part (Part I) of the paper examines the steady-state solution, which gives insight into the mechanisms that sustain the NADW OFF state in this coupled model; Part II deals with the transient behavior predicted by the evolution equation. The nonlinear behavior of the Antarctic Intermediate Water (AAIW) reverse cell is critical to the OFF state. Higher Atlantic salinity leads both to a reduced AAIW reverse cell and to a greater vertical salinity gradient in the South Atlantic. The former tends to reduce Atlantic salt export to the Southern Ocean, while the latter tends to increases it. These competing effects produce a nonlinear response of Atlantic salinity and salt export to salt forcing, and the existence of maxima in these quantities. Thus the authors obtain a natural and accurate analytical saddle-node condition for the maximal surface salt flux for which a NADW OFF state exists. By contrast, the bistability indicator proposed by De Vries and Weber does not generally work in this model. It is applicable only when the effect of the AAIW reverse cell on the Atlantic salt budget is weak.

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Michael Vellinga
,
Richard A. Wood
, and
Jonathan M. Gregory

Abstract

In an experiment with the latest version of the Hadley Centre climate model the model response has been analyzed after the thermohaline circulation (THC) in the Atlantic Ocean has been suppressed. The suppression is induced by a strong initial perturbation to the salinity distribution in the upper layer of the northern North Atlantic. The model is then allowed to adjust freely. Salinity gradually increases and deep water formation in the Greenland and Norwegian Seas restarts, later also in the Labrador Sea. The meridional overturning recovers after about 120 yr. In the first few decades when the overturning is very weak surface air temperature is dominated by cooling of much of the Northern Hemisphere and weak warming of the Southern Hemisphere, leading to maximum global cooling of 0.9°C. The disruption to the atmosphere's radiation balance results in a downward flux anomaly at the top of the atmosphere, maximally 0.55 W m–2 in the first decade then decreasing with the THC recovery.

The processes responsible for the recovery of the THC is examined in detail. In future model development this will help to reduce uncertainty in modeling THC stability. The recovery is driven by coupled ocean–atmosphere response. Northward salt transport by the subtropical gyre is crucial to the recovery of salinity in the North Atlantic. A southward shift of the ITCZ creates positive salinity anomalies in the tropical North Atlantic. This supports the northward salt transport by the subtropical gyre that helps to restart deep water formation and the THC.

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Sarah C. B. Raper
,
Jonathan M. Gregory
, and
Ronald J. Stouffer

Abstract

The role of climate sensitivity and ocean heat uptake in determining the range of climate model response is investigated in the second phase of the Coupled Model Intercomparison Project (CMIP2) AOGCM results. The fraction of equilibrium warming that is realized at any one time is less in those models with higher climate sensitivity, leading to a reduction in the temperature response range at the time of CO2 doubling [transient climate response (TCR) range]. The range is reduced by a further 15% because of an apparent relationship between climate sensitivity and the efficiency of ocean heat uptake. Some possible physical causes for this relationship are suggested.

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