Publications by Myles Allen

The influence of subseasonal wind variability on Tropical Instability Waves in the Pacific


RE Benestad, RT Sutton, MR Allen, DLT Anderson

Constraining climate model properties using optimal fingerprint detection methods

Climate Dynamics 18 (2001) 277-295

CE Forest, MR Allen, MR Allen, AP Sokolov, PH Stone

We present a method for constraining key properties of the climate system that are important for climate prediction (climate sensitivity and rate of heat penetration into the deep ocean) by comparing a model's response to known forcings over the twentieth century against climate observations for that period. We use the MIT 2D climate model in conjunction with results from the Hadley Centre's coupled atmosphere-ocean general circulation model (AOGCM) to determine these constraints. The MIT 2D model, which is a zonally averaged version of a 3D GCM, can accurately reproduce the global-mean transient response of coupled AOGCMs through appropriate choices of the climate sensitivity and the effective rate of diffusion of heat anomalies into the deep ocean. Vertical patterns of zonal mean temperature change through the troposphere and lower stratosphere also compare favorably with those generated by 3-D GCMs. We compare the height-latitude pattern of temperature changes as simulated by the MIT 2D model with observed changes, using optimal finger-print detection statistics. Using a linear regression model as in Allen and Tett this approach yields an objective measure of model-observation goodness-of-fit (via the residual sum of squares weighted by differences expected due to internal variability). The MIT model permits one to systematically vary the model's climate sensitivity (by varying the strength of the cloud feedback) and rate of mixing of heat into the deep ocean and determine how the goodness-of-fit with observations depends on these factors. This provides an efficient framework for interpreting detection and attribution results in physical terms. With aerosol forcing set in the middle of the IPCC range, two sets of model parameters are rejected as being implausible when the model response is compared with observations. The first set corresponds to high climate sensitivity and slow heat uptake by the deep ocean. The second set corresponds to low sensitivities for all magnitudes of heat uptake. These results demonstrate that fingerprint patterns must be carefully chosen, if their detection is to reduce the uncertainty of physically important model parameters which affect projections of climate change.

Identifying signals from intermittent low-frequency behaving systems


A Hannachi, MR Allen

Constraining uncertainties in climate models using climate change detection techniques

Geophysical Research Letters 27 (2000) 569-572

CE Forest, MR Allen, MR Allen, PH Stone, AP Sokolov

Predictions of 21(st) century climate by different atmosphere-ocean general circulation models depend on the sensitivities of the models to external radiative forcing and on their rates of heat uptake by the deep ocean. This study constrains these properties by comparing radiosonde-based observations of temperature trends in the free troposphere and lower stratosphere with corresponding simulations of a fast, flexible climate model, using objective techniques based on optimal fingerprinting. Parameter choices corresponding either to low sensitivity, or to high sensitivity combined with slow oceanic heat uptake are rejected provided the variability estimates used from the HadCM2 control run are correct. Nevertheless, the range of acceptable values is significantly wider than that usually quoted. The IPCC's range of possible sensitivities, 1.5 to 4.5 K, corresponds at best to only an 80% confidence interval. Therefore, climate change projections based on current general circulation models do not span the range of possibilities consistent with the recent climate record.

Optimal detection and attribution of climate change: sensitivity of results to climate model differences

CLIMATE DYNAMICS 16 (2000) 737-754

GC Hegerl, PA Stott, MR Allen, JFB Mitchell, SFB Tett, U Cubasch

Direct observations of skin-bulk SST variability

Geophysical Research Letters 27 (2000) 1171-1174

MJ Murray, MR Allen, MR Allen, CJ Merchant, AR Harris, CJ Donlon

Skin sea-surface temperatures from the first Along Track Scanning Radiometer (ATSR) are compared with coincident bulk temperatures from the Tropical Atmosphere Ocean (TAO) moored buoy array in the equatorial Pacific Ocean. The response of the skin-bulk sea-surface temperature difference (ΔT) to variations in wind speed and surface heat flux is examined. The use of remotely-sensed skin temperatures for this purpose is enabled by ATSR's unique design which permits the independent retrieval of ocean skin temperature to an accuracy of 0.3 K. For the four-year period considered (August 1991-August 1995), almost 6000 coincident skin and bulk sea surface temperature (SST) measurements were available; at night, the mean value of ΔT is -0.20 ± 0.46K, with a daytime mean value of +0.05 ± 0.51K. ΔT is found to depend on both net heat flux and local wind speed as predicted by the Saunders [1967] model and other formulations, and an estimate of the Saunders λ parameter is obtained.

External control of 20th century temperature by natural and anthropogenic forcings

SCIENCE 290 (2000) 2133-2137

PA Stott, SFB Tett, GS Jones, MR Allen, JFB Mitchell, GJ Jenkins

Sensitivity analysis of the climate of a chaotic system

Tellus, Series A: Dynamic Meteorology and Oceanography 52 (2000) 523-532

DJ Lea, MR Allen, TWN Haine

This paper addresses some fundamental methodological issues concerning the sensitivity analysis of chaotic geophysical systems. We show, using the Lorenz system as an example, that a naive approach to variational ('adjoint') sensitivity analysis is of limited utility. Applied to trajectories which are long relative to the predictability time scales of the system, cumulative error growth means that adjoint results diverge exponentially from the 'macroscopic climate sensitivity' (that is, the sensitivity of time-averaged properties of the system to finite-amplitude perturbations). This problem occurs even for time-averaged quantities and given infinite computing resources. Alternatively, applied to very short trajectories, the adjoint provides an incorrect estimate of the sensitivity, even if averaged over large numbers of initial conditions, because a finite time scale is required for the model climate to respond fully to certain perturbations. In the Lorenz (1963) system, an intermediate time scale is found on which an ensemble of adjoint gradients can give a reasonably accurate (O(10%)) estimate of the macroscopic climate sensitivity. While this ensemble-adjoint approach is unlikely to be reliable for more complex systems, it may provide useful guidance in identifying important parameter-combinations to be explored further through direct finite-amplitude perturbations.

Implications of changes in the Northern Hemisphere circulation for the detection of anthropogenic climate change

Geophysical Research Letters 27 (2000) 993-996

NP Gillett, GC Hegerl, MR Allen, PA Stott

The first principal component of Northern Hemisphere sea level pressure, known as the Arctic Oscillation (AO) index, has increased significantly in recent winters, and this change is associated with ~30% of Northern Hemisphere January-March warming. We examine the AO in a model used to detect anthropogenic influence on climate, and find that it exhibits no systematic trend in response to greenhouse gas, sulphate aerosol, or ozone forcing. To test the significance of this discrepancy for anthropogenic climate change detection, we include the spatio-temporal pattern of temperature change associated with the observed AO in the set of forcing-response 'fingerprints' used to account for observed changes, thus separating temperature change associated with the AO from a residual. We find that the detection of a global response to both anthropogenic greenhouse gases and sulphate aerosols is robust to this exclusion of AO-related warming.

How predictability depends on the nature of uncertainty in initial conditions in a coupled model of ENSO

JOURNAL OF CLIMATE 13 (2000) 3298-3313

Y Fan, MR Allen, DLT Anderson, MA Balmaseda

Modelled and observed variability in atmospheric vertical temperature structure

Climate Dynamics 16 (2000) 49-61

NP Gillett, MR Allen, SFB Tett

Realistic simulation of the internal variability of the climate system is important both for climate change detection and as an indicator of whether the physics of the climate system is well-represented in a climate model. In this work zonal mean atmospheric temperatures from a control run of the second Hadley Centre coupled GCM are compared with gridded radiosonde observations for the past 38 years to examine how well modelled and observed variability agree. On time scales of between six months and twenty years, simulated and observed variability of global mean temperatures agree well for the troposphere, but in the equatorial stratosphere variability is lower in the model than in the observations, particularly at periods of two years and seven to twenty years. We find good agreement between modelled and observed variability in the mass-weighted amplitude of a forcing-response pattern, as used for climate change detection, but variability in a signal-to-noise optimised fingerprint pattern is significantly greater in the observations than in a model control run. This discrepancy is marginally consistent with anthropogenic forcing, but more clearly explained by a combination of solar and volcanic forcing, suggesting these should be considered in future 'vertical detection' studies. When the relationship between tropical lapse rate and mean temperature was examined, it was found that these quantities are unrealistically coherent in the model at periods above three years. However, there is a clear negative lapse rate feedback in both model and observations: as the tropical troposphere warms, the mid-tropospheric lapse rate decreases on all the time scales considered.

The atmospheric response over the North Atlantic to decadal changes in sea surface temperature

Journal of Climate 12 (1999) 2562-2584

S Venzke, S Venzke, MR Allen, MR Allen, RT Sutton, RT Sutton, DP Rowell

Decadal fluctuations in the climate of the North Atlantic-European region may be influenced by interactions between the atmosphere and the Atlantic Ocean, possibly as part of a coupled ocean-atmosphere mode of variability. For such a mode to exist, a consistent atmospheric response to fluctuations in North Atlantic sea surface temperatures (SST) is required. Furthermore, this response must provide feedbacks to the ocean. Whether a consistent response exists, and whether it yields the required feedbacks, are issues that remain controversial. Here, these issues are addressed using a novel approach to analyze an ensemble of six integrations of the Hadley Centre atmospheric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949-93. Characterizing the forced atmospheric response is complicated by the presence of internal variability. A generalization of principal component analysis is used to estimate the common forced response given the knowledge of internal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Nino-Southern Oscillation and a further response related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circulation changes over the entire region, including a dipole pattern in sea level pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substantial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigated. Several potential feedbacks are identified but it has to be concluded that, because of their marginal consistency between ensemble members, a coupled mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability. Notwithstanding this conclusion, the authors' results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the higher latitude westerlies.

A comparison of competing explanations for the 100,000-yr ice age cycle

Geophysical Research Letters 26 (1999) 2259-2262

GH Roe, GH Roe, MR Allen, MR Allen, MR Allen

There currently exists no consensus as to the cause of the ice ages of the late Pleistocene. Many of the competing hypotheses have been formulated into mathematical models which enables a rigorous comparison to be made. Spectral analysis fails to distinguish rival models. Using regression analysis, we examine the relative performance of several models, each representative of a different type of modeling approach, as explanations of both the global ice volume and also the time rate of change of the global ice volume. We find there is no objective evidence in the record in favor of any particular model. The respective merits of the different theories must therefore be judged on physical grounds. Copyright 1999 by the American Geophysical Union.

Checking for model consistency in optimal fingerprinting

CLIMATE DYNAMICS 15 (1999) 419-434

MR Allen, SFB Tett

Causes of twentieth-century temperature change near the Earth's surface

NATURE 399 (1999) 569-572

SFB Tett, PA Stott, MR Allen, WJ Ingram, JFB Mitchell

Effects of subsurface ocean dynamics on instability waves in the tropical Pacific


SP Lawrence, MR Allen, DLT Anderson, DT Llewellyn-Jones

Actual and potential information in dual-view radiometric observations of sea surface temperature from ATSR

Journal of Geophysical Research C: Oceans 103 (1998) 8153-8165

MJ Murray, MR Allen, MR Allen, CT Mutlow, AM Závody, MS Jones, TN Forrester

The along-track scanning radiometer (ATSR) on ERS-1 has delivered a continuous global record of radiometric (skin) sea surface temperature (SST) since August 1991. We present a comprehensive analysis of the large-scale and low-frequency characteristics of the data set using direct comparison with other global SST analyses to develop a quantitative understanding of the various factors contributing to the accuracy of and sources of bias in the first 4 years of the ATSR SST record. Such a global analysis is a necessary complement to direct validation against in situ observations, since large-scale sources of bias may be indistinguishable from instrument noise or sampling uncertainty in individual validation campaigns. No large-scale features attributable to atmospheric contamination through aerosols or water vapor are discernible in the difference between the three-channel, dual-view ATSR SSTs and the National Oceanic and Atmospheric Administration (NOAA) blended analysis of advanced very high resolution radiometer (AVHRR) and in situ SST observations. Features in the difference field can be traced to known deficiencies in the data used in the NOAA operational analysis or tentatively related to skin-bulk temperature differences in certain regions. Atmospheric contamination is, however, evident in the ATSR two-channel SST retrieval (used in daytime and after the failure of the 3.7 /zm channel) compared to the NOAA operational analysis. The Pinatubo aerosol plume is the dominant feature of the difference field in the first year of ATSR operation. In all cases, however, the amplitude of the atmospheric signature is significantly lower in dual-view than in corresponding single-view ATSR SSTs, indicating that the potential remains for unbiased two-channel SST retrieval even in the presence of aerosol. Copyright 1998 by the American Geophysical Union.

Potential for improved ATSR dual-view SST retrieval

Geophysical Research Letters 25 (1998) 3363-3366

MJ Murray, MJ Murray, MR Allen, MR Allen, MR Allen, CJ Merchant, CJ Merchant, AR Harris, AR Harris

Recent validation studies have confirmed that the first along-track scanning radiometer (ATSR) can retrieve sea surface temperature (SST) to an accuracy of 0.3K even in the presence of heavy atmospheric aerosol. However, using the standard (pre-launch) retrieval, this accuracy is achieved only when data from all three thermal channels (3.7, 11 and 12 μm) are available; in the absence of 3.7 μm data, retrieved SSTs are subject to significant cold bias. As 3.7 μm data are useful only for nighttime observations, and ATSR's 3.7 μm channel failed in May 1992, only 11 and 12 μm data informed SST derivation for most of the 1991 - 1996 mission. We demonstrate the potential for improvement in this retrieval, based on comparison of observed brigthness temperatures with precise SSTs derived using 3.7 μm data. A reduction in global-mean cold bias from >0.6K to <0.1K is achieved, with standard deviation approximately halved. We also examine the treatment of optical pathlength variation around the ATSR swath. | Recent validation studies have confirmed that the first along-track scanning radiometer (ATSR) can retrieve sea surface temperature (SST) to an accuracy of 0.3 K even in the presence of heavy atmospheric aerosol. However, using the standard (pre-launch) retrieval, this accuracy is achieved only when data from all three thermal channels (3.7, 11 and 12 μm) are available; in the absence of 3.7 μm data, retrieved SSTs are subject to significant cold bias. As 3.7 μm data are useful only for nighttime observations, and ATSR's 3.7 μm channel failed in May 1992, only 11 and 12 μm data informed SST derivation for most of the 1991-1996 mission. We demonstrate the potential for improvement in this retrieval, based on comparison of observed brightness temperatures with precise SSTs derived using 3.7 μm data. A reduction in global-mean cold bias from >0.6 K to <0.1 K is achieved, with standard deviation approximately halved. We also examine the treatment of optical pathlength variation around the ATSR swath.

Decadal predictability of North Atlantic sea surface temperature and climate

Nature 388 (1997) 563-567

RT Sutton, RT Sutton, MR Allen

The weather at middle latitudes is largely unpredictable more than a week or so in advance, whereas fluctuations in the ocean may be predictable over much longer timescales. If decadal fluctuations in North Atlantic sea surface temperature could be predicted, it might be possible to exploit their influence on the atmosphere to forecast decadal fluctuations in climate. Here we report analyses of shipboard observations that indicate significant decadal predictability of North Atlantic sea surface temperature, arising from the advective propagation of sea-surface-temperature anomalies and the existence of a regular period of 12-14 years in the propagating signals. The same timescale can be identified in a dipole-like pattern of North Atlantic sea-level pressure variability. We propose a mechanism which may connect these oceanic and atmospheric fluctuations, possibly as part of a coupled ocean-atmosphere mode of variability. Our results are encouraging for the prospects of forecasting natural fluctuations in the climate of the North Atlantic region several years in advance.

Optimal filtering in singular spectrum analysis

Physics Letters, Section A: General, Atomic and Solid State Physics 234 (1997) 419-428

MR Allen, MR Allen, LA Smith

Singular spectrum analysis (SSA) provides a robust method of separating an arbitrary signal from "white" (independent, identically distributed) noise. In the presence of "coloured" noise, or any autocorrelated process, high-variance components of the noise can confuse the singular value decomposition, thereby obscuring genuine signals which are, in principle, detectable. A generalization of SSA is presented which yields both an optimal filter to discriminate against an arbitrary coloured noise and an objective method of quantifying uncertainty in signal reconstruction. The algorithm is applied to a simple synthetic signal-separation problem and used to resolve a degeneracy in the SSA of interannual and interdecadal variability of the Earth's global mean temperature. © 1997 Elsevier Science B.V.