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Richard G. Williams
,
Vassil Roussenov
,
M. Susan Lozier
, and
Doug Smith

Abstract

In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual-to-decadal time scales, which are associated with changes in both sea surface temperature and thermocline thickness; the subtropics are often warm with a thick thermocline when the subpolar gyre is cool with a thin thermocline, and vice versa. This climate variability is investigated using a semidiagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealized isopycnic model experiments. On time scales of typically 5 yr, the tendencies in upper-ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre but instead dominated by heat convergences associated with the meridional overturning circulation. In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing Labrador Sea density leads to an enhanced overturning and an influx of subtropical heat. Thus, upper-ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.

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Richard G. Williams
,
Vassil Roussenov
,
Doug Smith
, and
M. Susan Lozier

Abstract

Basin-scale thermal anomalies in the North Atlantic, extending to depths of 1–2 km, are more pronounced than the background warming over the last 60 years. A dynamical analysis based on reanalyses of historical data from 1965 to 2000 suggests that these thermal anomalies are formed by ocean heat convergences, augmented by the poorly known air–sea fluxes. The heat convergence is separated into contributions from the horizontal circulation and the meridional overturning circulation (MOC), the latter further separated into Ekman and MOC transport minus Ekman transport (MOC-Ekman) cells. The subtropical thermal anomalies are mainly controlled by wind-induced changes in the Ekman heat convergence, while the subpolar thermal anomalies are controlled by the MOC-Ekman heat convergence; the horizontal heat convergence is generally weaker, only becoming significant within the subpolar gyre. These thermal anomalies often have an opposing sign between the subtropical and subpolar gyres, associated with opposing changes in the meridional volume transport driving the Ekman and MOC-Ekman heat convergences. These changes in gyre-scale convergences in heat transport are probably induced by the winds, as they correlate with the zonal wind stress at gyre boundaries.

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Helen M. Hanlon
,
Gabriele C. Hegerl
,
Simon F. B. Tett
, and
Doug M. Smith

Abstract

Daily maximum and minimum summer temperatures have increased throughout the majority of Europe over the past few decades, along with the frequency and intensity of heat waves. It is essential to learn whether this rise is expected to continue in the future for adaptation purposes. A study of predictability of European temperature indices with the Met Office Hadley Centre Decadal Prediction System (DePreSys) has revealed significant skill in predictions of 5- and 10-yr average indices of the summer mean and maximum 5-day average temperatures based on daily maximum and minimum temperatures for a large area of Europe, particularly in the Mediterranean. In contrast, the decadal forecasts of winter mean/minimum 5-day average temperature indices show poorer skill than the summer indices. Significant skill is shown for the United Kingdom in some cases but less than for the European/Mediterranean regions.

Comparison of two parallel ensembles, one initialized with observations and one without initialization, has shown that the skill largely originates from external forcing. However, there were a few cases with hints of additional skill in forecasts of decadal mean indices due to the initialization.

Model realizations of extreme indices can have large biases compared to observations that are different from those of the mean climate indices. Several methods were tested for correcting biases, as well as for testing the significance and quantifying uncertainty of the results to rule out cases of spurious skill. Bias correction of each index individually is required as biases vary across different extremes.

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Doug M. Smith
,
Nick J. Dunstone
,
Adam A. Scaife
,
Emma K. Fiedler
,
Dan Copsey
, and
Steven C. Hardiman
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Doug M. Smith
,
Nick J. Dunstone
,
Adam A. Scaife
,
Emma K. Fiedler
,
Dan Copsey
, and
Steven C. Hardiman

Abstract

The atmospheric response to Arctic and Antarctic sea ice changes typical of the present day and coming decades is investigated using the Hadley Centre global climate model (HadGEM3). The response is diagnosed from ensemble simulations of the period 1979 to 2009 with observed and perturbed sea ice concentrations. The experimental design allows the impacts of ocean–atmosphere coupling and the background atmospheric state to be assessed. The modeled response can be very different to that inferred from statistical relationships, showing that the response cannot be easily diagnosed from observations. Reduced Arctic sea ice drives a local low pressure response in boreal summer and autumn. Increased Antarctic sea ice drives a poleward shift of the Southern Hemisphere midlatitude jet, especially in the cold season. Coupling enables surface temperature responses to spread to the ocean, amplifying the atmospheric response and revealing additional impacts including warming of the North Atlantic in response to reduced Arctic sea ice, with a northward shift of the Atlantic intertropical convergence zone and increased Sahel rainfall. The background state controls the sign of the North Atlantic Oscillation (NAO) response via the refraction of planetary waves. This could help to resolve differences in previous studies, and potentially provides an “emergent constraint” to narrow the uncertainties in the NAO response, highlighting the need for future multimodel coordinated experiments.

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Doug M. Smith
,
Nick J. Dunstone
,
Rosie Eade
,
David Fereday
,
Leon Hermanson
,
James M. Murphy
,
Holger Pohlmann
,
Niall Robinson
, and
Adam A. Scaife
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Amy Solomon
,
Lisa Goddard
,
Arun Kumar
,
James Carton
,
Clara Deser
,
Ichiro Fukumori
,
Arthur M. Greene
,
Gabriele Hegerl
,
Ben Kirtman
,
Yochanan Kushnir
,
Matthew Newman
,
Doug Smith
,
Dan Vimont
,
Tom Delworth
,
Gerald A. Meehl
, and
Timothy Stockdale

Abstract

Given that over the course of the next 10–30 years the magnitude of natural decadal variations may rival that of anthropogenically forced climate change on regional scales, it is envisioned that initialized decadal predictions will provide important information for climate-related management and adaptation decisions. Such predictions are presently one of the grand challenges for the climate community. This requires identifying those physical phenomena—and their model equivalents—that may provide additional predictability on decadal time scales, including an assessment of the physical processes through which anthropogenic forcing may interact with or project upon natural variability. Such a physical framework is necessary to provide a consistent assessment (and insight into potential improvement) of the decadal prediction experiments planned to be assessed as part of the IPCC's Fifth Assessment Report.

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Jeff R. Knight
,
Martin B. Andrews
,
Doug M. Smith
,
Alberto Arribas
,
Andrew W. Colman
,
Nick J. Dunstone
,
Rosie Eade
,
Leon Hermanson
,
Craig MacLachlan
,
K. Andrew Peterson
,
Adam A. Scaife
, and
Andrew Williams

Abstract

Decadal climate predictions are now established as a source of information on future climate alongside longer-term climate projections. This information has the potential to provide key evidence for decisions on climate change adaptation, especially at regional scales. Its importance implies that following the creation of an initial generation of decadal prediction systems, a process of continual development is needed to produce successive versions with better predictive skill. Here, a new version of the Met Office Hadley Centre Decadal Prediction System (DePreSys 2) is introduced, which builds upon the success of the original DePreSys. DePreSys 2 benefits from inclusion of a newer and more realistic climate model, the Hadley Centre Global Environmental Model version 3 (HadGEM3), but shares a very similar approach to initialization with its predecessor. By performing a large suite of reforecasts, it is shown that DePreSys 2 offers improved skill in predicting climate several years ahead. Differences in skill between the two systems are likely due to a multitude of differences between the underlying climate models, but it is demonstrated herein that improved simulation of tropical Pacific variability is a key source of the improved skill in DePreSys 2. While DePreSys 2 is clearly more skilful than DePreSys in a global sense, it is shown that decreases in skill in some high-latitude regions are related to errors in representing long-term trends. Detrending the results focuses on the prediction of decadal time-scale variability, and shows that the improvement in skill in DePreSys 2 is even more marked.

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Kevin J. Noone
,
Doug W. Johnson
,
Jonathan P. Taylor
,
Ronald J. Ferek
,
Tim Garrett
,
Peter V. Hobbs
,
Philip A. Durkee
,
Kurt Nielsen
,
Elisabeth Öström
,
Colin O’Dowd
,
Michael H. Smith
,
Lynn M. Russell
,
Richard C. Flagan
,
John H. Seinfeld
,
Lieve De Bock
,
René E. Van Grieken
,
James G. Hudson
,
Ian Brooks
,
Richard F. Gasparovic
, and
Robert A. Pockalny

Abstract

A case study of the effects of ship emissions on the microphysical, radiative, and chemical properties of polluted marine boundary layer clouds is presented. Two ship tracks are discussed in detail. In situ measurements of cloud drop size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside-cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to remotely sensed measurements of cloud radiative properties.

The authors examine the processes behind ship track formation in a polluted marine boundary layer as an example of the effects of anthropogenic particulate pollution on the albedo of marine stratiform clouds.

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Kevin J. Noone
,
Elisabeth Öström
,
Ronald J. Ferek
,
Tim Garrett
,
Peter V. Hobbs
,
Doug W. Johnson
,
Jonathan P. Taylor
,
Lynn M. Russell
,
Richard C. Flagan
,
John H. Seinfeld
,
Colin D. O’Dowd
,
Michael H. Smith
,
Philip A. Durkee
,
Kurt Nielsen
,
James G. Hudson
,
Robert A. Pockalny
,
Lieve De Bock
,
René E. Van Grieken
,
Richard F. Gasparovic
, and
Ian Brooks

Abstract

The effects of anthropogenic particulate emissions from ships on the radiative, microphysical, and chemical properties of moderately polluted marine stratiform clouds are examined. A case study of two ships in the same air mass is presented where one of the vessels caused a discernible ship track while the other did not. In situ measurements of cloud droplet size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aerosol in the two ship plumes are discussed;these indicate that combustion-derived particles in the size range of about 0.03–0.3-μm radius were those that caused the microphysical changes in the clouds that were responsible for the ship track.

The authors examine the processes behind ship track formation in a moderately polluted marine boundary layer as an example of the effects that anthropogenic particulate pollution can have in the albedo of marine stratiform clouds.

Full access