Publications


The American monsoon system in HadGEM3 and UKESM1

Weather and Climate Dynamics Copernicus Publications 1 (2020) 349-371

JL García-Franco, S Osprey, LJ Gray

The simulated climate of the American monsoon system (AMS) in the UK models HadGEM3 GC3.1 (GC3) and the Earth system model UKESM1 is assessed and compared to observations and reanalysis. We evaluate the pre-industrial control, AMIP and historical experiments of UKESM1 and two configurations of GC3: a low (1.875∘×1.25∘) and a medium (0.83∘×0.56∘) resolution. The simulations show a good representation of the seasonal cycle of temperature in monsoon regions, although the historical experiments overestimate the observed summer temperature in the Amazon, Mexico and Central America by more than 1.5 K. The seasonal cycle of rainfall and general characteristics of the North American monsoon of all the simulations agree well with observations and reanalysis, showing a notable improvement from previous versions of the HadGEM model. The models reasonably simulate the bimodal regime of precipitation in southern Mexico, Central America and the Caribbean known as the midsummer drought, although with a stronger-than-observed difference between the two peaks of precipitation and the dry period. Austral summer biases in the modelled Atlantic Intertropical Convergence Zone (ITCZ), cloud cover and regional temperature patterns are significant and influence the simulated regional rainfall in the South American monsoon. These biases lead to an overestimation of precipitation in southeastern Brazil and an underestimation of precipitation in the Amazon. The precipitation biases over the Amazon and southeastern Brazil are greatly reduced in the AMIP simulations, highlighting that the Atlantic sea surface temperatures are key for representing precipitation in the South American monsoon. El Niño–Southern Oscillation (ENSO) teleconnections, of precipitation and temperature, to the AMS are reasonably simulated by all the experiments. The precipitation responses to the positive and negative phase of ENSO in subtropical America are linear in both pre-industrial and historical experiments. Overall, the biases in UKESM1 and the low-resolution configuration of GC3 are very similar for precipitation, ITCZ and Walker circulation; i.e. the inclusion of Earth system processes appears to make no significant difference for the representation of the AMS rainfall. In contrast, the medium-resolution HadGEM3 N216 simulation outperforms the low-resolution simulations due to improved SSTs and circulation.


An evaluation of tropical waves and wave forcing of the QBO in the QBOi models

Quarterly Journal of the Royal Meteorological Society Wiley (2020) qj.3827

LA Holt, F Lott, RR Garcia, GN Kiladis, Y Cheng, JA Anstey, P Braesicke, AC Bushell, N Butchart, C Cagnazzo, C Chen, H Chun, Y Kawatani, T Kerzenmacher, Y Kim, C McLandress, H Naoe, S Osprey, JH Richter, AA Scaife, J Scinocca, F Serva, S Versick, S Watanabe, S Yukimoto


Uncertainty in the response of sudden stratospheric warmings and stratosphere-troposphere coupling to quadrupled CO2 concentrations in CMIP6 models

Journal of Geophysical Research: Atmospheres American Geophysical Union (AGU) (2020) e2019JD032345-e2019JD032345

AJ Charlton-Perez, B Ayarzagüena, S Watanabe, EM Volodin, P Hitchcock, IR Simpson, LM Polvani, N Butchart, AH Butler, EP Gerber, P Lin, B Hassler, L Gray, S Osprey, E Manzini, R Mizuta, C Orbe, F Lott, D Saint-Martin, M Sigmond, M Taguchi


Forecasting extreme stratospheric polar vortex events.

Nature communications 11 (2020) 4630-

L Gray, M Brown, J Knight, M Andrews, H Lu, C O'Reilly, J Anstey

Extreme polar vortex events known as sudden stratospheric warmings can influence surface winter weather conditions, but their timing is difficult to predict. Here, we examine factors that influence their occurrence, with a focus on their timing and vertical extent. We consider the roles of the troposphere and equatorial stratosphere separately, using a split vortex event in January 2009 as the primary case study. This event cannot be reproduced by constraining wind and temperatures in the troposphere alone, even when the equatorial lower stratosphere is in the correct phase of the quasi biennial oscillation. When the flow in the equatorial upper stratosphere is also constrained, the timing and spatial evolution of the vortex event is captured remarkably well. This highlights an influence from this region previously unrecognised by the seasonal forecast community. We suggest that better representation of the flow in this region is likely to improve predictability of extreme polar vortex events and hence their associated impacts at the surface.


The SPARC Quasi‐Biennial Oscillation initiative

Quarterly Journal of the Royal Meteorological Society Wiley (2020) qj.3820

JA Anstey, N Butchart, K Hamilton, SM Osprey


Historical Simulations With HadGEM3-GC3.1 for CMIP6

JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS 12 (2020) ARTN e2019MS001995

MB Andrews, JK Ridley, RA Wood, T Andrews, EW Blockley, B Booth, E Burke, AJ Dittus, P Florek, LJ Gray, S Haddad, SC Hardiman, L Hermanson, D Hodson, E Hogan, GS Jones, JR Knight, T Kuhlbrodt, S Misios, MS Mizielinski, MA Ringer, J Robson, RT Sutton


QBO changes in CMIP6 climate projections

Geophysical Research Letters American Geophysical Union (AGU) (2020)

N Butchart, Y Kawatani, JA Anstey, SM Osprey, JH Richter, T Wu


The Southern Hemisphere sudden stratospheric warming of September 2019

Science Bulletin Elsevier BV (2020)

X Shen, L Wang, S Osprey


The American Monsoon System in HadGEM3.0 and UKESM1 CMIP6

Weather and Climate Dynamics Copernicus GmbH (2020)

JL García-Franco, LJ Gray, S Osprey

<jats:p>&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Abstract.&amp;lt;/strong&amp;gt; The simulated climate in the American Monsoon System (AMS) in the CMIP6 submissions of HadGEM3.0 GC3.1 and the UKESM1 is assessed and compared to observations and reanalysis. Pre-industrial control and historical experiments are analysed to evaluate the model representation of this monsoon under different configurations, resolutions and with and without Earth System processes. The simulations exhibit a good representation of the temperature and precipitation seasonal cycles, although the historical experiments overestimate summer temperature in the Amazon, Mexico and Central America by more than 1.5&amp;amp;#8201;K. The seasonal cycle of rainfall and general characteristics of the North American Monsoon are well represented by all the simulations. The models simulate the bimodal regime of precipitation in southern Mexico, Central America and the Caribbean known as the midsummer drought, although with a stronger intraseasonal variation than observed. Austral summer biases in the modelled Atlantic Intertropical Convergence Zone (ITCZ), Walker Circulation, cloud cover and regional temperature distributions are significant and influenced the simulated spatial distribution of rainfall in the South American Monsoon. These biases lead to an overestimation of precipitation in southeastern Brazil and an underestimation of precipitation in the Amazon. El Ni&amp;amp;#241;o Southern Oscillation (ENSO) characteristics and teleconnections to the AMS are well represented by the simulations. The precipitation responses to the positive and negative phase of ENSO in subtropical America are linear in both pre-industrial and historical experiments. Overall, the UKESM has the same performance as the lower resolution simulation of HadGEM3.0 GC3.1 and no significant difference for the AMS was found between the two model configurations. In contrast, the medium resolution HadGEM3.0 GC3.1 N216 simulation outperforms the low-resolution simulations in temperature, rainfall, ITCZ and Walker circulation biases.&amp;lt;/p&amp;gt; </jats:p>


Prediction of the quasi‐biennial oscillation with a multi‐model ensemble of QBO ‐resolving models

Quarterly Journal of the Royal Meteorological Society Wiley (2020) qj.3919

TN Stockdale, Y Kim, JA Anstey, FM Palmeiro, N Butchart, AA Scaife, M Andrews, AC Bushell, M Dobrynin, J Garcia‐Serrano, K Hamilton, Y Kawatani, F Lott, C McLandress, H Naoe, S Osprey, H Pohlmann, J Scinocca, S Watanabe, K Yoshida, S Yukimoto


Tropospheric forcing of the 2019 Antarctic sudden stratospheric warming

Geophysical Research Letters American Geophysical Union (AGU) (2020)

X Shen, L Wang, S Osprey


The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

Journal of Advances in Modeling Earth Systems 12 (2020)

J Robson, Y Aksenov, TJ Bracegirdle, O Dimdore-Miles, PT Griffiths, DP Grosvenor, DLR Hodson, J Keeble, C MacIntosh, A Megann, S Osprey, AC Povey, D Schröder, M Yang, AT Archibald, KS Carslaw, L Gray, C Jones, B Kerridge, D Knappett, T Kuhlbrodt, M Russo, A Sellar, R Siddans, B Sinha, R Sutton, J Walton, LJ Wilcox

©2020. The Authors. Earth system models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth system components changes or improves the simulation of the physical climate is not well understood. Here we present a broad multivariate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal time scale evolution of important variables that span the North Atlantic climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intraseasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth system development, remains crucial in order to improve climate simulations.


On the Role of Rossby Wave Breaking in the Quasi-Biennial Modulation of the Stratospheric Polar Vortex during Boreal Winter

Quarterly Journal of the Royal Meteorological Society Wiley (2020)

H Lu, SM Osprey, MH Hitchman, JA Anstey, LJ Gray


Progress in simulating the Quasi-biennial Oscillation in CMIP models

Journal of Geophysical Research: Atmospheres American Geophysical Union (AGU) (2020) e2019JD032362-e2019JD032362

JH Richter, JA Anstey, N Butchart, Y Kawatani, GA Meehl, S Osprey, IR Simpson


Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern

Environmental Research Letters IOP Publishing 14 (2019) 054002-054002

K Kornhuber, S Osprey, D Coumou, S Petri, V Petoukhov, S Rahmstorf, L Gray


Slowdown of the Walker circulation at solar cycle maximum

Proceedings of the National Academy of Sciences National Academy of Sciences 116 (2019) 7186-7191

S Misios, L Gray, M Knudsen, C Karoff, H Schmidt, J Haigh

The Pacific Walker Circulation (PWC) fluctuates on interannual and multidecadal timescales under the influence of internal variability and external forcings. Here, we provide observational evidence that the 11-y solar cycle (SC) affects the PWC on decadal timescales. We observe a robust reduction of east–west sea-level pressure gradients over the Indo-Pacific Ocean during solar maxima and the following 1–2 y. This reduction is associated with westerly wind anomalies at the surface and throughout the equatorial troposphere in the western/central Pacific paired with an eastward shift of convective precipitation that brings more rainfall to the central Pacific. We show that this is initiated by a thermodynamical response of the global hydrological cycle to surface warming, further amplified by atmosphere–ocean coupling, leading to larger positive ocean temperature anomalies in the equatorial Pacific than expected from simple radiative forcing considerations. The observed solar modulation of the PWC is supported by a set of coupled ocean–atmosphere climate model simulations forced only by SC irradiance variations. We highlight the importance of a muted hydrology mechanism that acts to weaken the PWC. Demonstration of this mechanism acting on the 11-y SC timescale adds confidence in model predictions that the same mechanism also weakens the PWC under increasing greenhouse gas forcing.


The importance of stratospheric initial conditions for winter North Atlantic Oscillation predictability and implications for the signal‐to‐noise paradox

Quarterly Journal of the Royal Meteorological Society John Wiley and Sons, Ltd. 145 (2018) Part A, 131-146

C O'Reilly, A Weisheimer, T Woollings, L Gray, D Macleod

This study investigates the influence of atmospheric initial conditions on winter seasonal forecasts of the North Atlantic Oscillation (NAO). Hindcast (or reforecast) experiments – which differ only in their initial conditions – are performed over the period 1960–2009, using prescribed sea surface temperature (SST) and sea‐ice boundary conditions. The first experiment (“ERA‐40/Int IC”) is initialized using the ERA‐40 and ERA‐Interim reanalysis datasets, which assimilate upper‐air, satellite and surface observations; the second experiment (“ERA‐20C IC”) is initialized using the ERA‐20C reanalysis dataset, which assimilates only surface observations. The ensemble mean NAO skill is largest in ERA‐40/Int IC (r = 0.54), which is initialized with the superior reanalysis data. Moreover, ERA‐20C IC did not exhibit significantly more NAO hindcast skill (r = 0.38) than in a third experiment, which was initialized with incorrect (shuffled) initial conditions. The ERA‐40/Interim and ERA‐20C initial conditions differ substantially in the tropical stratosphere, where the quasi‐biennial oscillation (QBO) of zonal winds is not present in ERA‐20C. The QBO hindcasts are highly skilful in ERA‐40/Int IC – albeit with a somewhat weaker equatorial zonal wind amplitude in the lower stratosphere – but are incorrect in ERA‐20C IC, indicating that the QBO is responsible for the additional NAO hindcast skill; this is despite the model exhibiting a relatively weak teleconnection between the QBO and NAO. The influence of the QBO is further demonstrated by regressing out the QBO influence from each of the hindcast experiments, after which the difference in NAO hindcast skill between the experiments is negligible. Whilst ERA‐40/Int IC demonstrates a more skilful NAO hindcast, it appears to have a relatively weak predictable signal; this is the so‐called “signal‐to‐noise paradox” identified in previous studies. Diagnostically amplifying the (weak) QBO–NAO teleconnection increases the ensemble‐mean NAO signal with negligible impact on the NAO hindcast skill, after which the signal‐to‐noise problem seemingly disappears.


Observed and Simulated Teleconnections Between the Stratospheric Quasi-Biennial Oscillation and Northern Hemisphere Winter Atmospheric Circulation

Journal of Geophysical Research: Atmospheres 124 (2019) 1219-1232

MB Andrews, JR Knight, AA Scaife, Y Lu, T Wu, LJ Gray, V Schenzinger

©2019 Crown copyright. This article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland. The Quasi-Biennial Oscillation (QBO) is the dominant mode of interannual variability in the tropical stratosphere, with easterly and westerly zonal wind regimes alternating over a period of about 28 months. It appears to influence the Northern Hemisphere winter stratospheric polar vortex and atmospheric circulation near the Earth's surface. However, the short observational record makes unequivocal identification of these surface connections challenging. To overcome this, we use a multicentury control simulation of a climate model with a realistic, spontaneously generated QBO to examine teleconnections with extratropical winter surface pressure patterns. Using a 30-hPa index of the QBO, we demonstrate that the observed teleconnection with the Arctic Oscillation (AO) is likely to be real, and a teleconnection with the North Atlantic Oscillation (NAO) is probable, but not certain. Simulated QBO-AO teleconnections are robust, but appear weaker than in observations. Despite this, inconsistency with the observational record cannot be formally demonstrated. To assess the robustness of our results, we use an alternative measure of the QBO, which selects QBO phases with westerly or easterly winds extending over a wider range of altitudes than phases selected by the single-level index. We find increased strength and significance for both the AO and NAO responses, and better reproduction of the observed surface teleconnection patterns. Further, this QBO metric reveals that the simulated AO response is indeed likely to be weaker than observed. We conclude that the QBO can potentially provide another source of skill for Northern Hemisphere winter prediction, if its surface teleconnections can be accurately simulated.


Recent observed changes in the North Atlantic climate system with a focus on 2005-2016

International Journal of Climatology John Wiley & Sons, Inc. 38 (2018) 5050-5076

J Robson, A Archibald, F Cooper, M Christensen, L Grey, NP Holliday, C Macintosh, M McMillan, B Moat, K Carslaw, O Embury, D Feltham, D Grosvenor, S Josey, B King, A Lewis, GD McCarthy, C Merchant, AL New, C O'Reilly, S Osprey, K Read, A Scaife, A Shepherd

Major changes are occurring across the North Atlantic climate system, including in the atmosphere, ocean and cryosphere, and many observed changes are unprecedented in instrumental records. As the changes in the North Atlantic directly affect the climate and air quality of the surrounding continents, it is important to fully understand how and why the changes are taking place, not least to predict how the region will change in the future. To this end, this article characterizes the recent observed changes in the North Atlantic region, especially in the period 2005–2016, across many different aspects of the system including: atmospheric circulation; atmospheric composition; clouds and aerosols; ocean circulation and properties; and the cryosphere. Recent changes include: an increase in the speed of the North Atlantic jet stream in winter; a southward shift in the North Atlantic jet stream in summer, associated with a weakening summer North Atlantic Oscillation; increases in ozone and methane; increases in net absorbed radiation in the mid‐latitude western Atlantic, linked to an increase in the abundance of high level clouds and a reduction in low level clouds; cooling of sea surface temperatures in the North Atlantic subpolar gyre, concomitant with increases in the western subtropical gyre, and a decline in the Atlantic Ocean's overturning circulation; a decline in Atlantic sector Arctic sea ice and rapid melting of the Greenland Ice Sheet. There are many interactions between these changes, but these interactions are poorly understood. This article concludes by highlighting some of the key outstanding questions.


Preindustrial Control Simulations With HadGEM3-GC3.1 for CMIP6

JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS 10 (2018) 3049-3075

MB Menary, T Kuhlbrodt, J Ridley, MB Andrews, OB Dimdore-Miles, J Deshayes, R Eade, L Gray, S Ineson, J Mignot, CD Roberts, J Robson, RA Wood, P Xavier

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