Publications


Autonomous balloons take flight with artificial intelligence

Nature Nature Research 588 (2020) 33-34

SM Osprey


Forecasting extreme stratospheric polar vortex events

Nature Communications Springer Nature 11 (2020) 4630

LJ 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.


Revisiting gradient wind balance in tropical cyclones using dropsonde observations

Quarterly Journal of the Royal Meteorological Society Wiley (2020) qj.3947

J Garcia-Franco, J Schwendike

This study diagnoses the degree of gradient wind balance (GWB) in dropsonde observations of 30 tropical cyclones (TCs) divided into 91 intense observation periods. The diagnosed GWB in these observation periods are composited to investigate which characteristics of a TC are significantly related to departures from GWB. This analysis confirms that on average the flow above the boundary layer is approximately in GWB. Supergradient flow is more common near the radius of maximum wind (RMW) in the upper boundary layer than above in the free troposphere or outside the RMW and is also more common in strong storms than in weak storms. In contrast, the degree of GWB does not differ between intensifying, steady‐state and weakening storms. Storms with a peaked wind profile have a higher probability of showing supergradient winds than those with a flat wind profile. The comparison of two commonly used functions to fit observations shows that the diagnosing GWB from dropsonde observations is highly dependent on the analysis technique. The agradient wind magnitude and even sign is shown to depend on which of these functions is used to fit the observations. The use of a polynomial fit consistently diagnoses the presence of supergradient winds far more frequently than a piece‐wise function, and also shows a marked degree of imbalance above the boundary layer. Therefore, caution is warranted when determining the degree of GWB with a polynomial fit.


The Southern Hemisphere sudden stratospheric warming of September 2019

Science Bulletin Elsevier BV (2020)

X Shen, L Wang, S Osprey


Tropospheric forcing of the 2019 Antarctic sudden stratospheric warming

Geophysical Research Letters American Geophysical Union 47 (2020) e2020GL089343

X Shen, L Wang, S Osprey

The strongest and most persistent upward propagation of zonal wavenumber 1 (WN1) Rossby waves from the troposphere on record led to the rare Antarctic sudden stratospheric warming (SSW) in September 2019. The dynamical contribution from instantaneous anomalous WN1 and its linear interference with the climatological WN1 contributed equally to the event. The unprecedented WN1 planetary wave behavior is further attributed to a long‐lived midlatitude circumpolar Rossby wave train in the troposphere that was sustained by anomalous convection, first over the subtropical Pacific Ocean east of Australia and then over the eastern South Pacific. Besides the tropospheric wave forcing, the phase of the quasi‐biennial oscillation in the upper stratosphere also facilitated the weakening of polar vortex. Moreover, this SSW strongly influenced the tropospheric circulation via the Southern annular mode, favoring conditions linked to the 2019 bushfires in eastern Australia.


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

Quarterly Journal of the Royal Meteorological Society Wiley (2020)

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

A multi‐model study is carried out to investigate the ability of models to predict the evolution of the quasi‐biennial oscillation (QBO) up to 12 months in advance. All models are initialised from common reanalysis data, and forecasts run for a common set of 30 start dates over 15 years. All models have high skill in predicting the phase evolution of the QBO at 20–30 hPa, with slightly more variable results at higher and lower levels. Other aspects of the predicted QBO are of variable quality, and in some cases are consistently poor. QBO easterlies are too weak in all models at 20–50 hPa, while westerlies can be either too strong or too weak. This results in both a reduced amplitude of the QBO and a westerly bias in zonal‐mean winds, notably at 30 hPa. At 70 hPa models tend to have reduced QBO amplitude and an easterly bias. Despite these failings, a multi‐model ensemble of bias‐ and variance‐corrected forecasts can be used to give accurate and reliable QBO forecasts up to at least a year ahead. Analysis of the zonal momentum budget during the first month of the forecast shows that large‐scale forcing from Eliassen–Palm flux divergence and vertical advection are handled fairly well by the models, although vertical advection terms tend to be weaker than reanalysis estimates. Total tendencies show common errors, suggesting common failings in gravity‐wave drag treatments. Teleconnections from the QBO to Northern Hemisphere winter circulation are also examined, and do not appear to be realistic beyond the first month. Analysis of initialised forecasts is a powerful tool for diagnosing the accuracy of model processes driving the QBO.


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

JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS 12 (2020) ARTN e2020MS002126

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 Schroder, 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


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


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.


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>


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


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


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


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


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


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


Air quality in Mexico city during the fuel shortage of January 2019

Atmospheric Environment Elsevier 222 (2019) 117131

J Garcia-Franco

The closure of pipelines to tackle fuel-theft in central Mexico caused an unexpected fuel shortage that disrupted transport systems in Mexico City in January of 2019. Fuel sales in the Metropolitan Area and CO emissions from reanalysis showed a significant decrease during the fuel shortage of 7% and 6%, respectively. This study analyses the air quality and meteorological conditions during this period to evaluate whether these measures indirectly affected air quality in Mexico City. During the shortage, mean-ambient concentrations of nitric oxide (NO), nitrogen dioxide (NO2) and carbon monoxide (CO) were significantly lower than normal whereas levels of particulate matter (PM) were only modestly lower than usual. Daily-mean NO and CO had record-low anomalies of &#x2212;10 ppb and &#x2212;0.5 ppm from typical days, respectively. In contrast, ozone mean-levels were not significantly different than average. The percentage of days with PM mean concentrations above the World Health Organisation guidelines (5 and 19% for particles smaller than 2.5 and 10 &#x3BC;m, respectively) and the percentage of ozone 8-h rolling means above Mexican law (0.5%) were record lows. Meteorological factors, such as wind speed or the mixed-layer height were not significantly different than average. The anomalously low pollution levels were accentuated when each day was compared to days of similar flow patterns. This episode of better than usual air quality showcases how strategies addressing transport emissions could control air quality in Mexico City and highlights that improving ozone mean levels require comprehensive strategies that reduce emissions from all sectors.


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.


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.

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