WEATHER AND CLIMATE IN THE AMUNDSEN SEA EMBAYMENT,WEST ANTARCTICA:OBSERVATIONS, REANALYSES AND HIGH RESOLUTION MODELLING A thesis submitted to the School of Environmental Sciences of the University of East Anglia in partial fulfilment of the requirements for the degree of Doctor of Philosophy RICHARD JONES JANUARY 2018 © This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that use of any information derived there from must be in accordance with current UK Copyright Law. In addition, any quotation or extract must include full attribution. © Copyright 2018 Richard Jones iii ABSTRACT Glaciers within the Amundsen Sea Embayment (ASE) are rapidly retreating and so contributing 10% of current global sea level rise, primarily through basal melting. » Here the focus is atmospheric features that influence the mass balance of these glaciers and their representation in atmospheric models. New radiosondes and surface-based observations show that global reanalysis products contain relatively large biases in the vicinity of Pine Island Glacier (PIG), e.g. near-surface temperatures 1.8 ±C (ERA-I) to 6.8 ±C (MERRA) lower than observed. The reanalyses all underestimate wind speed during orographically-forced strong wind events and struggle to reproduce low-level jets. These biases would contribute to errors in surface heat fluxes and thus the simulated supply of ocean heat leading to PIG melting. Ten new ice cores show that there is no significant trend in accumulation on PIG between 1979 and 2013. RACMO2.3 and four global reanalysis products broadly reproduce the observed time series and the lack of any significant trend. The zonal pressure gradient between the Amundsen Sea and the Antarctic Peninsula is the main driver of variability in accumulation on PIG. Finally, the meteorological conditions associated with high heat flux events within coastal polynyas in the ASE are investigated. In three case studies high resolution 2 simulations produce turbulent heat fluxes that are 10-20% (up to 100 W m¡ ) larger than those in coarse resolution simulations. Representation of a hydraulic jump at the base of a slope is the key driver of the increased heat fluxes in a polynya close to PIG, with its presence leading to large differences in surface fluxes in the simulations. Overall it is shown that moving towards using higher resolution atmospheric products will both reduce the magnitude of coastal biases near PIG in reanalysis products, and allow complex orographic flows, important to coastal polynya processes, to be more adequately resolved. v ACKNOWLEDGEMENTS Firstly, I want to thank my primary supervisor Prof. Ian Renfrew for his guidance, support and constructive criticism. Also the occasional running hints and tips were much appreciated. To the rest of my supervisory team Ben Webber, Andrew Orr and Karen Heywood thankyou all for your feedback on my work throughout this PhD. Further thanks to Andy Elvidge, Denis Sergeev and Stuart Webster for their invaluable advice on setting up and running the Met Office Unified Model. Thanks also to all those onboard the JCR294 cruise, your enthusiasm for helping with the radiosonde launches was brilliant and helped to make my life easier. To all of the inhabitants of office 3.17, both past and present, it has been a pleasure to share the office with you over the last four years. Thanks to Tahmeena for sharing the multiple trips up and down the stairs each day to drink life giving tea, and for the useful advice on how to finish a thesis. Rhiannon, thanks for the crosswords and putting up with my boring matlab questions, Karin thanks for for your LaTeX help over the years and for showing me that a work-life balance is important. Umberto and Louise thanks for helping to stop me from going insane and teaching me about Seagliders. Honza, Dave, Laura, Alastair, Matt et al., thanks for putting up with me. My housemates for the last couple of years, Osgur and Barney, 55 Edinburgh Road will not be the same without you, it has been fantastic getting to know you both (too well) and it’s been fun to spend weekends concentrating on football and avoiding having to think about the return to the PhD thesis each Monday morning. To my Mum and Dad, thanks for supporting me throughout my studies at school and then university, without you I would never have gone on this meteorological journey. Thanks also for all your proof reading over the years - many embarrassing typos have been avoided (did I spell that correctly?). Finally my sister, Dr. Lizzy Jones, thanks for blazing the PhD trail in our family, you showed me that PhD life can be fun and that there are many opportunities to explore far away places along the way. Oh and final thanks to Leicester City, for showing us that miracles can happen, and bringing much happiness (though also stress) to the Jones/Wilson household. vii CONTENTS Abstract v Acknowledgements vii List of figures xiii List of tables xxv List of symbols and acronyms xxix 1 Introduction1 1.1 Glacial retreat in West Antarctica.........................2 1.2 Variability of glacial retreat in the Amundsen Sea...............5 1.3 The climate of the Amundsen Sea Embayment................7 1.3.1 The Amundsen Sea Low..........................7 1.3.2 Teleconnections and the influence of large scale climate modes..9 1.3.3 Precipitation and marine airmass intrusions............. 10 1.3.4 Katabatic winds and orographic flows................. 12 1.4 Sea ice and polynyas in the Amundsen Sea................... 13 1.4.1 Polynyas................................... 15 1.4.2 How do polynyas affect the atmosphere?................ 16 1.4.3 How do polynyas affect the ocean?................... 18 1.4.4 Coastal polynyas and ice shelves around Antarctica......... 19 1.4.5 A changing sea ice season in the Amundsen Sea........... 20 1.5 Lack of in-situ observations............................ 20 1.6 Thesis aims and structure............................. 21 2 Observational data, reanalyses and NWP model setup 23 2.1 Observations..................................... 23 2.1.1 Radiosondes................................. 23 ix x CONTENTS 2.1.2 Meteorological conditions during the JCR cruise........... 25 2.1.3 Research vessel observations....................... 27 2.1.4 AWS observations.............................. 27 2.1.5 Ice core accumulation observations................... 30 2.2 Reanalysis Products and RACMO......................... 32 2.2.1 Reanalysis products............................ 32 2.2.2 RACMO2.3.................................. 33 2.3 Met Office Unified Model details and configuration............. 35 3 An Evaluation of four global reanalysis products using in situ observations in the Amundsen Sea Embayment 39 3.1 Motivation...................................... 39 3.2 Meteorological reanalysis skill in Antarctica.................. 41 3.3 Observations, Reanalyses and Methodology.................. 43 3.3.1 Observations................................. 43 3.3.2 Reanalyses.................................. 45 3.3.3 Comparison Methodology......................... 46 3.4 AWS Comparison.................................. 49 3.4.1 Seasonal Temperature Comparison................... 49 3.4.2 Wind speed and Humidity comparison................. 54 3.5 Comparison to research vessel meteorological data............. 58 3.6 Radiosonde profile Comparison......................... 65 3.6.1 Mean profile biases............................. 65 3.6.2 Temperature biases............................. 66 3.6.3 Wind speed biases............................. 67 3.6.4 Biases in profiles containing a temperature inversion........ 69 3.6.5 Examples: 13th and 15th Feb 2014.................... 71 3.7 Overview of reanalysis products performance in the Amundsen Sea... 76 3.7.1 Strengths and weaknesses common to all products......... 76 3.7.2 ERA-Interim................................. 76 3.7.3 JRA-55..................................... 77 3.7.4 CFSR...................................... 77 3.7.5 MERRA.................................... 78 3.7.6 Implications................................. 78 CONTENTS xi 3.8 Conclusions..................................... 79 4 Assessing recent trends in accumulation on PIG using new ice core observations, global reanalyses and RACMO 81 4.1 Introduction..................................... 81 4.2 Motivation...................................... 82 4.3 Aims.......................................... 83 4.4 Methods........................................ 84 4.5 Results......................................... 86 4.5.1 The spatial distribution of accumulation on PIG........... 90 4.5.2 West Antarctic atmospheric circulation in low and high accumulation years............................. 94 4.5.3 What are the important meteorological processes and atmospheric phenomena controlling the accumulation on PIG?...................................... 97 4.6 Discussion...................................... 102 4.6.1 Which products most accurately reproduce the ice core observed accumulation on PIG?..................... 102 4.6.2 Atmospheric circulation in high and low accumulation years and implications for 21st century accumulation changes......... 105 4.7 Conclusions..................................... 108 5 Modelling the meteorological conditions during high heat flux events over the Amundsen Sea Embayment 111 5.1 Introduction..................................... 111 5.2 Aims.......................................... 112 5.3 Control case study: Feb