Multiple studies in recent years have documented cooling on the Antarctic Peninsula since the late 1990s, opposing the general longer-term warming trend previously reported for this region. Multi-decadal temperature trends in West Antarctica, in comparison, are more difficult to evaluate, as Byrd is the only long-term station in the area, compared to many long-term staffed stations on the Peninsula. The study presented here aims to puts temperature changes in the Antarctic Peninsula and West Antarctica into a larger spatial and temporal perspective, predominantly focusing on observational data due to the unreliability of reanalyses in the southern high-latitudes prior to 1979. The primary data source comes from monthly stationbased surface temperature observations across the extratropical Southern Hemisphere. Temperature trends are evaluated over various periods, with the longest period from 1957-2016 (1957 is the International Geophysical Year), and then a more recent period of 1979-2016 that is split into 1979-1997 and 1999-2016, to examine the Peninsula cooling. The spatial and temporal aspects of this study, with a maximum duration of 60 years and 59 stations included, and its focus on observational data are what makes it unique. HadISST1 is utilized for sea surface temperatures, and the ERA-Interim reanalysis provides MSLP data for circulation pattern analysis; both of these are used to identify large-scale patterns that correspond with the observational temperatures. Our results confirm the statistically significant cooling in both station observations and sea surface temperature trends throughout the entire Antarctic Peninsula region in the past 17 years (1999-2016), caused by an abnormal pressure pattern driving southerly winds across this area. However, the full 60-year period shows statistically significant, widespread warming across the Southern Hemisphere mid-and high-latitudes, including the iv Antarctic Peninsula and West Antarctica. Positive SST trends broadly reflect these warming trends, especially in the mid-latitudes. Furthermore, we confirm the known prominent influence of the Southern Annular Mode (SAM) on southern mid-and high-latitude climate variability, the positive SAM trend in recent decades, and the related cooling over East Antarctica. Expanding on these results, when we remove the influence of the SAM from the station temperature records, we find statistically significant warming across all of the extratropical Southern Hemisphere, including East Antarctica, revealing strong background warming. If the SAM begins trending less positive in the future (as a result of Antarctic stratospheric ozone recovery, for example), this background warming could become more evident, and could have much greater implications for global climate. v Dedication To my parents: for all your love and support over the years vi Acknowledgements

Capulse Summary The Year of Polar Prediction in the Southern Hemisphere had a Special Observing Period (SOP) during the 2018-2019 austral summer. Activities during and resulting from the Antarctic SOP are described.

West Antarctica (WA), especially the Ross Ice Shelf (RIS), has experienced more frequent surface melting during the austral summer recently. The future is likely to see enhanced surface melting that will jeopardize the stability of ice shelves and cause ice loss. We investigate four major melt cases over the RIS via Polar Weather Research and Forecasting (WRF) simulations (4 km resolution) driven by European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) reanalysis data and Moderate Resolution Imaging Spectroradiometer (MODIS) observed albedo. Direct warm air advection, recurring foehn effect, and cloud/upper warm air introduced radiative warming are the three major regional causes of surface melting over WA. In this paper, Part I, the first two factors are identified and quantified. The second paper, Part II, discusses the impact of clouds and summarizes all three factors from a surface energy balance perspective. With a high‐pressure ridge located westward towards the Sulzberger Ice Shelf (77° S, 148° W) and a low‐pressure center located between 165° and 180° W, warm marine air from the Ross Sea is advected towards the coastal RIS and leads to surface melting. When the high‐pressure ridge is located farther east towards Marie Byrd Land (120–150° W), the foehn effect can cause a 2–4°C increase in surface temperature on the leeside of the mountains. For three of four melt cases, more than 40% of the melting period experiences foehn warming. Isentropic drawdown is usually the dominant foehn mechanism and contributes up to a 14°C temperature increase, especially when strong low‐level blocking occurs on the upwind side. The thermodynamic mechanism can be important depending on the strength of moisture uptake and condensation on the windward side. Meanwhile, sensible heat flux contributes less to foehn warming, but still plays an important role in the melting. The prediction of future stability of the RIS should include foehn warming as a major driver.

The Ross Ice Shelf (RIS) buttresses ice streams from the Antarctic continent and restrains the grounded ice sheet from flowing into the ocean, which is important for the stability of the ice sheet. In recent decades, West Antarctic ice shelves, including the RIS, have experienced more frequent surface melting during summer. We investigated the role of warm, descending föhn winds in a major melt event that occurred on the RIS in January 2016. Only a few summer melt events of this magnitude have been observed since 1979. Backward trajectories from the area of earliest melting were constructed using the Antarctic Mesoscale Prediction System to investigate the dominant mechanisms at the beginning of the melt event, mainly from 10 to 13 January. Analysis was conducted over two distinct areas. The föhn effect contributed around 2–4 °C to the surface temperature increase over the coastal mountains of Marie Byrd Land and around 1 °C over the much lower Edward VII Peninsula. Most of the föhn warming was caused by isentropic drawdown of air aloft. On 10 January, the second‐most important contributor for both mountain ranges was the thermodynamic mechanism. On 11 January, the second‐most important mechanism was the sensible and radiative heat flux. This study contributes to a better understanding of surface melt events over the RIS and benefits research associated with the stability of West Antarctic ice shelves.

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