The Antarctic Peninsula (AP), with its relatively mild climate, has experienced significant temperature changes. While warming trends were observed between the 1950s and 2000, more recent studies indicate a cooling trend. These temperature fluctuations significantly impact the AP's ecosystems, ice shelf mass balance, and ultimately, global sea levels. The AP's cordillera creates distinct climatic zones: a mild, humid marine climate on the west coast and a cooler continental climate on the east coast. Warming events on the northwest side are linked to strengthened northerly flow, while those on the northeast side are associated with foehn winds. The Esperanza Base, located on the northeastern AP, has experienced long-term warming and episodic extreme warm events, including a 17.5 °C temperature on March 24, 2015, attributed to a foehn event and atmospheric river. The unprecedented 18.3 °C temperature recorded on February 6, 2020, at Esperanza, coincided with record-breaking surface melt on the Larsen C Ice Shelf. This study aims to examine the thermodynamic mechanisms—horizontal heat advection, vertical motion, and diabatic heating—that contributed to this record-high temperature.

Previous research has highlighted the warming trend on the Antarctic Peninsula, with studies showing both warming and subsequent cooling phases. The influence of strengthened northerly flow due to increasing South Atlantic pressure and the Amundsen Sea low has been established for the northwest side. On the northeast side, strong circumpolar westerlies and resulting foehn winds have been identified as major contributors to warming events. Specific events, such as the 17.5°C temperature in March 2015 at Esperanza, have been studied, demonstrating the role of foehn events triggered by atmospheric rivers. However, detailed thermodynamic analyses of such extreme events have been limited.

This study utilizes surface observations from the Esperanza Base (1973-present), focusing on air temperature, wind speed, direction, and relative humidity (3-hourly data). The ERA5 reanalysis dataset (1979-present) provides hourly data with 0.25° x 0.25° spatial resolution and 37 pressure levels, used to analyze atmospheric circulation (2m temperature, 10m wind, mean sea level pressure, 500hPa geopotential height and wind, 850hPa temperature and geopotential height, temperature, wind and vertical velocity at 925hPa and 975hPa, surface pressure and geopotential). The thermodynamic equation was applied to quantify the contributions of horizontal advection, vertical motion, and diabatic heating to temperature changes. To determine the causes of foehn warming, a heat budget analysis was conducted using Lagrangian trajectory model output from three points along the air mass trajectory. Additionally, 196 extreme warm events (December-February, 1973-2020) were identified using a 95th percentile threshold. The HYSPLIT model was used to compute 10-day backward trajectories for each event, which were then subjected to cluster analysis. Composite analyses of 500hPa geopotential height anomalies were performed for these extreme warm event clusters.

The 2020 record-high temperature event at Esperanza was characterized by a dramatic 9-hour temperature increase from -0.7°C to 18.3°C. Analyses of sea level pressure, 500hPa geopotential height, and wind fields revealed a high-pressure ridge and a blocking high over the Drake Passage, directing warm Pacific air towards the AP. The thermodynamic equation analysis showed that vertical motion was the dominant factor driving the temperature increase, with a negative contribution from horizontal advection (advection of cold air from the southwest). The heat budget analysis revealed that sensible heat and radiation were the primary mechanisms contributing to foehn warming, exceeding the contributions from isentropic drawdown and offsetting negative contributions from the thermodynamic mechanism and pressure gradient. While an atmospheric river was present, it was not the primary driver. The cluster analysis of 196 extreme summer warm events at Esperanza indicated that 89% of air masses originated from the Pacific Ocean, with Cluster 5 (containing the February 2020 event) exhibiting the highest temperatures and a relatively short path from the Pacific. The composite 500hPa geopotential height anomaly for Cluster 5 shows a positive pressure anomaly over the Drake Passage, similar to the pattern observed in February 2020. This suggests that the record-breaking event is representative of other extreme warm events in the region.

This study demonstrates that the record-high temperature at Esperanza in February 2020 was primarily driven by vertical air flows in a foehn warming event, rather than solely by large-scale warm air advection. Although large-scale circulation patterns contributed to directing warm air towards the Antarctic Peninsula, the local orography, creating favorable conditions for foehn winds, played a crucial role in the extreme warming. The dominance of sensible heat and radiation in the foehn warming process is significant, and the analysis of past events shows this mechanism's frequent occurrence. The prevalence of Pacific air mass origins in extreme warm events highlights the importance of understanding the interactions between tropical Pacific climate variability and the Antarctic Peninsula's climate.

The study conclusively demonstrates the dominant role of vertical air flows (foehn) in producing the record high temperature at Esperanza station. While large-scale circulation patterns played a role in steering warm air towards the Antarctic Peninsula, the local topographic effects amplified the warming significantly. Future research should focus on higher-resolution modeling to better resolve the complex interactions between the atmosphere, ocean, and cryosphere in this region, and on investigating the connection between large-scale climate variability and localized foehn events. The frequency of such extreme events could increase with future climate change, warranting further investigation into their potential impacts.

The study's reliance on ERA5 reanalysis data, with its inherent limitations in spatial resolution, might have led to some underestimation of the detailed processes involved in the foehn effect. Future research utilizing higher-resolution models is necessary for a more thorough understanding of the complex interactions involved. The analysis focuses primarily on the Esperanza station and doesn't necessarily represent the entirety of the Antarctic Peninsula's temperature patterns during the events.