Atmospheric river brings warmth and rainfall to the northern Antarctic Peninsula during the mid-austral winter of 2023

Recent research has extensively analyzed summertime atmospheric river (AR) events in the Antarctic Peninsula (AP) using ground-based and atmospheric observations, yet a significant gap remains in understanding the occurrence and impacts of ARs during the Antarctic winter. This study focuses on an extraordinary warming event in the AP between 1 and 3 July 2023, utilizing data from recent wintertime field campaigns and ERA5 reanalysis. On 2 July, the Frei station in northern AP recorded a remarkable daily maximum near-surface air temperature of 2.7°C, significantly higher than the mean winter value of -3.8°C and surpassing the winter 99th percentile of 1.8°C. On 2-3 July, at least 6 hours of liquid precipitation were recorded, as corroborated by ERA5 data, leading to notable rain-on-snow and melt events. This occurrence challenges conventional expectations, as liquid precipitation during the depths of the southern winter is exceedingly rare in Antarctica. Radiosonde observations indicated a substantial elevation of the freezing level to about 650 meters, a stark contrast to the 20 meters observed before the event. These observations also revealed a moist and nearly saturated atmospheric profile. The event was synoptically characterized by a distinct trough over the Bellingshausen Sea and a pronounced northwest-southeast oriented blocking ridge from the southwestern Atlantic to the Weddell Sea, resulting in a dipole-like pressure pattern around the AP. These conditions were instrumental in the development of an AR with a north-to-south flow. This flow was marked by maximum integrated vapor transport values exceeding 500 kg m^-1 s^-1, channeling warm, moisture-laden air from continental South America towards the AP. A long-term winter trend analysis reveals a significant strengthening of the dipole pattern, which correlates with increased frequencies of ARs and consequently leads to notable warm temperature anomalies over the northern AP. The study underscores the importance of understanding the complex relationship between local, synoptic conditions, and the dynamics of ARs in influencing winter climate patterns in the AP. This study's ongoing high-resolution simulations and isotope analysis aim to uncover the detailed characteristics and isotopic signatures of this extraordinary warming event, enhancing our understanding of its origins and impacts.