Synoptic situation during the ARTofMELT 2023 spring expedition
Authors: Murto Sonja , Tjernström Michael 
Published: March 2024

A 6-week long expedition ARTofMELT (Atmospheric rivers and the onset of Arctic sea-ice melt) with the Swedish Icebreaker Oden took place in the Arctic Ocean during late winter and spring of 2023. The aim was to collect observations and study processes leading up to the sea-ice melt onset. One of the targets was to assess the role of atmospheric rivers (ARs), i.e., southerly warm and moist-air injections, in advancing the melt-timing. This paper presents the synoptic situation during the expedition, based on observations measured onboard Oden and reanalysis data (ERA5). Additionally, the origin and paths of airmasses reaching Oden are determined using 7-day backward trajectories computed with the Lagrangian analysis tool LAGRANTO. The meteorological conditions were quite dynamic during these 35 days, strongly influenced by several (at least 6) surface cyclones passing Oden and only two warming events accompanied by rather weak ARs were observed, the latter one leading to the melt onset at the end of the expedition.&#160;Based on meteorological conditions from 6-hourly launched radiosoundings, the expedition can be divided into six periods. The first short period encompasses the first days of the expedition, when Oden was located at the marginal ice zone. The winds were variable, mainly southerly, and it was moist with slightly below-freezing temperatures. As Oden was moving northwestwards, a one-week cold (~-15 - -10) and dry period followed. This period was mainly governed by northerly winds, guided by a persistent family of surface cyclones located over the Laptev and Kara Seas. The first major storm, that coincided with an atmospheric blocking over Scandinavia, was related to a cyclone forming to the southwest of Greenland and moving northeast, bringing winds over 25 m/s as it hit Oden on 13 May. &#160;Northerly winds followed after the stormed had passed, guided by a surface pressure dipole between a high over Greenland and a low over the Arctic Ocean.&#160;The first one-week long ice camp was built at the end of the second period, extending into the third period. A low-pressure over Greenland and high-pressure and an upper-level blocking over Scandinavia resulted in a pathway for a transient warm-air mass from the south, and melting was observed for the first-time. However, this warming was only temporary, as temperatures dropped below freezing after the AR had passed. Several weaker storms governed this third milder period, ending with the second major storm associated with a cyclone on 25 May. Again, winds turned northerly after the storm passed, which made the entry to the fourth longer, colder and drier period. The second 2-week long ice camp was established at the beginning of this period and expanded over the two last periods. These captured the forecasted (6 June) and the real melt onset (10 June). A surface pressure dipole with a high over Greenland and a low over the Arctic Ocean dominated at the beginning of the fifth period, and warm but dry air aloft was observed. As the winds turned southerly, the melt-onset period was characterized as warm and moist.