Upper Arctic Ocean properties and water mass pathways during the year-round MOSAiC expedition in the context of historical observations

The Arctic Ocean is characterized by complex processes coupling the atmosphere, cryosphere, ocean and land, and undergoes remarkable environmental changes due to global warming. To better understand this system of physical, biogeochemical and ecosystem processes, as well as recent changes was the aim of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) ice drift conducted year-round from autumn 2019 to autumn 2020. Here, we focus on the properties and circulation pathways of upper Arctic Ocean water masses that have been found to change in recent decades, likely in response to changes in sea ice, surface fluxes, and advection of air masses under Arctic amplification.

We use hundreds of hydrographic profiles obtained with two Conductivity Temperature Depth (CTD) systems mounted to rosette water samplers from the drifting ship and at a remote location on the ice to investigate the properties of the polar mixed layer, halocline waters and warm water of Atlantic origin (“Atlantic Water”) in the Eurasian Arctic during the MOSAiC campaign. Additionally, we analyse chemical tracers (noble gases and anthropogenic tracers CFC-12 and SF6) measured from water samples taken with both CTD/Rosette systems to identify pathways of the water masses. We compare these observations with a comprehensive dataset of historical hydrographic data from the region to put our findings into a long-term context.

We find a shoaling and thickening of the Atlantic-Water layer compared to historical observations, as well as signatures of interleaving at the core of the warm Atlantic Water that slowly get eroded during the drift. Along the MOSAiC track the hydrographic data show convective lower halocline waters that are typically formed north of Fram Strait and further downstream, as well as advective-convective lower halocline waters typically formed in the Barents Sea. We see a change in lower halocline properties in the eastern Amundsen Basin compared to historical observations, that could either be caused by local formation or a change in circulation. Further, we use the chemical tracers to investigate possible pathways and formation regions of the observed water masses.