Blog from the RV Polarstern: MOSAiC trace gas fluxes update 5.3

By Byron Blomquist

We now have about one week of science activity remaining in MOSAiC. It's widely expected we will begin the return transit to Germany via Svalbard on September 20th, so the final pack-up will probably commence at the end of next week. I wasn't sure what to expect from this leg, but things have worked out quite well overall. We've been getting great data at MetCity from the flux tower, SODAR and ARM radiation suite, without significant interruptions. Our bulk trace gas concentration measurements from the bow have been operating normally and we've had several days of suitable flux conditions for the ship system. Thus far only a couple bear visits, mostly after working hours.

Photo: Met City prior to the freeze

We were able to capture conditions at the end of the melt season, and ice measurements show we lost a few cm of thickness over the first couple weeks at the Leg 5 camp. Fresh water layers remained a common feature over most leads and melt ponds up to the end of melt season. We have transitioned to the refreeze season, with current temperatures near -10°C, and the sun, when visible, is very low on the horizon. Melt ponds are well-frozen now and open leads become ice covered within 24-36 hours. A precipitation event this last week left several cm of new snow, which has since drifted over all the ponds and ridges. Once familiar features of the ice terrain are now totally transformed. The helicopter has been unable to fly for most of this leg due to fog and icing conditions, so small drones have been invaluable assets for mapping the camp layout and the evolution of the ice floe during our stay.

The ATMOS team was able to capture a recent precipitation event with an increased frequency of radiosonde launches, and we plan a similar intensive during a second precip event forecast for this weekend. We have almost all of our on-ice assets deployed and running, including the two ASFS surface energy budget sleds and two AWI flux sleds. The 'miss piggy' tethered balloon is deployed for temperature profiling with the fiber-optic sensor, but has struggled with icing problems in the freezing fog. We have already lost one balloon to the ice and I see this morning the replacement is again weighed down by ice, hovering near ground level.

Eddy correlation fluxes of CO2, CH4 and DMS have been very low over most of this leg - at or below the detection limit. The atmospheric DMS concentration has been mostly below 40 ppt and is now 10 ppt or less since the onset of freezing. We've been conducting almost daily measurements with the flux chamber, but also see very little gas transfer since the onset of refreeze. We sampled one mixing event in an open lead with the chamber, when strong winds were finally able to mix out the fresh surface layer and salinity was 29 psu over the upper 10m.

However, my impression of the central Arctic from observations during MOSAiC is that this is an environment where turbulent transfer between the surface ocean and troposphere is severely limited by stable stratification on both the air and water side of the interface. While sea ice may be permeable to gas transfer, the fluxes are very small, and fresh water ice layers in the newly refrozen floe only serve to further inhibit gas transfer. Gas transfer rates are certainly much larger in the marginal ice zone and over the open ocean, and we sampled a bit of this during the period between legs 4 and 5. These areas are increasing as the extent of summer sea ice loss advances with the warming climate, and the physical characteristics of new sea ice over the open ocean should be different from the refrozen floes with fresh water layers we have seen during MOSAiC.