Periodic Reporting for period 5 - TERIFIC (Targeted Experiment to Reconcile Increased Freshwater with Increased Convection)
Berichtszeitraum: 2023-11-01 bis 2024-10-31
The ocean Meridional Overturning Circulation (MOC) is responsible for poleward heat transport, and deep storage of heat and carbon. Climate models generally predict that a slowdown of the MOC will occur this century, with dramatic regional and global climate changes, but how the slowdown will occur is subject to debate. It is widely recognised that convection – the downward limb of the MOC – is sensitive to freshwater fluxes, and recent investigations have suggested that the increased melt from the Greenland Ice Sheet has already reduced convection. Yet in 2015, convection returned, and was anomalously strong. Despite the expectation that the MOC is sensitive to freshwater forcing, we do not understand the processes by which freshwater inputs influence the MOC.
The two key gaps in our understanding are: how freshwater reaches the regions of deep convection, and the relative importance of freshwater to convection and restratification. Numerical simulations give conflicting results on freshwater pathways, and convective parameterisations neglect small-scale restratifying processes. Traditional observational approaches cannot capture the spatial and temporal variability of these processes without inordinate cost.
TERIFIC addresses these gaps in understanding through new observations, leveraging recent advances in small-scale electronics to deploy large numbers of mini-drifters on the shelves of Greenland, and em- ploying subsurface and surface autonomous platforms to characterise the balance of processes controlling convection and restratification. The analysis of these observations will answer fundamental questions about how freshwater reaches and affects the regions of deep convection, and enable a critical ground truth of numerical simulations of these processes for climate models.
The two key gaps in our understanding are: how freshwater reaches the regions of deep convection, and the relative importance of freshwater to convection and restratification. Numerical simulations give conflicting results on freshwater pathways, and convective parameterisations neglect small-scale restratifying processes. Traditional observational approaches cannot capture the spatial and temporal variability of these processes without inordinate cost.
TERIFIC addresses these gaps in understanding through new observations, leveraging recent advances in small-scale electronics to deploy large numbers of mini-drifters on the shelves of Greenland, and em- ploying subsurface and surface autonomous platforms to characterise the balance of processes controlling convection and restratification. The analysis of these observations will answer fundamental questions about how freshwater reaches and affects the regions of deep convection, and enable a critical ground truth of numerical simulations of these processes for climate models.
The main fieldwork for TERIFIC has been completed. We have deployed 203 surface drifters which measure their position and properties at the ocean's surface once every hour. These drifters were deployed during different seasons (December 2019, March 2020, September 2020 and August-September 2021). The majority of drifters were deployed at the southwest corner of Greenland where the salty offshore water meets the fresh shelf currents, with the remaining ~25 in Davis Strait where they were in Arctic/Greenland-origin freshwater. The drifters then are carried downstream by ocean currents, tracking the fate of the fresh shelf currents and transmitting their position and surface ocean properties once every hour. We additionally deployed two autonomous underwater vehicles ('gliders') and a surface vehicle to overwinter in the Labrador Sea. The two gliders carried out their missions from December 2019 to May 2020, and again from December 2021 - June 2022, transiting the ocean while repeatedly diving to 1000m then surfacing and transmitting their data back to a basestation located in the UK. A surface vehicle collected surface meteorological data (winds and air temperature) as well as surface ocean properties (temperature, salinity and wave height), and transmitted this data back via satellite, however it was damaged during a storm and not retrieved. Datasets are being quality controlled and made open access via online repositories: the drifter data are immediately available via the Global Drifter Program website with quality-controlled data available ~6 months later. The first glider dataset is now quality-controlled and is available from an open repository; the second glider dataset will be appended to the same repository. The autonomous surface vehicle data available (before the vehicle was lost) will be submitted to the same repository.
Initial results from the drifter datasets show that the export from the shelf to open-ocean is sporadic, with wind playing a dominant role in the March deployment, and a marginal role in the other deployments. Additionally, the majority of the Davis Strait drifters remained on the Labrador shelf rather than exiting the shelf. These results were presented at international conferences and are in the process of being prepared for publication in a journal article. The glider datasets showed the expected development of convection and deep mixing layers in the wintertime (published in grey literature, awaiting final form). During the restratification period, deep submesoscale instabilities were identified (below 500m) which is unique to this region and contributed to the continuous restratification process. These small-scale lateral mixing processes were responsible for bringing both fresh and warm water into the open Labrador Sea. Continuing research is ongoing into the role of mixing and restratification during wintertime deep convection.
Initial results from the drifter datasets show that the export from the shelf to open-ocean is sporadic, with wind playing a dominant role in the March deployment, and a marginal role in the other deployments. Additionally, the majority of the Davis Strait drifters remained on the Labrador shelf rather than exiting the shelf. These results were presented at international conferences and are in the process of being prepared for publication in a journal article. The glider datasets showed the expected development of convection and deep mixing layers in the wintertime (published in grey literature, awaiting final form). During the restratification period, deep submesoscale instabilities were identified (below 500m) which is unique to this region and contributed to the continuous restratification process. These small-scale lateral mixing processes were responsible for bringing both fresh and warm water into the open Labrador Sea. Continuing research is ongoing into the role of mixing and restratification during wintertime deep convection.
The novelty in TERIFIC lies in the intensity of the observations: giving full seasonal coverage at high resolution using autonomous platforms to deliver an observational view of key processes at unprecedented resolution. Drifters were deployed on the Greenland shelves upstream of the Labrador Sea, to capture the seasonal variability of freshwater exchange at the surface. Each drifter will record its position and transmit data giving insight into the surface circulation changes, enabling documentation of seasonally-varying cross-shelf exchange, regardless of where the exchange happens – in contrast to traditional, moorings-based approaches. Glider missions will resolve small spatial scales, profiling at horizontal spacing of 2–4 km, depending on piloting, and diagnosing both hydrographic changes during convection nd the vertical velocities and turbulent mixing during convective plumes. Using autonomous vehicles enables a full season of measurements (rather than a time-limited snapshot from a ship), but at high enough spatial resolution to investigate smaller scale vertical and lateral processes. A concurrent surface vehicle mission, deployed near the shelf and piloted to the convective region, will provide meteorological conditions at sub-daily resolution as well as hydrographic properties near the surface. An experiment of this kind was not possible even 5 years ago; it relies on new technological development to capture both the surface and subsurface conditions, and large numbers of drifters with satellite communications, to be able to resolve the small scales of relevant processes while enduring long enough to capture seasonal evolution. This observational approach will overcome challenges with traditional observations, and deficiencies in numerical modelling approaches.