A document by Diego Bruciaferri. Click on the document to view its contents.

Projections of future sea-level change are characterized by both quantifiable uncertainty and by ambiguity. Both types of uncertainty are relevant to users of sea-level projections, particularly those making long-term investment and planning decisions with multigenerational consequences. Communicating information about both types is thus a central challenge faced by scientists who generate sea-level projections to support decision-making. Diverse approaches to communicating uncertainty in future sea-level projections have been taken over the last several decades, but the literature evaluating these approaches is limited and not systematic. Here, we review how the Intergovernmental Panel on Climate Change (IPCC) has approached uncertainty in sealevel projections in past assessment cycles and how this information has been interpreted by national and subnational assessments, as well as alternative approaches used by recent US subnational assessments. The evidence reviewed here generally supports the explicit approach to communicating both types of uncertainty adopted by the IPCC Sixth Assessment Report (AR6).

Air-sea turbulent heat fluxes play a fundamental role in generating and dampening sea surface temperature (SST) anomalies. To date, the turbulent heat flux feedback (THFF) is well quantified at basin-wide scales (~20 W~m^{-2}~K^{-1}) but remains unknown at the oceanic mesoscale (10-100 km). Here, using an eddy-tracking algorithm in three configurations of the coupled climate model HadGEM3-GC3.1, the THFF over mesoscale eddies is estimated. The THFF magnitude is strongly dependent on the ocean-to-atmosphere regridding of SST, a common practice in coupled models for calculating air-sea heat flux. Our best estimate shows that the mesoscale THFF ranges between 35 and 45 W~m^{-2}~K^{-1} globally, across different eddy amplitudes. Increasing the ratio of atmosphere-to-ocean grid resolution can lead to an underestimation of the THFF, by as much as 75% for a 6:1 resolution ratio. Our results suggest that a large atmosphere-to-ocean grid ratio can result in an artificially weak dampening of mesoscale SST anomalies.

The marginal sea ice zone (MIZ) is a complex interface between the open ocean and the pack ice, where ocean-ice-atmosphere interactions are extremely complex. With a width of about 100 km, similar to the grid size of many climate models, the MIZ is not resolved in CMIP5 climate scenarios. In recent years, coupled climate models have been developed with ocean components at higher resolution, such as the high resolution version of the Met Office Global Coupled Model GC3 based on the GO6 configuration of the ORCA12 1/12° ocean model. We compare the MIZ representation in a coupled simulation with a simulation using the same ocean component forced by observed atmospheric data. Biases in the MIZ position and width are of the same order of magnitude in the coupled and forced model. The sea ice edge is strongly influenced by the ocean circulation and is often found at the wrong location, even when an observed atmospheric state is used to force the model. Despite a possible mismatch between atmosphere and ice/ocean in the forced model, due to the absence of feedback between sea ice and atmospheric temperature, surface heat fluxes in the MIZ are similar in amplitude in the coupled and forced simulations.  Our analysis focuses on the Greenland Sea because it is region of deep water formation, a major control of the Atlantic Meridional Overturning Circulation and thus very important for climate scenarios. The strong interannual variability of sea ice in the Greenland Sea is examplified by the Odden tongue, a protrusion of sea ice extending northeastward away from the Greenland continental slope. The coupled model exhibits such interannual variability, with a sea ice concentration larger than observed on average. The relationship between atmosphere, ice concentration and mixed layer depth is analyzed to assess the performance of both coupled and forced 1/12° models to represent deep water formation in the Nordic seas.