Buoyancy forcing: a key driver of northern North Atlantic sea surface temperature variability across multiple timescales

Analyses of observational data (from year 1870 AD) show that seasurface temperature (SST) anomalies along the pathway of Atlantic Watertransport in the North Atlantic, the Norwegian Sea and the Iceland Sea arespatially coherent at multidecadal timescales. Spatially coherent SSTanomalies are also observed over hundreds of thousands of years during partsof the Pliocene (5.23-5.03, 4.63-4.43, and 4.33-4.03 Ma). However, wheninvestigating CMIP6 (Coupled ModelIntercomparison Project 6) SSP126 (Shared Socioeconomic Pathway) future scenario runs (next century) and otherPliocene time intervals, the following three additional SST relations emerge: (1) theNorwegian Sea SST anomaly is dissimilar to the North Atlantic and theIceland Sea SST anomalies (Pliocene; 4.93-4.73 and 3.93-3.63 Ma), (2) theIceland Sea SST anomaly is dissimilar to the North Atlantic and theNorwegian Sea SST anomalies (Pliocene; 3.43-3.23 Ma), and (3) the North AtlanticSST anomaly is dissimilar to the SST anomalies of the Norwegian and Iceland seas(future trend). Hence, spatially non-coherent SST anomalies may occur inequilibrium climates (Pliocene), as well as in response to transient forcing(CMIP6 SSP126 low-emission future scenario). Since buoyancy is a keyforcing for the inflow of Atlantic Water to the Norwegian Sea, we investigatethe impacts of buoyancy forcing on spatial relations between SST anomaliesseen in the North Atlantic and the Norwegian and Iceland seas. This is done byperforming a range of idealized experiments using the MassachusettsInstitute of Technology general circulation model (MITgcm). Through theseidealized experiments we can reproduce three out of four of the documentedSST anomaly relations: being spatially coherent under weak to intermediatefreshwater forcing over the Nordic Seas, the Iceland Sea being dissimilar to theNorth Atlantic and the Norwegian Sea under weak atmospheric warming over theNordic Seas, and the North Atlantic being dissimilar to the Norwegian and Icelandseas under strong atmospheric warming over the Nordic Seas. We suggest thatthe unexplained SST anomaly relation, when the Norwegian Sea is dissimilarto the North Atlantic and the Iceland Sea, may reflect a response to aweakened Norwegian Atlantic Current compensated for by a strong IrmingerCurrent or an expanded East Greenland Current.