Reply on RC1

Abstract

Analyses of observational data (from year 1870 AD) show that Sea Surface Temperature (SST) anomalies along the pathway of Atlantic Water transport in the North Atlantic, the Norwegian Sea and the Iceland Sea are in-phase at multidecadal time scales. In-phase SST anomaly relationships are also observed over hundreds of thousands of years during parts of the Pliocene (5.23–5.03, 4.63–4.43 and 4.33–4.03 Ma). However, when investigating CMIP6 SSP126 future scenario runs (next century) and Pliocene reconstructions (5.23–3.13 Ma), three additional phase relations emerge: 1) The Norwegian Sea is out of phase with the North Atlantic and the Iceland Sea (Pliocene; 4.93-4.73 and 3.93–3.63 Ma); 2) The Iceland Sea is out of phase with the North Atlantic and the Norwegian Sea (Pliocene; 3.43–3.23 Ma); 3) The North Atlantic is out of phase with the Norwegian and Iceland Seas (future trend). Hence, out of phase relationships seem to be possible in equilibrium climates (Pliocene) as well as in response to transient forcing (CMIP6 SSP 126 low-emission future scenario). Since buoyancy is a key forcing for inflow of Atlantic Water to the Norwegian Sea, we investigate the impacts of buoyancy forcing on the phase relation between SST anomalies in the North Atlantic, Norwegian and Iceland Seas. This is done by performing a range of idealized experiments using the Massachusetts Institute of Technology general circulation model (MITgcm). Through these idealized experiments we can reproduce three out of four of the documented phase relations: in-phase relationships under weak to intermediate fresh water forcing over the Nordic Seas; the Iceland Sea out of phase with the North Atlantic and the Norwegian Sea under weak atmospheric warming over the Nordic Seas; and the North Atlantic out of phase with the Norwegian and Iceland Seas under strong atmospheric warming over the Nordic Seas. We suggest that the unexplained phase relation, when the Norwegian Sea SSTs are out of phase with the North Atlantic and the Iceland Sea, may reflect a response to a weakened Norwegian Atlantic Current compensated by a strong Irminger current, or an expanded East Greenland Current.