Role of the re-emerging SST anomalies on the winter-to-winter persistence of the NAO (Invited Presentation)

In the extratropics, thermal anomalies stored in the deep winter oceanic mixed layer persist at depth through summer where they are insulated from surface fuxes. They become reentrained back in the deepening mixed layer during the following winter and their reemergence explains part of the winter-to-winter persistence of the extratropical Sea Surface Temperature anomalies. The forcing of the oceanic reemergence on the atmosphere is investigated here in the North Atlantic using a simplified coupled model (Atmospheric Global Circulation Model coupled to a Mixed Layer Ocean Model and to a thermodynamical ice component). Such a model configuration takes the vertical oceanic processes into account and correctly represents the physics of the ocean-atmosphere interaction at the interface. Estimation of the thermal anomalies created by late-winter/early spring atmosphere and stored below the hightly stratified summer thermocline are obtained from a long control simulation. They strongly project on the so-called North Atlantic tripole associated with the phase of the North Atlantic Oscillation (NAO). These thermal anomalies are applied below 40meter depth and north of 25N, in the oceanic initial conditions of a 60-member ensemble of 1yr integration starting in August. We show that their reemergence occurs in November/December and has a significant impact on the model atmosphere. The reentrainment of the subsurface oceanic anomalies tends to favor the same phase of the NAO which created them during the previous winter. The simulated atmospheric response would confirm the hypothesized role of the oceanic reemergence in the weak but significant year-to-year persistence of the wintertime NAO. Model results suggest a probable role of the high-frequency atmospheric eddies. Storminess is clearly modify in the perturbed ensemble and eddy-mean flow interactions would explain the large-scale and persistent atmospheric response to the extratropical North Atlantic reemergence.