Decadal air-sea interaction in the North Atlantic based on observations and modeling results

The decadal, 12-14-yr cycle observed in the North Atlantic SST and tide gau ge data was examined using the NCEP-NCAR reanalyses, the Comprehensive Ocea n-Atmosphere Data Set, and an ocean model simulation. SST data are contrast ed with tide gauge data, which shows that in SST the decadal mode is nonsta tionary with strong variability for the last 40-50 yr, but the sea level at the southeastern U.S. coast exhibits a robustly regular variability at thi s period for the last 75 yr. Thus, sea level variability gives credibility to the existence of a decadal mode. The main finding is that this 12-14-yr cycle can be constructed based on the leading mode of the surface heat Bur, which implicates the participation of the thermohaline circulation. This c ycle has potential to be a coupled mode because three necessary aspects of a coupled mode are found: a positive feedback between the atmosphere and oc ean in the subpolar gyre, a negative feedback of the overturning variabilit y on itself, and a delayed adjustment through midlatitude Rossby waves to t hese processes. During the cycle, starting from the positive index phase of the North Atlantic oscillation (NAO), positive SST and oceanic heat conten t anomalies exist in the subtropics. The warm anomalies advect to the subpo lar eyre where they are amplified by local heat flux, that is, a positive f eedback between the atmosphere and ocean. At the same time the advection of warm anomalies to the subpolar gyre constitutes of a negative feedback of the thermohaline circulation on itself. The effect of this internal feedbac k of the ocean is amplified by the positive feedback between the atmosphere and ocean. Consequently the oceanic thermohaline circulation slows down an d the opposite cycle starts. The adjustment to the changes in overturning i s mediated by midlatitude Rossby waves. They are responsible for the subtro pical heat content anomalies that later advect to the high latitudes. This analysis suggests that the two principal modes of heat flux variability, co rresponding to patterns similar to the NAO and the Western Atlantic, are pa rt of the same decadal cycle.