Atmosphere–Ocean Interaction in the North Atlantic: Near-Surface Climate Variability*

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 208 and 608N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months. Coupling does not change the mean wintertime model climatology (December–February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air–sea interaction has a greater impact on seasonally averaged variance than monthly variance. There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4–0.6) with anomalies during the following winter. These autocorrelations are consistent with the ‘‘re-emergence’’ mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies. The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with ‘‘decreased thermal damping.’’ When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.