ATMOSPHERE-OCEAN INTERACTION IN THE NORTH-ATLANTIC - NEAR-SURFACE CLIMATE VARIABILITY

The impact of an interactive ocean on the midlatitude atmosphere is ex amined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 20 degrees and 60 degrees N) cou pled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle o f sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months. Coupling does not change the mean win tertime model climatology (December-February); however, it does have a significant impact on model variance. Air temperature and mixing rati o 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 en hancement in the persistence of air temperature anomalies on interannu al timescales as a result of coupling. In the North Atlantic sector, s urface air and ocean temperature anomalies during late winter are unco rrelated with the following summer but are significantly correlated (0 .4-0.6) with anomalies during the following winter. These autocorrelat ions are consistent with the ''re-emergence'' mechanism, where late wi nter ocean temperature anomalies are sequestered beneath the shallow s ummer 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 tem perature anomalies on monthly timescales also increases with coupling and is likely associated with ''decreased thermal damping.'' When coup led to the atmosphere, the ocean is able to adjust to the overlying at mosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.