FREQUENCY-RESPONSE PROPERTIES OF FORCED CLIMATIC SST ANOMALY VARIABILITY IN THE NORTH-ATLANTIC

Frequency response properties of North Atlantic (5 degrees-57 degrees N) sea surface temperature anomaly (T-sa) variability with periods of several months to 20 years are characterized using the Comprehensive O cean-Atmosphere Data Set (GOADS). Significant direct forcing of T-sa v ariability by the anomalous wind field (primarily through the resultin g anomalous surface turbulent heat flux) is observed in the westerly w ind and trade wind belts. To characterize properties of the large-scal e climatic T-sa response to this forcing over the entire frequency ban d resolved, it is necessary to consider the dual role of anomalous sur face heat flux as both the dominant local forcing mechanism and the do minant damping mechanism, the latter through a negative linear feedbac k (Newtonian relaxation). At frequencies where wind forcing is importa nt, good agreement exists between the frequency response function esti mated from data and the same function theoretically predicted by a sim ple stochastic forcing model where the locally forced response is damp ed by a negative linear feedback with a decay time scale of 3 mo. To m ake this comparison, the total anomalous surface heal flux represented by the standard bulk formula was decomposed into two components, one primarily representing the local wind forcing and the other primarily representing negative feedback damping. In the westerlies, wind forcin g is effective over periods from several months to 8 yr, primarily 2-4 yr, and is ineffective at periods of 8-20 yr. These fluctuations are primarily forced in the western part of the basin then propagate to th e east and northeast across the Atlantic at a characteristic speed of 6 km day(-1). When time series of winter-only T-sa are analyzed, howev er, wind forcing of winter to winter T-sa variability remains signific ant at decadal and longer periods. In the trades, wind forcing is effe ctive over periods from 8 mo to 13.3 yr, primarily 2-3 yr and 7-13.3 y r, and significant seasonal differences are not observed.