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Abstract

We describe in this paper a mechanism for decadal climate variability that can lead to decadal climate cycles in the North Pacific and North Atlantic Oceans. A hierarchy of numerical models and observations are used to understand the fundamental dynamics of these decadal cycles. They are generated by large-scale ocean-atmosphere interactions in mid-latitudes and must be regarded as inherently coupled modes. The memory of the coupled system, however, resides in the ocean and is associated with slow changes in the subtropical ocean gyres.

When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the western boundary current and its extension, leading to a positive SST anomaly in mid-latitudes. The atmospheric response to this SST anomaly involves a weakened storm track and the associated changes at the air-sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The atmospheric response, however, consists also of a wind stress curl anomaly which spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The existence of such decadal cycles provides the basis of long-range climate forecasting at decadal time scales.