Decadal variability in the North Atlantic is studied in a series of global coupled ocean-atmosphere model simulations aided by a modeling surgery approach. Both observations and the model show the North Atlantic exhibits two distinctive modes: an interannual to decadal tripole mode and a decadal to multidecadal basin-scale monopole mode. Our modeling surgery study found that the tripole mode originates predominantly from atmospheric internal variability known as the North Atlantic Oscillation with no need to invoke ocean-atmosphere coupling. In contrast, the decadal variability of the monopole depends critically on ocean-atmosphere coupling. The transition from the monopole to the tripole tends to be associated with the mean oceanic circulation. Analogy to the North Pacific decadal variability is then discussed.