The oceanic orgins of the Atlantic multidecadal Oscillation

Friday, December 15, 2017 - 09:00
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The relative roles of the ocean and the atmosphere in driving the Atlantic Multidecadal Oscillation (AMO) have been debated extensively. Past approaches to attribution have often focused on dynamic atmosphere and ocean processes in isolation. Here, we turn our attention to the fully-coupled atmosphere-ocean system, and how the nature of interactions between the atmosphere and ocean may drive low-frequency variability over the North Atlantic. We introduce a novel experimental design to isolate, in turn, the impact of atmospheric dynamics and the impact of ocean circulation variability on fully-coupled air-sea interactions over the North Atlantic. We introduce a partially-coupled framework in which SST anomalies induced by ocean circulation variability are removed from coupling, such that only passive SST variations driven by surface fluxes induced by the atmosphere remain. By comparing this to a fully-coupled simulation, we decompose the fully-coupled North Atlantic SST and surface flux variability into a part due to ocean-driven variability and another that is atmosphere-driven. Variable SST anomalies induced by the ocean circulation show more power at low frequencies (25- to 50-yr periods) while those due to the atmosphere have more power at higher frequencies (10- to 20-yr periods). The ocean-induced low frequency variability dominates air-sea interaction in the fully-coupled simulation, such that surface turbulent fluxes have more power at low frequencies in the fully-coupled simulation compared to the partially-coupled. This ocean-forced surface flux variability damps the oceanic SST anomaly inducing it, such that the fully-coupled low frequency power is only slightly stronger than that due to the atmosphere by itself (i.e. in the partially-coupled simulation). Our results suggest that low frequency variability of the AMO depends on the strength of the air-sea coupling among models, and that this variability is largely driven by the ocean.

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