MULTIDECADAL VARIABILITY IN THE NORTH ATLANTIC OCEAN
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Climate change in the North Atlantic Ocean has wide-spread implications for Europe, Africa, and the Americas. Our studies demonstrate that recent warming over the North Atlantic is linked to both anthropogenic climate change and multidecadal variability (~50-80 years). Multi-decadal fluctuations are prevalent in the upper 3000 m of the North Atlantic Ocean. Spatially averaged temperature and salinity from the 0-300 m and 1000-3000 m layers vary in opposition: prolonged periods of cooling and freshening (warming and salinification) in one layer are generally associated with opposite tendencies in the other layer, which is consistent with the notion of thermohaline overturning circulation. For example, in the 1990s, widespread cooling and freshening was a dominant feature in the 1000-3000 m layer, whereas warming and salinification generally dominated the upper 300 m layer, except for the subpolar North Atlantic where complex exchanges with the Arctic Ocean occur. The single sign basin-scale pattern of multi-decadal variability is evident from our analyses. Our results suggest a general warming trend of 0.012C per decade in the upper 3000 m North Atlantic over the last 55 years of the 20th century, although during this time there are periods in which short-term trends are strongly amplified by multi-decadal variability. The multidecadal variability accounts for ~60% of North Atlantic warming since 1970. In contrast, the overall long-term warming trend exhibits a pattern of cooling in regions associated with major northward heat transports, consistent with a slowdown of the North Atlantic circulation. This localized cooling has been masked in recent decades by warming during the positive phase of multidecadal variability. The next cold phase could induce a cooler North Atlantic, having serious implications for climate over Europe. Finally, since the North Atlantic Ocean plays a crucial role in establishing and regulating global thermohaline circulation, the multi-decadal fluctuations of the heat and fresh water balance should be considered when assessing long-term climate change and variability, both in the North Atlantic and at global scales.