Multidecadal variability:




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North Atlantic temperature & salinity anomalies:

  • Vary in opposition in 0-300 and 1000-3000m layers;
  • Vary coherently.
  • Overall warming/salinification since the mid-1940s;
  • Cooling/freshening since mid-1970s.
Time evolution of key parameters of the North Atlantic climate system. Striking similarity is apparent.

Temperature and salinity spatial multidecadal anomalies:

  • Single-sign.
  • Basin-wide.
Regression coefficients of (top) SST, (middle) sensible heat flux, and (bottom) SLP associated with long-term climate change and the multidecadal variability. Areas of cooler water are associated with the major North Atlantic Current system (top left panel) and this pattern is consistent with the hypothesis that the circulation is slowing due to climate change.
Patterns of zonal average North Atlantic Ocean temperature variability associated with the nonlinear trend (left) and multidecadal variability (right). Panel A shows EOF-1 (insert shows PC1 and CO2 anomalies). Panel C shows EOF-2 (insert shows PC2). The high-latitude (>50°N) long-term cooling (left) provides support for weakening of the North Atlantic circulation.
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.