A research team from the Bjerknes Centre for Climate Research in Bergen, Norway, has studied the climate in the North Atlantic region over the past 600 years using the Bergen Climate Model and the observed temperature evolution. They point to changes in the solar intensity and explosive volcanic eruptions as important causes for climate variations in the North Atlantic during this period.
The sun, volcanoes or ocean currents?
The traditional and common view is that climate variations in the North Atlantic lasting adecade and more, is governed by changes in the large-scale ocean circulation. The presented analysis supports this common perception, but only when the climate effects from changes in the solar intensity and volcanic eruptions are left out. When the scientists include actual changes in the solar forcing and the climate effect of volcanic eruptions in their model they find a strong causal link between these external forcings and variations in the Atlantic surface temperature. In particular, the study highlights volcanic eruptions as important for long-term variations in the Atlantic climate both through their strong cooling effect, but also through their direct impact on atmosphere and ocean circulation.
Regional climate variations and ocean temperatures
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| Odd Helge Otterå |
A wide range of regional climate variations of high societal importance have been linked to temperature variations in the North Atlantic. These include periods of prolonged droughts in the U.S., changes in European summer temperatures, long-term changes in the East-Asian monsoon and variations in the intensity of Atlantic hurricanes. The governing mechanisms behind such long-term variations are, however, not well understood.
The study, which was presented in Nature Geoscience, provides new insight into the causes and timing of long-term variations in the Atlantic and, consequently, for the potential for developing decadal prediction schemes for the Atlantic climate.
Warming during the 20th century
The study also shows that the observed warming in the North Atlantic during the 20th century cannot be explained by the solar and volcanic forcings alone. In the model the increased emissions of CO2 and other well-mixed greenhouse gases to the atmosphere since the onset of the industrial revolution have to be included in order to simulate the observed temperature evolution.
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Figure1. Simulated Atlantic multidecadal variability. The upper panel shows the variations in North Atlantic Ocean basin wide sea surface temperatures in a simulation that includes historical variations in total solar irradiance and volcanic aerosols (blue), and in a simulation that in addition to the natural external forcings also include anthropogenic forcings for the last 150 years (red). Up to year 1900, the blue curve is consistent with available temperature observations, whereas only the red curve matches the observed temperature evolution in the 20th century. The lower panel shows variations in the large-scale ocean circulation in the Atlantic (black) in the simulation that includes natural external forcings for the last 600 years. Large volcanic eruptions (illustrated on the time-axis) tend to strengthen this large-scale ocean circulation. |