Regional climate variations such as the long period of draught in the USA during the 1930s, heat waves in Europe during recent years and variations in the strength of tropical hurricanes are connected with temperature variations in the North Atlantic. Little is known however, about why these sea-surface temperature variations occur.

Scientists have previously assumed that decadal climate variations in the Atlantic are the result of changes in large ocean currents.  Climate researchers from the UiB, the Bjerknes Centre and the Nansen Centre have now shown that violent volcanic eruptions influence sea-surface temperatures in the North Atlantic.

- The effect that volcanic eruptions have on the climate is largely due to the emission of sulphurous gasses into the atmosphere.  These gasses are quickly transformed into small particles that reflect some of the incoming sunlight so that the average global temperature is lowered, says Odd Helge Otterå who is a scientist at the Bjerknes Centre for Climate Research.

The results of the study were recently published in Nature, Geoscience, with Otterå’s name mentioned first.

Volcanoes affect the winds
But it appears that volcanoes also give some areas of the planet warmer weather.

- A big volcanic eruption in the tropics will give warmer winters and more rainfall in North West Europe.  This is because volcanic eruptions strengthen westerly winds, causing more storms and more warm-air currents to develop in the Atlantic – especially during the winter, says Professor Helge Drange of the Geophysical Institute, UiB.

But the effect of a volcanic eruption is largely dependent upon where it takes place.  In the tropics, volcanic particles are easily and effectively transported up into the higher levels of the atmosphere by the rising air masses, and are subsequently transported to both polar areas.  The particles will therefore be spread over larger areas and have a greater influence than volcanic eruptions closer to polar areas.

- The eruption of Eyafjallajökull this spring is only a firecracker when compared with big volcanic eruptions in the tropics, like the eruption in the Philippines in 1991 or Indonesia in 1815, says Odd Helge Otterå.

Simulated 600 years
Otterå and Drange were part of a research group that compared, observed and simulated climate variation in the North Atlantic 600 years back in time.  The group used the Bergen climate model to simulate the climate.  This model couples data from the atmosphere and ocean using mathematical models.

Ocean circulation became the most important factor for understanding the simulated temperature variations in the ocean, when external climate influences from the sun, volcanoes and greenhouse gasses were excluded.

On the other hand, when the scientists took historical variations in solar and volcanic activity into consideration, it became apparent that they had a big influence on the ocean temperature in the Atlantic.

- It’s perhaps not so surprising that the sun has an effect on ocean temperature. That volcanoes play such an important role concerning both wind and ocean circulation in the North Atlantic, is perhaps more surprising, says Otterå.

Gives better weather forecasting
The most important result of this study with regard to research is that we must take into account both volcanoes and the sun if we are to understand weather analyses from earlier periods in time, according to Helge Drange.

- We will in addition have a much better weather forecasting system in a10 to 20 years perspective, when changes in solar irradiation and the effect of volcanic activity are taken into account, according to Drange.




The upper part of the diagram shows variations in sea-surface temperature in the North Atlantic in a model that includes historical variations in solar irradiance and the effects of volcanic eruption (blue curve), and in a model that in addition to the natural climate drivers also includes mankind’s emissions of climate gasses during the last 150 years (red curve).  The bottom diagram shows variations in the strength of ocean circulation in the Atlantic Ocean.  This often increases after a violent volcanic eruption.

The warming-up process that was observed in the North Atlantic during the 1900s is not only due to solar and volcanic activity.  Mankind’s emission of CO2 and other greenhouse gasses must also be included in the model in order to show a temperature development that corresponds with the observations.