Hunga Tonga impacts: Will the eruption alter our winter?

On January 15th 2022, the strongest volcanic eruption in recent history took place in the South Pacific. However, research suggests that the Hunga Tonga eruption could in fact determine our weather this winter.

It is now thought that the Hunga Tonga eruption might affect Earth's climate, following research from a team at the California Institute of Technology this summer. Since the eruption released little sulphur dioxide, scientists assumed that the normal cooling effect of large volcanic eruptions would hardly be felt.

However, the volcano also emitted unusually large amounts of water vapour into the stratosphere, which could have the opposite effect. This is because water vapour is a very powerful greenhouse gas, and any significant increase in it would inevitably lead to warming of the lower layers of the atmosphere.

A measurable cooling would then be expected in the upper, dry stratosphere, which is what researchers have been able to prove; the beginning of a large-scale cooling of the stratosphere over Earth's southern hemisphere.

This stratospheric cooling has now reached unprecedented levels, and this effect could spread into the northern hemisphere over the coming months.

This may affect the behaviour of a phenomenon called the North Atlantic Oscillation (NAO), increasing the tendency to develop a negative NAO index. This would mean that the pressure difference between the low-pressure area near Iceland and the Azores high will decrease, leading to more blocking weather patterns in Europe.

Blocking flow patterns in the winter months can often, but not always, lead to rather cold weather conditions in north-west Europe.

However, because the Hunga Tonga submarine volcanic eruption is a complex phenomenon that has not yet been observed on a large scale, any conclusions drawn from this event should be interpreted with great caution.

Because on the flip side, a permanently blocking high-pressure area could also lead to a very mild winter, if, for example, it anchors over south-eastern Europe, instead of over the British Isles. Additionally, the expected effects on the NAO could be delayed or postponed until spring, so winter could be more Atlantic driven, i.e. mild and humid.

So, the question on this winter remains open, with only tentative possibilities based on the scarce findings from the Tonga eruption.