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A recent study published in Geophysical Research Letters has found that volcanic eruptions in tropical regions, specifically those occurring at 23°N/S of the equator, can have a significant impact on global-scale climate cycles in the Indian Ocean. The research reveals that these eruptions can disrupt ocean-atmosphere climate interactions, such as El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), for almost a decade before returning to pre-eruption baseline levels.

The Indian Ocean Dipole (IOD) is characterized by a contrast in sea surface temperatures, with cooler temperatures in the eastern Indian Ocean and warmer temperatures in the western region. During the positive phase of IOD, this contrast leads to changes in temperature, precipitation, and wind patterns in nearby regions, resulting in floods in East Africa and drought in East Asia and Australia. The negative phase of IOD reverses these conditions. The study found that intense volcanic eruptions in the tropics induce a negative IOD in the year of the eruption, followed by a positive phase the next year. These IOD anomalies persist for 7-8 years after the eruption before returning to pre-eruption conditions.

Another climate cycle, the Interdecadal Pacific Oscillation (IPO), which spans both hemispheres, also influences the impact of volcanic eruptions on IOD. When the IPO is in a positive phase, the tropical Pacific Ocean becomes warmer, while northern regions become cooler. A negative IPO phase leads to a stronger negative IOD, and a positive IPO enhances positive IOD responses. The strength of the initial IOD response is dependent on the tropical Pacific sea surface temperature during IPO.

The study also highlights the influence of volcanic eruptions on El Niño and La Niña conditions associated with ENSO. Following large tropical eruptions, there is a temperature gradient between land and sea in Africa and the Indian Ocean, affecting westerly trade winds and resulting in El Niño warming in the first year after the eruption. Thereafter, La Niña conditions dominate. The simulations showed that the ENSO response lags behind the positive IOD by 2 months, and negative IOD coincides with strong La Niña conditions from years 3 to 5 post-eruption.

The depth of the thermocline, an abrupt temperature gradient, within the Indian and Pacific oceans also affects sea surface temperatures and climate responses. Positive IPO conditions result in a shallower thermocline in the eastern Indian Ocean, weakening the sea surface temperature gradient and neutralizing the post-eruption IOD. Negative IPO conditions, on the other hand, lead to a deeper thermocline, strengthening the sea surface temperature gradient and predisposing the Indian Ocean basin for stronger negative IPO events post-eruption. These impacts are most pronounced in the first year after the eruption and then taper off.

It is worth noting that the timing of volcanic eruptions also plays a role in their climatic impact. Eruptions occurring in boreal spring (March-May) are most likely to impact the IOD/ENSO response in the same year, while those occurring later may have a delayed or less pronounced climatic impact.

In addition to affecting climate, volcanic eruptions release aerosols that impact global radiative forcing, the balance between incoming and outgoing solar radiation. This results in atmospheric cooling post-eruption, which can last for months or years. Therefore, the forcing on IOD/ENSO must be strong for it to outweigh the impact of reduced temperatures.

Understanding the impact of tropical volcanic eruptions on global climate cycles is crucial for regions prone to eruptions. It provides valuable insights for conducting risk assessments and preparing for the resulting extreme climate events. By being prepared, the environmental and societal impacts of these eruptions can be mitigated.

Source: Benjamin H. Tiger et al, Tropical Volcanic Eruptions and Low Frequency Indo-Pacific Variability.

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