Strengthening of West African Monsoon during Green Sahara period may have affected El Niño Southern Oscillation

Changes in the El Niño Southern Oscillation (ENSO) -- an important driver of large-scale climate variability -- have broad impacts on society and eco-systems globally. Both observations and model simulations suggest that with the current trend in global warming, we may see changes in ENSO behaviour. Understanding how ENSO has varied historically and the causes of this variability is paramount for predicting the future.

Many paleoclimate records from the warm Mid-Holocene (4,000 -- 7,000 yrs BP) show that variations in ENSO were reduced by 30%-60% compared to pre-industrial times. This is not accurately captured by most climate models, which show a modest reduction of 10% using only changes in Earth's orbital parameters.

Our study accounts for a vegetated and less dusty Sahara. This reduces the variability of the Mid-Holocene ENSO with up to 25% compared to the pre-industrial, more than twice the decrease found by using orbital forcings alone, says Francesco S.R. Pausata, researcher at the Department of Meteorology at Stockholm University (MISU).

In the study four model simulations with varying forcings were compared, from using orbital parameter changes alone to a simulation that included orbital forcings, added vegetation and reduced dust emissions. The researchers found a tight link between the intensity of the climatological West African Monsoon, the strength and position of the Walker circulation and the variability of ENSO.

The results of the study show that the strengthening of the West African Monsoon, associated with the greening of the Sahara, alters the tropical Atlantic mean state and variability. This in turn affects ENSO activity through changes in the Walker circulation, explains Francesco S.R. Pausata.

Therefore, vegetation and dust feedbacks are important players in amplifying ENSO's response to insolation forcing.

More proxy records from both the Pacific and the Atlantic Ocean are critically needed to capture the natural variability of ENSO and its teleconnections with the Atlantic basin. These will provide a better understanding of the ENSO spatio-temporal characteristics through time. Together with improved model simulations that account for vegetation and dust changes, this will improve our prediction of future climate change, Francesco S.R. Pausata concludes.