Mid-Holocene ENSO Variability reduced by northern African vegetation
changes: a model intercomparison study
Abstract
The relationship between the mean state of the Pacific Ocean and El Niño
Southern Oscillation (ENSO) and its variability through time is
inadequately understood, especially on longer timescales. Several
studies have indicated that the mid-Holocene (6,000 years before
present) was characterized by stronger east-west temperature contrast
and lower ENSO variability relative to the present day. While climate
models show a reduction in ENSO variability, they underestimate this
reduction compared to many paleoclimate reconstructions. Further, the
drivers behind these changes remain unclear. In this work, we use five
global climate models to show that incorporating vegetation changes over
northern Africa during the mid-Holocene are vital to capturing global
circulation changes. Greening the Sahara alters the Walker Circulation,
enhancing zonal temperature and pressure gradients in the equatorial
Pacific and driving it to a La Niña-like state. Incorporating Green
Sahara boundary conditions leads to reductions in interannual
variability in all Niño index regions relative to orbital and GHG
changes, with reductions of up to 18% in the Niño3.4 region. Our work
highlights the importance of the Atlantic influence on ENSO and provides
paleoclimatic evidence for this synergistic teleconnection.