Altered ENSO flavors during the mid-Holocene revealed by model-proxy synthesis

Abstract:

The great majority of paleoclimate records from the eastern Pacific suggest a drastic reduction in ENSO variability during the mid-Holocene (around 6 ka BP). However, new coral records from the central Pacific challenge this notion; they show that changes in mid-Holocene ENSO variability lie within the range of variability seen in the Late Holocene. Using a combination of proxy records, modern observations, and climate model simulations we show that conflicting evidence from the east and the central Pacific could be explained by a differential response of the so-called ENSO flavors (Central and Eastern Pacific ENSO) to orbital forcing. We first re-interpret the proxy records based on their location: we argue that eastern Pacific proxies record solely the impact of EP ENSO events on sea surface temperature and precipitation, while western Pacific coral records most likely capture their influence on sea surface salinity. The central Pacific is more complex since CP and EP events have an impact there via temperature and salinity respectively. Aided by a quantitative method that combines observed patterns of variance we show that the proxies are consistent with a large reduction in ENSO’s EP flavor and a not so dramatic increase in the CP flavor. These results are in agreement with climate model simulations of the mid-Holocene, which indeed show a large reduction in the frequency of EP events, and a small increase in the frequency of CP events in response to orbital forcing.