The Madden-Julian oscillation (MJO) is the most prominent mode of intraseasonal variability in the tropical and even global climate system, with quasi-oscillating periods of 30-90 days.
Its variability over longer time scales and its changes with the increasing background temperature are important for predicting future climate anomalies and variability under global warming scenarios.
The short time-span of modern instrumental data limits our understanding of the MJO and obtaining MJO information from natural archives could extend this greatly.
Recently, a research team led by Prof. YAN Hong from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences detected the Madden-Julian Oscillation Signal in a natural archive of Giant Clam Shell (Tridacna spp.) for the first time.
The Giant Shell was collected from the northern South China Sea, in the western Pacific, with a life span about two years (from January 29, 2012 to December 9, 2013).
The researchers developed several daily to hourly resolution biological and geochemical proxy records, including the daily growth rate, hourly Fe/Ca, Sr/Ca and fluorescence intensity, to compare these with local weather/climate records.
They found that daily to hourly resolution biological and geochemical proxy records could clearly record MJO variability in the tropics. Significant MJO periods were detected from the daily growth rate (DGR), hourly Fe/Ca and fluorescence intensity time series.
The connection between the daily growth rate and MJO was possibly linked through the local effective solar radiation, while the connections between the Fe/Ca, fluorescence intensity and MJO were possibly linked through the local wind speed intensity and associated vertical mixing in the upper ocean.
Their findings provide a tool with the potential to study the MJO beyond the instrumental records. Fossil Tridacna shells from beyond the period of instrumental records may be used to investigate a fuller range of variability of MJO activity.