Characteristics of Madden-Julian Oscillation during the mid-Holocene

Abstract

Using Community Earth System Model version 2.1(CESM2.1), this study investigated the changes in Madden-Julian Oscillations (MJO) in response to the natural forcing [orbital forcing & natural variability in GreenHouse Gas(GHG) concentrations]. To elucidate the variability in MJO, we have conducted two simulations, one is referred to the Pre-Industrial control (piC) and another one is mid-Holocene (MH) simulation (6000 years ago). The distinct boundary conditions of mid-Holocene from the Pre-Industrial simulation are orbital forcing and greenhouse gas(GHG) concentrations, and other conditions such as aerosol concentrations, geographical configuration and continental ice sheet extent are the same as those used in the Pre-Industrial simulation. Difference in GHG concentrations between mid-Holocene and Pre-Industrial is assumed due to natural variability. The results discussed in this study are November to April season (peak MJO). The CESM2.1 model able to simulate the mean rainfall compared to the recent observations. Seasonal mean rainfall is very similar in both periods except few pockets over Indo-Pacific warm pool region, similar results were found in MJO filtered rainfall anomalies also. During mid-Holocene Sea Surface Temperature(SST) and Surface air temperature were colder than Pre-Industrial this can be attributed to the reduction in solar radiation and less GHG concentrations during this period. MJO phase speed is ~3% weaker during mid-Holocene when compared to the Pre-Industrial, which is consistent with the previous studies which find the increase in the phase speed in warming climate. The zonal scale (wave number) of MJO is slightly higher in mid-Holocene which is consistent with weaker phase speed. The slow eastward propagation of the MJO and weaker signal strength in MJO wave band during mid-Holocene compared to the Pre-Industrial is associated the reduction in meridional moisture gradient. The weaker meridional moisture gradient induces lesser diabetic heating (Matsuno-Gill response) and inturn altering the horizontal moisture gradient and playing a critical role in modulating of MJO propagation and wave numbers.