MJO-like Variability in the Eastern Pacific during the Pliocene.

Abstract

The Madden-Julian Oscillations (MJO) is an important oscillation mode of intraseasonal variability in the tropics and interacts with many weather systems globally. The MJO is characterized by a slowly (typically 5m/s) eastward propagation band of convection. Previous studies that have found the effects of the increasing of tropospheric temperatures in association with increasing greenhouse gases(GHG) concentration on the MJO revealed that its phase speed, amplitude and precipitation variance increasing with tropospheric warming. However, the variability of MJO in a warming environment largely unknown. In this study using Community Earth System Model version 2.1(CESM2.1) we focused on the variability of MJO during Pliocene(5-2 Million years ago) which is analog of near future climate. Global temperatures during Pliocene are 3 C warmer than the present climate and CO2 concentrations are 350-500 ppm. To elucidate the variability in MJO, we have conducted two simulations, one is referred to the Pre-Industrial control (piC) and another one is Pliocene simulation. The results discussed in this study are November to April season (peak MJO). Usually, MJO associated precipitation initiates over the western Indian Ocean and moves eastward through the Maritime continent, and dissipates over the central Pacific as it encounters cooler sea surface temperatures(SST), we found that during Pliocene MJO activity over central and eastern Pacific significantly increased whereas decreased over the Indian Ocean and western Pacific. The decrease in zonal and meridional temperature gradients in the tropics during Pliocene significantly reduced the rainfall in warm pool region. The abnormal warming during Pliocene over the cold tongue region (east tropical Pacific) favored the eastward propagation of the MJO. The phase speed of MJO increased by nearly 2.5 m/s. The variance of rainfall significantly decreased during Pliocene over the Indian Ocean and warm pool region and increased over eastern Pacific when compared to the pre-Industrial period. It is also found that, MJO associate convective activity is mostly confined to the north of the equator