Abstract
With widespread influence on global climate and weather extremes, the Madden-Julian Oscillation (MJO) plays a crucial role in subseasonal prediction. Our latest global climate models (GCMs), however, have great difficulty in realistically simulating the MJO. This model inability is largely due to problems in representation of MJO’s cumulus organization. This study, based on a series of idealized aqua-planet model experiments using an atmospheric-only GCM, clearly demonstrates that MJO propagation is strongly modulated by the large-scale background state in which the lower-tropospheric mean moisture gradient and zonal winds are critical. Therefore, when tuning climate models to achieve improved MJO simulations, particular attention needs to be placed on the model large-scale mean state that is also significantly affected by cumulus parameterizations. This study indicates that model biases in representing MJO propagation may be related to the widely reported double-ITCZ (intertropical convergence zone) problem in climate models.
Highlights
First detected in the 1970s and named after its two discoverers, the well-known Madden-Julian Oscillation (MJO)[1] is characterized by slow eastward propagating large-scale convective fluctuations along the equator with characteristic periods of 30–60 days, and has been recognized to have tremendous influence on global weather extremes[2]
Distinct MJO propagation in responding to sea surface temperatures (SSTs) patterns Forced by various zonally uniform SST distributions with their meridional profiles transitioning from QOBS to FLAT, a series of idealized experiments based on the ECHAM atmosphere-only GCM (AGCM) are conducted under an aqua-planet configuration
How the propagation of tropical intraseasonal variability relates to the large-scale environment is demonstrated based on a series of idealized model experiments using the ECHAM AGCM
Summary
First detected in the 1970s and named after its two discoverers, the well-known Madden-Julian Oscillation (MJO)[1] is characterized by slow eastward propagating large-scale convective fluctuations along the equator with characteristic periods of 30–60 days, and has been recognized to have tremendous influence on global weather extremes[2]. The observed eastward propagation of MJO convection and associated circulations could only be simulated by a limited number of global climate models (GCMs). As shown, the ECHAM atmosphere-only GCM (AGCM) produces weak propagation of MJO convection over the Indian Ocean when forced by observed climatological sea surface temperatures (SSTs). This is in contrast to the observed systematic eastward propagation of MJO convection from the Indian Ocean to western Pacific. The improved MJO representation achieved by tuning using such methods often occurs at the cost of a degraded model mean state and other climate phenomena[9]
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