MJO and the Maritime Continent – Air-Sea-Land Interactions and Impact on the Diurnal Cycle and MJO Propagation

Abstract:

The Madden-Julian Oscillation (MJO) is a dominant mode of intraseasonal variability in the tropics. Large-scale convection fueling the MJO is initiated over the tropical Indian Ocean and propagates eastward across the Maritime Continent (MC), toward the western Pacific Ocean. Observational studies have shown that 40-50% of MJO events dissipate over the MC, and some weaken in the process – this is known as the MC barrier effect. As an eastward-propagating MJO event passes over the MC, the MJO’s nature may be altered by its interaction with the complex topography and strong diurnal circulation over the islands. In turn, the MJO passage and its large-scale circulation can modulate precipitation and circulation patterns over the MC.

This study aims to improve our understanding of the interactions between the diurnal cycle (DC) of precipitation, complex topography of the MC, and the passage of MJO over the region. We use a combination of observations (TRMM TMPA, GPM IMERG, CMORPH), global model reanalysis products (ERA5, MERRA-2), and a regional coupled atmosphere-ocean-land model at convection-permitting resolution. Satellite observations and reanalysis fields provide a robust climatology over 20 years. The coupled model simulation of the Nov-Dec 2011 MJO event is compared with model experiments based on scenarios where topography is flattened, and where land is replaced with ocean, which allow us to better understand the physical processes controlling the DC and MJO propagation across the MC.

Satellite observations show that the DC of precipitation over the MC is strongly driven by diurnally varying land-sea and mountain-valley circulations. An afternoon precipitation maximum develops over the coast and propagates inland, while morning precipitation propagates offshore. During MJO passage, MC precipitation is enhanced, especially over water, in the early morning. In reanalysis products, precipitation is exaggerated over land, and peak precipitation timing is shifted compared to observations; the coupled model control simulation captures the timing and location of the DC, though the amount of precipitation is exaggerated. Model simulations indicate that the presence of land, and its associated DC, is more detrimental to MJO propagation than the presence of high terrain over the MC.