Surface precipitation anomalies over Maritime Continent islands typically lead oceanic precipitation by a week in the form of dipolar pattern before the arrival of Madden-Julian Oscillation (MJO) convective phase. The authors focus on this dipolar pattern over Borneo during the boreal winter MJO event in January-February 2017 using cloud-permitting modeling, observation and reanalysis datasets. The diurnal cycles of precipitation are analyzed during the local growing and decaying stages of this MJO event. Both the observation and simulation show positive precipitation anomaly at the southwestern Borneo and negative anomaly at the northeastern Borneo associated with the MJO easterly in the growing stage, while the pattern reverses in the decaying stage. Due to relatively high terrain, the low-level flows over Borneo split near the topography on the diurnal time scale. During the late afternoon and night (17–20 Local Solar Time), the splitting flow induced wake vortices and thermally-driven sea breezes tend to converge at the leeside, both contributing to leeward convergence and precipitation, which peaks at the midnight. Subsequent offshore propagation during midnight and early morning develops from the leeward inland convection, and propagates northwestwards in the growing stage on the west Borneo, and eastwards in the decaying stage on the east Borneo. Offshore propagation lasts until next noon when sea breezes and island convection initiate. The timing and location of the offshore propagation suggest that it is not an independent convective mode. Instead, it is tied to the dipolar distribution of island precipitation modulated by MJO.

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The precipitation anomaly over Borneo features a dipolar pattern under the influence of the Madden‐Julian Oscillation (MJO). To understand the formation mechanisms of this pattern, a linear theory is developed to study the factors controlling its diurnal cycles and the dipolar pattern. The theory predicts that the prevailing wind is primarily responsible for the asymmetry over an idealized island, while the topography plays a critical role in the leeward convergence and convection asymmetry. The results are largely consistent with the observed composites of the dipole at Borneo. Nonlinear cloud‐permitting simulations are further conducted to test the effects of island topography and solar radiative heating in different MJO phases. The results show that the island topography can cause the mesoscale flow to split around the mountain due to the topographic blocking, and favor the development of the lee side sea breeze. These processes strengthen the low‐level convergence and convection at the leeside of the island during the late afternoon and night, which is very important to the formation of island dipolar precipitation anomaly. In contrast, the inland convergence is weakened and the dipole disappears when the terrain is flattened. The diurnal cycle of solar insolation is the dominant factor driving the land‐sea breeze circulation, which intensifies the island convection at the leeside. These results indicate that the MJO wind anomaly, island topography and solar insolation play distinct roles in the formation of the dipolar pattern of Borneo precipitation.

The Maritime Continent disrupts eastward propagation of the Madden Julian Oscillation (MJO). This study surveys the impact of the disruption – the barrier effect. The MJO propagation may be broadly categorized based on whether the MJO precipitation crosses the Maritime Continent (MC) during extended boreal winter seasons: successfully propagating across the Maritime Continent (MJO-C) and being blocked by MC (MJO-B). Compositing atmospheric circulation upon these two categories reveals that precipitation anomalies of MJO-C are stronger and more coherent than those of MJO-B, while their phase speed and lifetime are comparable. MJO-C and MJO-B excite distinct extratropical responses due to their diabatic heating in the deep tropics. Mid-latitude circulation displays stronger and long-lasting negative geopotential anomalies in the northern Pacific Ocean 5-14 days after phase 7-8 of MJO-C, but significantly weaker anomalies for MJO-B. The extratropical water vapor transports of MJO-B and MJO-C differs markedly after phase 2. The Pacific-North American (PNA) pattern and North Atlantic Oscillation (NAO) both show significant response after phase 6 of MJO-C as its precipitation anomaly over tropical Pacific during this period is stronger, while MJO-B has little impact on both. Surface air temperature (SAT) at high-latitudes during MJO-B and MJO-C are also significantly different. SAT is weaker and delayed in MJO-B in comparison to MJO-C, likely due to different meridional eddy heat fluxes.

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