Cross-equatorial northerly surge (CES) is a synoptic-scale weather system responsible for devastating hydrological extremes and significant losses of life and property in southern Indonesia and northern Australia during the boreal winter. It is characterized by a rapid acceleration of low-level northeasterly moist monsoonal winds crossing the equator in the western Maritime Continent (MC). Whilst significant progress has been made in the past decade to understand its dynamics, drivers, and impacts, few studies have sought to understand its response to global warming and the associated impacts.

Here, we use an ensemble of high-resolution simulations from the HighResMIP project, conducted by seven modeling groups, to examine changes in CES characteristics (including frequency, intensity, duration, precipitation and associated large-scale environmental controls) in the present-day and future climates. The present-day climate experiments are forced by time-dependent SSTs and sea-ice from the observational dataset, while future climate simulations are forced by the SST warming projected by the ensemble mean of the CMIP5 future climate simulations under the RCP8.5 scenario. Analysis of historical simulations indicates that these high-resolution climate models capture the characteristics of these high-impact weather events reasonably well in the present-day climate, giving us confidence in using them for future projections. The results indicate that future CES events may become less frequent in a warmer climate, decreasing from ~21.9 CES days per winter (1979–1999) to ~16.7 CES days per winter (2030–2050). However, significant increases in both the intensity and the total amount of precipitation from CES events are projected. Such changes are hypothesized to be driven by the more humid and convectively unstable lower troposphere. As a result, the contribution of CES events to both total precipitation and extreme precipitation is projected to increase considerably in the vicinity of southern Indonesia and northern Australia. Specifically, a twofold increase in the probability of extreme precipitation (defined as the 95th percentile of daily mean precipitation) has been found in these regions, making individual CES events more threatening in a warmer climate. The strengthening of CES is also consistent with intensified low-level convection, leading to the strengthening of the local Hadley circulation over the MC under a warming climate. Given the significant socio-economic impacts of CES events, our analysis could provide insights into hydrological changes caused by these high-impact synoptic-scale weather systems for the coming decades, which may require effective mitigation and adaptation strategies.