Climatology of Borneo Vortices in the HadGEM3-GC3.1 General Circulation Model

Ju Liang,Kevin I Hodges,James M Haywood,Mou Leong Tan,Matthew Hawcroft,Jennifer L Catto

doi:10.1175/jcli-d-20-0604.1

Abstract

AbstractBorneo vortices (BVs) are intense precipitating winter storms that develop over the equatorial South China Sea and strongly affect the weather and climate over the western Maritime Continent because of their association with deep convection and heavy rainfall. In this study, the ability of the Hadley Centre Global Environment Model 3–Global Coupled, version 3.1 (HadGEM3-GC3.1), global climate model to simulate the climatology of BVs at different horizontal resolutions is examined using an objective feature-tracking algorithm. The HadGEM3-GC3.1 at the N512 (25 km) horizontal resolution simulates BVs with well-represented characteristics, including their frequency, spatial distribution, and lower-tropospheric structures when compared with BVs identified in a climate reanalysis, whereas the BVs in the N96 (~135 km) and N216 (~65 km) simulations are much weaker and less frequent. Also, the N512 simulation better captures the contribution of BVs to the winter precipitation in Borneo and the Malay Peninsula when compared with precipitation from a reanalysis data and from observations, whereas the N96 and N216 simulations underestimate this contribution because of the overly weak low-level convergence of the simulated BVs. The N512 simulation also exhibits an improved ability to reproduce the modulation of BV activity by the occurrence of northeasterly cold surges and active phases of the Madden–Julian oscillation in the region, including increased BV track densities, intensities, and lifetimes. A sufficiently high model resolution is thus found to be important to realistically simulate the present-climate precipitation extremes associated with BVs and to study their possible changes in a warmer climate.

Highlights

During boreal winter, the western ‘‘Maritime Continent,’’ including the regions of Borneo, the Malay Peninsula, and Sumatra, experiences pronounced rainfall seasons due to the domination of the northeasterly winter monsoon (Johnson and Houze 1987; Chang et al 2005a)
The modulations of Borneo vortices (BVs) and associated precipitation by cold surge (CS) and the Madden–Julian oscillation (MJO) represented by ERA5 and HadGEM3-GC3.1 are discussed
D When compared with the simulations at N96 and N216 (;65 km) horizontal resolutions, the HadGEM3-GA3 simulation at N512 (;25 km) resolution shows the best performance in representing the BV characteristics when compared with ERA5, especially for the BV frequencies, the magnitudes of BV genesis/track densities, horizontal mean sea level pressure (MSLP) gradients, and convergence near the BV center

Summary

Introduction

The western ‘‘Maritime Continent,’’ including the regions of Borneo, the Malay Peninsula, and Sumatra, experiences pronounced rainfall seasons due to the domination of the northeasterly winter monsoon (Johnson and Houze 1987; Chang et al 2005a). One of the main weather systems behind the connection between CSs and precipitation extremes in the western Maritime Continent is the near-equatorial cyclonic disturbances that develop offshore in the downstream region of the CSs near the north of Borneo (Cheang 1977; Chang et al 1979, 2003; Johnson and Houze 1987; Chen et al 2015a,b; Isnoor et al 2019) These disturbances, usually referred to as Borneo vortices (BVs) or cold surge vortices (Chen et al 2002, 2013, 2015a,b, 2017), are features with a closed cyclonic circulation in the lower troposphere and associated with intense rainfall with deep cumulus convection in the vicinity of the cyclone center (Koseki et al 2014). Such synoptic systems usually have a shallow vertical structure (Robertson et al 2011; Trilaksono et al 2012) and their motion is usually quasi stationary (Chang et al 2005b)

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Conclusion