Investigating the dynamics of Borneo vortices and their impacts

Project description

Borneo vortices are synoptic-scale cyclonic disturbances with distinct structure and behaviour and unique to the Maritime Continent during boreal winter. They can often be found north or west of Borneo (Fig. 1). They are associated with major impacts including heavy rainfall, landslides and rough seas in Malaysia, Vietnam, and Thailand, which makes it essential for forecasters in Southeast Asia to be able to understand and forecast them.

Fig. 1: Example of a Borneo vortex on 12 January 2028 at 00 UTC which is located in the northwest of Borneo. The streamlines at 850 hPa are based on ERA5 reanalysis data, and the rainfall is from GPM IMERG. The asterisk marks the centre of the vortex.

Vortices in the vicinity of Borneo have been referred to by various names – Borneo vortices (Chang et al 2005), cold surge vortices (Chen et al., 2012, 2013, 2015), Southeast Asian sea–Maritime Continent vortices (Nugyen et al., 2016) and South China Sea vortices (Howard et al., 2021). Crook et al. (2024) have shown recently that disturbances named “Borneo vortices” can be split into two types with different behaviour: westward propagating vortices to the north of Borneo and also static vortices near the equator, often west of Borneo but sometimes crossing south of the equator and affecting southern Borneo and Java. This project aims to investigate the dynamics of Borneo vortices with different tracks in detail.

There have been a few case studies on Borneo vortices (e.g. Juneng et al. 2007; Ooi et al. 2011; Isnoor et al. 2019; Hardy et al. 2023), but there still remain fundamental questions about the dynamics of Borneo vortices in relation to their formation, intensification and track and how the westward propagating and equatorial types differ. A statistical link with high impact weather is established, but it is not understanding where it preferentially occurs relative to the vortex or why.

Cold surges occur during the Asian northeast monsoon (in winter) and characterize stronger pulses of cold air intruding deep into the Maritime Continent.  Cold surges affect the location of Borneo vortices (Chang et al., 2005, Koseki et al 2014, Chen et al., 2015) propagation and perhaps intensity. The formation of these vortices may be due to enhanced vorticity near Borneo due to the channelling of strong north-easterly winds by the land masses (Koseki et al., 2014), or, for those forming further north, shear instability (Hardy et al., 2023) or an easterly disturbance (Chen et al 2015) may be required. Crook et al. (2024) have shown that frequency, intensity and rainfall can all be modified by westward moving equatorial, while eastward moving Kelvin waves can greatly enhance the rainfall in such events.

Objectives

The scientific objectives of the PhD project are:

1. Conduct a detailed investigation into the processes that lead to the formation, intensification, track and impacts of Borneo vortices;
2. Develop an understanding of how large-scale drivers impact Borneo vortices;
3. Investigate the impact of orography on the formation and track of Borneo vortices.

In particular, according to your particular research interests, the studentship could involve the following:

Year 1: Structure and dynamics of Borneo vortices with different tracks

We will start by analysing representative case studies of the five clusters of Borneo vortices identified in Crock et al. (2024). We analyse the structure and dynamics of the Borneo vortices in the different regimes and compare the results from different vortex groups. We will also be able to investigate whether the mechanisms behind vortex formation, maintenance and intensification are different for systems forming over the South China Sea or nearer to the equator, including over Borneo. We will also investigate the physical processes responsible for the mesoscale distribution of high-impact weather within the Borneo vortex. In Addition, we will address the following two questions: (i) Where does the convection occur in BVs (mainly along the shear line between cold surge and vortex) and why? And (ii) How is heavy convection linked to cold surges and Borneo vortices?

We will focus on the season between October to March (years 2018 – 2021) as this is the season when the most South China Sea and Borneo vortices occur (Howard et al., 2021). We will build on the work of Hardy et al. (2023) which is based on the in-depth analysis of one South China Sea vortex in October 2018. In this work the semi-geotriptic balance approximation tool developed by Cullen (2018) has been used to show that the vortex is well approximated by a balance flow, even though it is deep in the Tropics, the vortex circulation is confined to the lower troposphere and that the dynamical coupling to latent heat release is important to the development of the disturbance on the south side of the larger scale cold surge flow.

Year 2: Effect of orography on the location of cold surges and Borneo vortices

We will carry out sensitivity experiments with the MetUM using varying roughness length over land and ocean, as well as varying orography. We hypothesise that boundary layer friction is a very important element in the turning of flow from the cold surges, enabling them to cross the equator and also in the formation of vortices. Simulations of the same systems will be compared in the global and convection-permitting limited area MetUM simulations, investigating the importance of resolution in representation of coastal flows and their relation to Borneo vortices.

We will use the semi-geotriptic balance approximation tool (Cullen, 2018) to investigate the effects of the frictional boundary layer on the wind field of the South China Sea vortices by varying the turbulent eddy diffusivity in the boundary layer and using the tool to attribute the effects on the flow. We will run these sensitivity experiments for distinct types of vortices identified in year 1.

Year 3: Borneo vortices and high impact weather

In year 3 we will investigate the impact of cold surges on the formation, intensity, and track of Borneo vortices. We will address the following questions:

• Why do Borneo vortices form?
• What is the link to large scale weather systems such as equatorial waves on the strengths, formation location, intensity, path, and movement of Borneo vortices?

You will have the opportunity to spend time at the Met Office and learn how to run regional model simulations, and to visit Met Malaysia.

Potential for high-impact outcome

The Met Office has a close working relationship with the Meteorology, Climatology, and Geophysical Agency (BMKG) of Indonesia, the national weather service in Malaysia (Met Malaysia), and the National Center for Hydrometeorological Forecasting (NCHMF) in Vietnam, Indonesia, Malaysia and Vietnam are heavily affected by Borneo vortices. Understanding the dynamics of Borneo vortices and forecasting their track and impact is a key challenge for NWP models such as the MetUM, in both global and regional configurations. This project provides a way to investigate Borneo vortex dynamics in detail, to evaluate how certain dynamical processes are represented in the model with the aim of improving our ability to forecast these weather systems more accurately, ultimately benefiting people at risk. The project will generate results for several papers, with at least one being suitable for submission to a high impact journal.

Training

The student will be able to visit the Met Office in Exeter for a placement in the team working on Southeast Asia, split across several visits with timing depending on the research undertaken (for example conducting new model runs). The student will have access to a broad spectrum of training in:

• Dynamics of the tropical atmosphere;
• Numerical modelling and use of cutting-edge supercomputers;
• State-of-the-science application and analysis of global atmospheric reanalysis data;
• A computer programming language (e.g. Python) to perform complex analysis techniques;
• Effective written and oral communication skills.

Further reading

Crook, J., Hardy, S., Methven, J., Schwendike, J., Hodges, K.I., Harvey, B., Yik, D.J., Yang, G.-Y. (2023). The structure of Borneo vortices and their relationship with cold surges and equatorial waves. Accepted at QJRMS.

Cullen, M. P. J. (2018): The use of semigeostrophic theory to diagnose the behaviour of an atmospheric GCM, Fluids, 3, 72, https://doi.org/10.3390/fluids3040072.

Hardy, S., Methven, J., Schwendike, J., Harvey, B., and Cullen, M. (2023). Examining the dynamics of a Borneo vortex using a balance approximation tool. Weather