Projected changes in ENSO-driven regional tropical cyclone tracks

Abstract

Simulations and projections of the El Niño Southern Oscillation’s (ENSO’s) influence on TC track variability was analysed globally using Coupled Model Intercomparison project Phase 5 (CMIP5) models. The ability of these models to simulate the historical (1970–2000) ENSO–TC track relationship and inform us of the likely projected changes resulting from high carbon emissions (RCP8.5) in a climate projection (2070–2100) was determined through cluster analysis. The number of seasonal TC occurrences during traditional ENSO events (“El Niño” and “La Niña”) in each cluster were used to determine whether each cluster was “El Niño dominant”, “La Niña dominant” or “neither”. Only seven out of a combined total of 28 clusters across all basins were found to disagree in terms of “ENSO dominance” between the observed records and historical model simulations. This suggests that models can simulate the ENSO and TC track relationship reasonably well. Under sustained high carbon emissions, La Niña TCs were projected to become dominant over El Niño TCs in the central South Indian Ocean (~ 60–100°E), the southern Bay of Bengal and over straight-moving TCs in the South China Sea. El Niño TCs were projected to increase and become dominant over La Niña TCs in a larger area of the western South Pacific (~ 160°E–165°W) and central North Pacific (~ 160°E–145°W) Oceans. Projections of track directions and lifetimes, while less robust, indicated that El Niño TCs would track westward more often in the Coral Sea (150–165°E), while El Niño TCs that took an eastward track here would have longer lifetimes (~ 3 days).

Appendix: OWZ detection and tracking

The OWZ detection system consists of six parameters (Table 9): minimum thresholds of OWZ at the 850- and 500 hPa levels, relative humidity (RH) at the 950- and 700 hPa levels, specific humidity (SpH) at the 950 hPa level and a maximum threshold of vertical wind shear (VWS) between 850- and 200 hPa. The OWZ variable is a low deformation vorticity parameter used to identify regions favourable for TC formation at the centre of a semi-closed circulation (i.e. a ‘marsupial pouch’; Dunkerton et al. 2009), within the lower- to mid-troposphere. More precisely, it is the product of absolute vorticity and the Okubo–Weiss parameter (Okubo 1970; Weiss 1991) normalised by the vertical components of relative vorticity squared such that:

$$OWZ = \text{sgn} (f) \times (\zeta + f) \times \hbox{max} \left[ {\frac{{\zeta^{2} - (E^{2} + F^{2} )}}{{\zeta^{2} }},0} \right],$$

(1)

where f is the Coriolis parameter, $\zeta = \frac{\partial v}{\partial x} - \frac{\partial u}{\partial y}$ the vertical component of relative vorticity, $E = \frac{\partial u}{\partial x} - \frac{\partial v}{\partial y}$ the stretching deformation, and $F = \frac{\partial v}{\partial x} + \frac{\partial u}{\partial y}$ the shearing deformation.

Table 9 Parameter threshold values for the two sets of the OWZ-Detector’s detection criteria, subscripts refer to hPa level

The OWZ detection and tracking scheme is concisely summarized in five dot points below, with further detail accessible in other studies (Tory et al. 2013a; Bell et al. 2018).

Each 1° × 1° grid point is assessed based on the initial threshold values of each OWZ-Detector parameter every 12-h.

When at least two neighbouring grid points satisfy the initial thresholds of each OWZ-Detector parameter, these points are considered to represent a single circulation at that point in time.

The circulations from step (b) are linked through time by estimating their position in relation to the circulation’s expected position based on an averaged 4° × 4° steering wind at 700 hPa.

Tracks are terminated when no circulation match is found in the next two time-steps within a generous (~ 350 km) latitude dependent radius.

The core thresholds are then applied to each storm track, and if they are satisfied for 48-h, a TC is declared.