Eastern North Pacific tropical cyclone activity in historical and future CMIP5 experiments: assessment with a model-independent tracking scheme

Abstract

The sensitivity of tropical cyclone (TC) projection results to different models and the detection and tracking scheme used is well established in the literature. Here, future climate projections of TC activity in the Eastern North Pacific basin (ENP, defined from 0° to 40°N and 180° to  ~ 75°W) are assessed with a model- and basin-independent detection and tracking scheme that was trained in reanalysis data. The scheme is applied to models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments forced under the historical and Representative Concentration Pathway 8.5 (RCP8.5) conditions. TC tracks from the observed records and models are analysed simultaneously with a curve-clustering algorithm, allowing observed and model tracks to be projected onto the same set of clusters. The ENP is divided into three clusters, one in the Central North Pacific (CNP) and two off the Mexican coast, as in prior studies. After accounting for model biases and auto-correlation, projection results under RCP8.5 indicated TC genesis to be significantly suppressed east of 125°W, and significantly enhanced west of 145°W by the end of the twenty-first century. Regional TC track exposure was found to significantly increase around Hawaii (~ 86%), as shown in earlier studies, owing to increased TC genesis, particularly to the south-east of the island nation. TC exposure to Southern Mexico was shown to decrease (~ 4%), owing to a south-westward displacement of TCs and overall suppression of genesis near the Mexican coastline. The large-scale environmental conditions most consistent with these projected changes were vertical wind shear and relative humidity.

Appendix

1.1 OWZ detection and tracking

The OWZ detection system consists of six parameters (Table 6): 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 component of relative vorticity squared such that:

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

(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 6 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. 2018a).

• (a) Each 1º × 1º grid point is assessed based on the initial threshold values of each OWZ-detector parameter every 12-h.

• (b) 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.

• (c) 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 4º × 4º steering wind at 700 hPa.

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

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