Remote forcing of water levels by tropical cyclones in southwest Australia Matthew Eliot, Charitha Pattiaratchi n School of Environmental Systems Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia article info Article history: Received 18 January 2010 Received in revised form 30 May 2010 Accepted 3 June 2010 Available online 19 June 2010 Keywords: Tropical cyclones Continental shelf wave Sea level South-western Australia abstract Tropical cyclones (termed hurricanes and typhoons in other regions), are extreme events associated with strong winds, torrential rain and storm surges (in coastal areas) and cause extensive damage as a result of strong winds and ﬂooding (caused by either heavy rainfall or ocean storm surges) in the immediate area of impact. The eastern Indian Ocean, particularly in the northwest region of Australia, is impacted by up to 10 tropical cyclones during the cyclone season, although direct impact of cyclones along the west and southwest coastlines is rare. However, the sub-tidal frequency component of sea level records along the west and south coasts of Western Australia indicates lagged correspondence with the occurrence of tropical cyclones. It is demonstrated that the tropical cyclones generate a continental shelf wave which travels along the west and south coasts of Australia up to 3500 km with speeds of 450–500 km day 1 (5.2–5.8 ms 1 ) with maximum trough to crest wave height of 0.63 m, comparable with the mean daily tidal range in the region. The shelf wave is identiﬁed in the coastal sea level records, initially as a decrease in water level, 1–2 days after the passage of the cyclone and has a period of inﬂuence up to 10 days. Amplitude of the shelf wave was strongly affected by the path of the tropical cyclone, with cyclones travelling parallel to the west coast typically producing the most signiﬁcant signal due to resonance and superposition with local forcing. Analysis of water levels from Port Hedland, Geraldton, Fremantle and Albany together with cyclone paths over a ten year period (1988–1998) indicated that the tropical cyclones paths may be classiﬁed into 6 different types based on the amplitude of the wave. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction The often destructive forces of wind and storm surge associated with tropical cyclones (also named hurricanes or typhoons) when the region of maximum winds nears the coastal region are well documented (BMRC, 1983; Pielke et al., 2008). Along the northwest shelf of Australia wind gusts up to 414 km h 1 (115 ms 1 ) and maximum storm surges of up to 3.6 m have been measured (Buchan et al., 1999; Egis Consulting Australia, 1999). The energy input by cyclonic winds and the resulting water circulation patterns have the potential to generate coastally trapped waves, deﬁned as a wave that travels parallel to the coast, with maximum amplitude at the coast and decreasing offshore (Gill and Schumann, 1974; Fandry and Steedman, 1994; Ly, 1994; Tang and Grimshaw, 1995; Tang et al., 1997; Yankovsky, 2009). Examples of such waves include continental shelf waves (CSWs) and internal Kelvin waves (LeBlond and Mysak, 1978), which are governed through conservation of vorticity (Huyer, 1990). These waves travel to the left (right) along the coast in the southern (northern) hemisphere. The characteristics of CSWs depend on only the cross-shelf bathymetric proﬁle; the vertical density proﬁle controls the structure of an internal Kelvin wave (Huyer, 1990). CSWs are common along the Australian coastline and, along the south and east coasts, are generated by the alongshore component of wind stress (Church and Freeland, 1987; Grifﬁn and Middleton, 1991). Continental shelf waves have the potential to inﬂuence the water circulation and shelf circulation thousands of kilometres from the generating region (O’Callaghan et al., 2007). CSWs were identiﬁed within the sea level time series at Fremantle, Western Australia (Fig. 1), which indicated the presence of anomalous surge events during the Australian summer (December to March) that were not correlated with local weather conditions. These surges occurred within several days after the genesis, passage and decay of a tropical cyclone, a signiﬁcant distance away (1000s of km) from Fremantle. The aim of this paper is to examine the inﬂuence of tropical cyclones generated in the eastern Indian Ocean, particularly in the northwest region of Australia, on summer water levels along the micro-tidal mid-west and southwest coasts of Western Australia. Comparatively few studies have reported the generation of CSWs by tropical cyclones (or hurricanes/typhoons). Merriﬁeld (1992) reported the shelf wave response to hurricanes tracking parallel to the southern California coast. Ly (1994) and Yankovsky (2008) have described the generation of alongshore propagating waves in the Gulf of Mexico whist Dukhovskoy et al. (2009) have Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.06.002 n Corresponding author. Tel.: + 61 0 8 6488 3179; fax: + 61 0 8 6488 1015. E-mail address: chari.pattiaratchi@uwa.edu.au (C. Pattiaratchi). Continental Shelf Research 30 (2010) 1549–1561