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FREMANTLE

TROPICAL CYCLONES AFFECTING FREMANTLE

Historical tropical cyclone information of a general nature is available for Fremantle as early as 1910, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1961. Table XI-24 contains a descriptive history of all 14 tropical cyclones and hurricanes passing within 180 nmi of Fremantle during the 38-year period 1961-1998. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Fremantle are based on the data set used to compile Table XI-24.

The majority of tropical cyclones with a "classical track" have originated in the area to the north of Western Australia, have followed a west/southwest track parallel to or just off the northwest coast, and then have turned to the south or southeast. A few of the tropical cyclones have actually moved out of the central Indian Ocean on a more east-southeastward track (Holland, 1983). Very few tropical storms keep intensifying after moving past 20° S. Note that tropical cyclones in the Southern Hemisphere have a clockwise wind circulation, and at the latitude of Fremantle, have a much higher speed of advance than in the tropics.

Although the tropical cyclone season for Australia is considered to extend from November through April, Table XI-25 shows that Fremantle's season is essentially limited to the period of January through early May. No storms were recorded during November and December and only one storm was recorded during May throughout the 38-year period 1961-1998. April had the greatest tropical cyclone activity, with 6 of 14 storms (43 %) occurring during that month. Of the 14 storms in the data set, only one was of hurricane strength ( 64 kt) within 180 nmi of Fremantle. Table XI-25 also shows the average direction towards which storms were moving when they were at CPA to Fremantle. The average movement for all storms is 127° at 24 kt. The April storms moved faster, however, with an average speed of 27 kt. This speed of advance is two to three times faster than typically experienced in the tropics.

Figure XI-48 shows the annual distribution of the 14 tropical cyclones that entered the 180 nmi threat radius around Fremantle in seven-day increments. The period of maximum activity, from January 22nd through May 13th, is clearly shown.

Figure XI-49 depicts the chronology of the 14 tropical cyclones that passed within 180 nmi of Fremantle during the 38-year period 1961-1998. As shown in the figure, there is an average occurrence rate of 0.37 per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 2.7 years. There have been several multiple-year periods since 1961 when no tropical cyclone entered Fremantle's 180 nmi threat radius. The eight-year period from 1961 through 1968, seven-year period from 1992 through 1998, and six-year period from 1983 through 1988 had no tropical cyclones passing within 180 nmi of Fremantle.

Figure XI-50 shows the octants from which the 14 tropical cyclones were approaching when they were at CPA to Fremantle. As shown in the figure, the predominant threat direction is from the northwest. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the tropical cyclone's initial movement towards Fremantle. Note that both the threat direction and the high speed of movement are important considerations for evasion at sea.

As shown in Figure XI-51, the movement of the 14 tropical cyclones that entered Fremantle's 180 nmi radius can be described as fairly consistent. The dark circles in the figure indicate the locations of the 14 tropical cyclones when they were 72 hours from CPA to Fremantle. Note that the dark circles are generally grouped together and the tropical cyclones generally approach Fremantle from the northwest. However, a few tropical cyclones do approach from the west in a different pattern.

Figure XI-52 is a subset of Figure XI-51, and shows the track of tropical cyclone Alby, the one tropical cyclone during the 1961-1998 period that contained at least hurricane force ( 64 kt) sustained winds while within Fremantle's 180 nmi threat radius. The dark circle in the figure indicates the location of the tropical cyclone when it was 72 hours from CPA to Fremantle.

Figure XI-52 is the statistical summary of threat probability based on tropical cyclone tracks for the period 1961-1998. The data is presented with solid lines representing "percent threat" for the 180 nmi radius circle surrounding Fremantle. The heavy dashed lines represent approximate approach times to Fremantle based on the climatological speeds of movement. In Figure XI-53 for example, a tropical cyclone located near 18°S 109°E has approximately a 10 % probability of passing within 180 nmi of Fremantle. In addition, this tropical cyclone would reach Fremantle in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Fremantle. Note the acceleration of the speed of advance as tropical cyclones approach Fremantle.

Wind

Wind measuring instruments are located at the Fremantle Port Authority administration building at Fremantle. The instruments are not blocked or affected by other buildings, and indicate winds that are representative throughout the port area.

The sequence of wind directions from a tropical cyclone passing to the east is from the east then south and then west. For a tropical cyclone passing to the west it is from north and then west.

Locations most vulnerable to wind damage are berths on the North Quay of the Inner Harbor and on the eastern and southeastern side of Cockburn Sound.

The sea-breeze effect is predominant from October through April on practically a daily basis. At approximately sunrise the wind blows off the land (easterly wind) until about 1000 to 1200. Then the sea breeze normally comes in from the south-southwest at speeds averaging 19-26 knots by mid-afternoon. Extremes can be greater than 30 knots. This breeze usually begins to dissipate around midnight.

Waves

The entrance to the Inner Harbor and the Gage Roads anchorage area are particularly affected by ocean-wave motion. Additionally, waves from the southwest to northwest are refracted into Cockburn Sound. Vessels can surge at the berths in Cockburn Sound.

The maximum significant wave heights recorded at berths and sheltered anchorages at times of severe winter storms (not tropical cyclones) are in the vicinity of 6.6 ft (2m).

Storm Surge

Water levels throughout the port are very susceptible to atmospheric pressure changes with surges of 1 ft (30 cm) to 1.3 ft (40 cm) being common with the passage of normal winter frontal systems. There was a surge of 3 ft (90 cm) during a storm in 1994. However, there are no records of vessels or port facilities suffering damage due to storm surges.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/australi/fremantl/text/sect7.htm