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MACKAY

TROPICAL CYCLONES AFFECTING MACKAY

Australia's tropical cyclone season extends from November through April. Historical tropical cyclone information of a general nature for Mackay is available as early as 1888, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1958. Table XI-8 contains a descriptive history of all 31 tropical storms and hurricanes passing within 180 nmi of Mackay during the 40-year period 1958-1997. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Mackay are based on the data set used to compile Table XI-8.

Although the official tropical cyclone season for Australia is considered to extend from November through April, Table XI-9 shows that Mackay's season is effectively limited to the period of December through March. No storms were recorded during November, and only three storms were recorded during April during the 40-year period 1958-1997. All of the April storms had sustained winds of hurricane force (64 kt) when at their closest point of approach (CPA) to Mackay. January is the month of greatest tropical cyclone activity for Mackay, with 10 of the 31 storms (32%) occurring during that month. Of the 31 storms in the data set, six (19%) were of hurricane strength (64 kt) at CPA. Table XI-9 also shows the average heading towards which storms were moving when they were at CPA to Mackay. The average movement for all storms is 172° at 10 kt. The average monthly speed of movement of all storms did not vary much from the seasonal average. The January tropical cyclones had an average movement direction of 140°, while the average movement for February and March was more southerly.

During the 40-year period 1958-1997 there were 31 tropical cyclones that entered Mackay's 180 nmi threat radius. Figure XI-12 shows the annual distribution of the storms in seven-day increments.

Figure XI-13 depicts the chronology of the 31 tropical cyclones that passed within 180 nmi of Mackay during the 40-year period 1958-1997. As shown in the figure, there is an average occurrence rate over the 40-year period of 0.77 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 1.3 years. However, for hurricane strength (64 kt) storms, the average occurrence rate is 0.15 storms per year, with a recurrence rate of 6.7 years. There have been several multiple-year periods since 1958 when no tropical cyclone entered Mackay's 180 nmi threat radius.

Figure XI-14 shows the octants from which the 31 tropical cyclones were moving when they were at CPA to Mackay. As shown in the figure, the predominant threat direction is the northwest octant, although a similar number of cyclones (11) approached Mackay from the north and northeast octants combined. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the storm's initial movement toward Mackay.

The movement of the 31 tropical cyclones that entered Mackay's 180 nmi radius can best be described as inconsistent. The lack of steering flow is apparent in many of the cases, and as a result many storms took meandering or, in some cases, looping tracks. Figure XI-15 which depicts the tracks of the 10 storms that had hurricane force winds ( 64 kt) at some time while within 180 nmi of Mackay, shows that the erratic movement is particularly evident when the tropical cyclones were north of 20°S. The dark circles in the figure indicate the locations of eight of the tropical cyclones when they were 72 hours from CPA to Mackay. Two tropical cyclones were either not detected or were too weak to be tracked 72 hours prior to CPA.

Figure XI-10 of the Brisbane evaluation depicts the track of Tropical Cyclone Justin and illustrates the meandering nature of some of the tropical cyclones occurring over the Coral Sea. Justin occurred in March 1977, but because Justin did not enter Mackay's 180 nmi threat radius, the storm is not included in Mackay's 31-storm data set. The text accompanying Figure XI-10 discusses the 17-day life of Justin.

Wind

Winds at the Port of Mackay are measured at the Port Officer's Office, and are considered to be representative of conditions at the ship's berths. Wind records at the port are not archived, however, so historical wind data is available only for the Mackay Airport.

Wind data for Mackay Airport are available for the 24 tropical cyclone seasons occurring during the period of December 1973 through February 1997. During that period, 21 tropical cyclones of tropical storm or hurricane strength passed within 180 nmi of Mackay. Of those, nine caused sustained winds 22 kt to be recorded at Mackay during their passage. Of those nine storms, only two, Tropical Cyclone Kerry in 1979 and Tropical Cyclone Aivu in 1989, caused sustained winds of gale force (34 kt) to be recorded. The maximum sustained wind recorded during the period for which observations are available for Mackay is 44 kt, which occurred during the passage of Tropical Cyclone Kerry.

Callaghan (1997b) indicates that the Mackay Meteorological Office experienced storm force (47 kt) winds in March 1940. The same document states that Tropical Cyclone Kerry brought sustained winds (10-minute average) of 50 kt with gusts to 76 kt to the airport Meteorology Office in March 1979, but the observations are not recorded in available wind record information. It is probable that the strongest winds occurred between regular hourly observations, which are the only ones included in the available database.

Waves

Wave data for the area outside the breakwaters at the Port of Mackay are not available. While tropical cyclones are capable of generating open ocean waves well over 40 ft (12 m) in height, the shallow water covering the Great Barrier Reef that lies east of Mackay would significantly hinder the propagation of the larger deep ocean swell waves across it. Some swell energy might reach the port, but most of the wave motion near the Port of Mackay would likely be wind waves. The mouth of Mackay Harbor is open to the east-northeast, so any wave motion approaching from approximately 010° through 120° could pass through the harbor entrance. The greatest effect would be felt at Berths 2 through 4. Although records are not maintained, a local harbor authority estimates that the maximum observed wave height at the port is approximately 5 ft (1.5 m).

Storm Surge

Storm surge is a water rise that is the combined result of an atmospheric low pressure area (in this case, a strong tropical cyclone) that causes a dome in the water surface, and other factors, such as the horizontal scale of the storm, the direction and speed of advance (SOA) of the storm, and the coastal geometry and bottom topography near the area of interest. As the storm moves toward land, the dome of water invades coastal areas. The problem is compounded by the large waves usually accompanying a strong tropical cyclone. The waves are driven across shoaling water by high winds, piling them on the shore. These steep, breaking waves are driven so hard by the wind that they create a general landward-flowing surface current that moves faster than the surplus water can return seaward along the bottom. The result is a flooded coast (Bascom, 1980). In the Southern Hemisphere the highest storm surge commonly occurs in the region of strongest winds to the left of the path of the tropical cyclone.

Historical records indicate that Mackay is vulnerable to storm surge. In 1918, when a tropical cyclone crossed the coast just north of Mackay, the town experienced a storm surge of 11.8 ft (3.6 m). One observer reported that a 25 ft (7.6 m) wall of water swept over the beaches toward Mackay at the height of the cyclone. In 1987, a survivor of the 1918 storm recalled seeing 7.9 to 8.9 ft (2.4 to 2.7 m) waves breaking in the center of the city. More recent tropical cyclones have caused 3.3 ft (1 m) storm surges to areas just north and south of Mackay.

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