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CAIRNS

TROPICAL CYCLONES AFFECTING CAIRNS

Australia's tropical cyclone season extends from November through April. Historical tropical cyclone information of a general nature for Cairns is available as early as 1878, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1958. Table XI-14 contains a descriptive history of all 47 tropical storms and hurricanes passing within 180 nmi of Cairns 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 Cairns are based on the data set used to compile Table XI-14.

Although the official tropical cyclone season for Australia is considered to extend from November through April, Table XI-15 shows that Cairns' season doesn't begin until December. No storms were recorded during November during the 40-year period covered by the table. January and February are the months of greatest tropical cyclone activity for Cairns, with 27 of the 47 storms (57%) occurring during those two months. Of the 47 storms in the data set, only six (13%) were of hurricane strength (64 kt) at their closest point of approach (CPA). Table XI-15 also shows the average direction towards which tropical cyclones were moving when they were at CPA to Cairns. The average movement for all 47 tropical cyclones is 170° at 11 kt, but there are considerable differences in movement directions and speeds during the months represented in the table. Average monthly directions vary from southeast in January to southwest in February and March. Average speeds vary from a low of 9 kt in December to 12 kt in March and April.

During the 40-year period 1958-1997 there were 47 tropical cyclones that entered the 180 nmi threat radius around Cairns. Figure XI-25 shows the annual distribution of the storms in seven-day increments. The figure clearly shows Cairns' well defined tropical cyclone season.

Figure XI-26 depicts the chronology of the 47 tropical cyclones that passed within 180 nmi of Cairns during the 40-year period 1958-1997. As shown in the figure, there is an average occurrence rate of 1.17 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 1.1 years. 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 Cairns' 180 nmi threat radius. The four-year period from the 1991-1992 season through the 1994-1995 season had no storms passing within 180 nmi of Cairns.

Figure XI-27 depicts the octants from which the 47 tropical cyclones were moving when they were at CPA to Cairns. The figure shows that there are two primary threat axes at Cairns. Eighteen storms (38.3%) approached Cairns from the west or northwest octants. Nineteen storms (40.4%) approached Cairns from the northeast and east octants. Those approaching from the west or northwest either moved eastward from the Gulf of Carpentaria or recurved after moving onshore north of Cairns. Those from the northeast and east likely either formed over the Coral Sea or passed over the Coral Sea after forming in the Pacific Ocean east of the Coral Sea. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the storm's initial movement toward Cairns.

The movement of the 47 tropical cyclones that entered Cairns' 180 nmi radius can only be described as inconsistent. The lack of steering flow is apparent in many of the cases, and as a result many storms took meandering and, in some cases, looping tracks. Figure XI-28, which depicts the tracks of the nine storms that had hurricane force winds (64 kt) at some time while within Cairns' 180 nmi threat radius, shows the erratic movement typical of many of the storms affecting Cairns. The dark circles in the figure indicate the locations of eight of the storms when they were 72 hours from CPA to Cairns. One storm was either not detected or was too weak to be tracked back 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, and is included in Cairns' 47-storm data set. The text accompanying Figure XI-10 discusses the 17-day life of Justin.

Wind

The Port of Cairns has no wind measuring equipment. All wind observations are obtained from the Cairns Airport, but winds at the airport are considered to be representative of port conditions. Wind observation records are available for the Cairns Airport for the 24 tropical cyclone seasons occurring from March 1973 through March 1997 . During that period, 35 tropical cyclones passed within 180 nmi of Cairns. Of those, only five caused sustained winds 22 kt at Cairns. Two storms, Tropical Cyclone Joy in 1990 and Tropical Cyclone Justin in 1997, caused sustained winds of gale force (34 kt). The strongest wind contained in the available observation records, occurring with Tropical Cyclone Joy in 1990, is 39 kt sustained with gusts to 56 kt. Historical records contained in Callaghan (1997b) documents other events of strong winds associated with passing tropical cyclones.

The predominant wind direction at Cairns is southeasterly. Bureau of Meteorology authorities state that southeasterly winds can occur at any time of the year, but are more prevalent during the months of April through October, with speeds averaging 15 to 25 kt. From late October to early November, the southeasterly flow weakens. The monsoon season occurs during the warmer months of December through March when winds are usually from northwest to northeast. Light winds and moderate afternoon northeasterly sea breezes are common.

Local harbor authorities state that some of the worst winds at the Port of Cairns occur when winds spill over the tops of the hills surrounding Cairns and flow downslope on the lee side. The resultant winds are strong and gusty. The seaward end of the entrance channel is also subject to gusty winds. It was stated that during normal southeasterly 15-25 kt trade winds, there might be gusts to 50 kt at the channel entrance as the wind blows perpendicular to the channel orientation. The same persons said that their experiences indicate that Cairns is subject to southeasterly winds that funnel northward through the Mulgrave River Valley south-southeast of Cairns.

Navigating the entrance channel can be difficult during periods of southeasterly winds of 20-25 kt. A ship entering the channel will experience easterly winds as the winds round the north end of Cape Grafton (Figure XI-23), and have wind on the ship's port quarter. As the ship progresses southward through the channel, the winds funnel across the flat lands south of Mission Bay and cross the channel at 90° to the channel orientation. There are relatively high elevations east of the channel between False Cape and the harbor entrance, causing the wind to spill over the top and get very strong and gusty. Finally, as the ship enters the harbor, the winds funnel northward through the Mulgrave River Valley south-southeast of the port, and reach the ship as southerly.

Waves

Wave motion is not a significant problem at the Port of Cairns. The location of the Port on Trinity Inlet limits fetch from all directions except north-northeast. Waves from that direction would have to pass through the shallow waters over the Great Barrier Reef and the mud flats adjacent to the Entrance Channel, which would greatly mitigate their effects at the port. Any wave motion that did enter Trinity Inlet would be parallel to the wharves along which ships moor, and pose little threat to the vessels.

Storm Surge

Anecdotal records of tropical cyclone activity near Cairns is available as early as 1878 (Callaghan, 1997b). However, most descriptions include generalities about wind damage and flooding during tropical cyclone passages, but references to storm surges are sparse. The few references to storm surges refer to locations near to, but not at, Cairns. One reference mentions that in 1918, when a tropical cyclone passed directly over Innisfail, the sea rose 9.8 ft (3.0 m) above high water at Maria Creek.

Innisfail is approximately 43 nmi southeast of Cairns. A 1920 event recorded "photographic evidence of sea water inundation at Cairns." Another states that, in 1934, a tropical cyclone moved onshore near Cape Tribulation (approximately 52 nmi north of Cairns) and caused a 29.9 ft (9.1 m) storm surge at nearby Bailey Creek and a 5.9 ft (1.8 m) storm surge at Port Douglas (approximately 26 nmi north of Cairns). In an interview with a Cairns Port Authority Piermaster, it was revealed that Tropical Cyclone Justin caused a 2.0 ft (0.6 m) storm surge above high tide levels in 1997. In three of the cases cited, the term storm surge is used, but it is not known how much, if any, of the water rise was due to heavy precipitation runoff or to storm surge effects. Although no mention is made of the storm's effects at Cairns in the Innisfail and Cape Tribulation events, the close proximity of each occurrence indicates that Cairns is located in an area that is vulnerable to storm surge, and if a strong tropical cyclone moved onshore just north of Cairns, a storm surge of considerable height is possible.

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