The Cocos (Keeling) islands are a series of 27 coral islands, formed into two large coral atolls, situated in the Indian Ocean, approximately 2770 kilometres north west of Perth. Located at 12.2S 96.8E, they cover an area of 14 square kilometres. Most of the population live on Home Island and West Island. Rainfall observations began in 1907 while temperature, wind and other records began in 1952. From 1952 to 2005 there have been 27 tropical cyclones which have caused severe wind gusts of at least 90 km/h and just four to have caused destructive winds gusts of at least of 125 km/h (Doreen January 1968, Annie November 1973, Pedro November 1989 and Harriet February 1992). On average this equates to about one causing damaging winds every 2 years and one causing destructive winds every 14 years. The highest wind gust recorded was 176 km/h during Doreen in January 1968. Historically by far the most significant cyclone to affect the Islands occurred in 1909 when a wind gust of 225 km/h was estimated and a pressure of 945 hPa was recorded.
By contrast Christmas Island (10.5S 105.7E) is the summit of a submarine mountain, rising steeply to a central plateau dominated by stands of rainforest. This plateau reaches heights of up to 361 metres and consists mainly of limestone and layers of volcanic rock. The island's 80 kilometre coastline is an almost continuous sea cliff, of up to 20 metres in height. In a few places, the cliff gives way to shallow bays with small sand and coral shingle beaches. The largest of these bays forms the island's only port - Flying Fish Cove. Rainfall and wind observations began in 1972.
The incidence of cyclones at Christmas Island is less than at Cocos Islands which is further south and west. The map of the average annual number of tropical cyclones shows the area around Christmas Island having an annual incidence of about 0.2 or about one every five years. This is also significantly less than the area off the northwest coast where the incidence exceeds 1.0. Between 1972 and 2005 there there have been 13 tropical cyclones which have passed within 220 kilometres of the Island. On average this equates to one about one every two to three years. However, in the 20 years of wind records, there has only been one severe gust recorded of at least 90 km/h. That was 107 km/h in December 1980 during tropical cyclone Dan. While this suggests that Christmas Island has a low cyclone risk, there is still the potential for a major wind impact on Christmas Island.
Figure 1. Tracks of cyclones which came within 220 kilometres of the Cocos Islands and were Category 2 or stronger in the period 1952-2005. Click on image to enlarge.
Figure 2. Monthly occurrence of cyclones causing severe wind gusts at the Cocos Islands. Click on image to enlarge.
Figure 3. Tracks of cyclones which came within 220 kilometres of Christmas Island and were Category 2 or stronger in the period 1970-2004. Click on image to enlarge.
Figure 4. Monthly occurrence of cyclones within 220 km of Christmas Island. Click on image to enlarge.
See also the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.
Heavy rainfall can occur at any time of the year on Christmas and Cocos Islands. Cyclone Pedro caused 298 mm of rain on Cocos Islands in November 1989. However, flooding is not considered to be a significant issue particularly on Cocos Islands where water drains away. Of particular concern on Christmas Island is the impact that heavy rain can have on destabilizing hillsides. As a result landslips and rock falls are a potential danger during cyclone events.
The Cocos Islands are flat atolls in the Indian Ocean and as a result are quite vulnerable to storm surge. The account of the cyclone in 1909 below demonstrates the community's susceptability when waves estimated to be 15 m high swept over the island. However, it does need an intense cyclone approaching from the right direction and in conjunction with a high tide for the island to be significantly affected by storm surge. This is rare and there is no evidence to suggest there has been a significant storm surge since 1909.
In direct contrast Christmas Island is a island with volcanic origins and which rises to an elevation of 361 m above sea level. As most of the coastline is an almost continuous sea cliff, of up to 20 metres in height means that the storm surge risk is low. However, Flying Fish Cove on the northeast side of the island where the port facilities are located is particularly exposed to large seas and swell and storm surge from the northwest. Tropical cyclones that approach from the west or northwest such as Emma (1984) are the type most likely to generate a damaging storm surge. Fortunately these types of systems are rare. Even vigorous northwesterly monsoon winds may generate sufficient swell waves to adversely impact this area of the island.
Some Notable Cyclones Impacting the Cocos Islands
On November 27, 1909, a cyclone of unprecedented violence swept over the Islands. The damage is vividly described in the Governor's own words:
"The barometer fell as low as 27.92- lower than I have ever known it to fall before. We knew the day before that we were sure to have a very heavy storm, and we worked all day to prepare and secure for it; but unfortunately the whole of the preparations came to naught. At about six o'clock in the evening the cyclone was on top of us, and by eleven o'clock that night the centre passed over our islands. After that we had a lull for about half an hour, and then came the final blast, which carried away and finished everything. The cyclone was accompanied by great waves (sea -water, carrying sand with it, passed through the tower of my house, which is fifty feet high), and these waves left hardly anything standing. The whole of the villages, as well as the working sheds and stores, were levelled to the ground, or wasted and carried away by the seas. I could scarcely credit my eyes when daylight came; the wreckage was so thorough and complete that the islands were unrecognizable. Of my plantations of coconuts I am sure that not more than one tree in a hundred stood it; in consequence I have lost 800,000 trees more or less. As for lighters and boats, more than 40 per cent are lost, broken, or damaged.
Altogether only five buildings stood upright, and these without roofs upon them. The whole of that night we were practically under water or in the water, wet and miserably bedraggled. Fortunately only one man was killed right off by a falling tree and another died of exposure. It was simply miraculous how the people were saved, and the tales of their sufferings during that night are beyond belief. They were scattered by the seas all over the islands, and one can only say that providence guided them into safety.
We were plagued by mosquitoes, flies and other insects, all of varieties that I do not even recognize-and they pester the whole group, and make life a burden.
It is a sad duty to add this letter to the description I have given of the prosperity and order of the settlement as I last saw it. If it is permissible to extract interest from such a disaster, I would call attention to the importance of the arrival of presumed new species of insects with the wind. Again, the fact that the waves swept right over the islands, and deposited sand upon a tower fifty feet high, will serve to bring home to those that do not believe in the power of waves to shape atolls, and move "negro heads," that the waves of the storm driven blue ocean are not to be pictured at an inland fireside."
|Tropical Cyclone||Central Pressure (hPa)||Wind Gust (km/h)||Impact Description|
|c. 1862||Records show the settlement was badly damaged.|
|Jan. 28 1876||26.5 inches or approx 900 hPa||Storm surge destroyed the storehouses, oil mills and most of the houses in the kampong.|
|Feb. 4 1893||Over 300 000 palms uprooted as well as other damage occurred over 2 days.|
|Mar. 4 1902||Lasting only a few hours and less violent than previous storms, the cyclone destroyed over 300 000 palms.|
|Nov 27. 1909||945||225 (est.)||The peak lasted about 6 hours and was accompanied by a storm surge which left only five buidlings standing, about 800 000 palms were uprooted or decapitated. See above for a more detailed account.|
|Doreen Jan. 1968||970||176||Unknown.|
|Harriet Feb. 1992||975||163||Unknown.|
Some Notable Cyclones Impacting Christmas Island
There have been no notable wind impacts recorded from tropical cyclones recorded at Christmas Island to date. However, there have only been 20 years of wind records during the period from 1972-81 and 1985-2005 and undoubtedly there have been some historical cyclone events to have impacted the island.