The effects of the strong El Niño event suppressed tropical cyclone activity in the eastern Indian Ocean during the season. Overall the number of cyclones was below average in the Perth Tropical Cyclone Warning Centre Region (90°E - 125°E). Three tropical cyclones occurred in the whole region (all off the northwest coast) compared to the long term average of 6 (average number near the northwest coast is 4). All three were classified as severe cyclones. There were no impacts along the northwest coast compared to the average number of 2. Two other significant lows affected the coastal regions of the north Kimberley during the season, ex-TC Les in early February and a tropical low in April. Both produced gale force winds for a period along the north Kimberley coast but were not classified as tropical cyclones since the gale areas did not surround the low centres.
Details of each Tropical Cyclone in the northwest Australian region 1997-98
Severe tropical cyclone Selwyn 26 December 1997 - 03 January 1998
TC Selwyn formed about 650km eastsoutheast of Christmas Island from an area of convection within the monsoon trough. During the same period TC Sid developed near the NT coast and moved into the Gulf of Carpentaria. Selwyn intensified rapidly to cyclone status early on the morning of 27 December and reached maximum intensity during 28 December. Apart from increasing swell along the NW coast the cyclone had no direct impact on the mainland or Island communities.
Selwyn was a reasonably small cyclone during its lifetime and thus was able to intensify rapidly in its development stage but was also was susceptible to vertical wind shear. It formed a well developed eye during 28 December but during that night the cloud field around the cyclone became elongated along a NW-SE axis. A low level centre becoming exposed on the northern flank of the cloud mass overnight on 29 December.
Severe tropical cyclone Tiffany 22 January - 02 February 1998
TC Tiffany formed early on 24 January off the northwest Kimberley coast and was the most severe cyclone of the season. It occurred during an active monsoon period when TC Katrina intensified in the Coral Sea and TC Les formed in the Gulf of Carpentaria. It developed rapidly into a small but intense cyclone with a radius of gales between 100 and 130km and moved generally west or westsouthwest, remaining offshore from the Pilbara coast at a distance of approximately 200 kilometres. Tiffany caused strong to near gale force winds on exposed areas of the Pilbara coast and storm force winds at North Rankin during 27 January but had no other effect on coastal WA. Tiffany weakened as it continued to move westwards. Winds were estimated to have decreased below gale force by early on 30 January. The low-level centre then moved westnorthwest and continued to weaken, some 700km southsoutheast of Cocos Islands.
Tiffany was a small system during its lifetime and intensified rapidly to severe cyclone status in a region of low windshear. Using observations from two offshore sites the radius to gale force winds was estimated to be 100 to 130 kilometres. Storm force winds occurred within 50 kilometres of the centre and hurricane force winds extended outwards a distance of only 30 kilometres. The gale radius was non-symmetric and expanded outwards as spiral rainbands passed across the observing sites. In the early stages of development the wind profile had an asymmetric distribution towards the southern side due to a surge in pressure to the south. This resulted in strong winds in the Port Hedland area well outside the core of strong winds associated with the cyclone centre.
A new product, the tropical cyclone threat map was trialed during this event.
Ex-tropical cyclone Les 24 January - 02 February
Les formed in the Gulf of Carpentaria from a low pressure system that had been in the Gulf area for the previous 4 days. It passed over the southern part of Groote Island before crossing the coast of Arnhem Land and weakening into a monsoon low. It continued to track across the Top End of the NT and produced record rainfall and severe flooding in the Katherine and Daly regions. It then re-emerged over water in the Timor Sea. Monsoon gales were observed in coastal regions as the centre moved inland across the north Kimberley. The centre remained over land and was unable to re-intensify to tropical cyclone strength with gales completely surrounding the centre. However gale force winds with severe squalls continued in coastal areas as it tracked along the northwest Kimberley coast. The centre eventually moved southwards just to the east of Broome, then close to Telfer before moving further inland and weakening. Its poorly developed central structure made it difficult to track by both radar and satellite imagery.
Severe tropical cyclone Victor 08 - 17 February 1998
TC Victor formed from a tropical low that passed across northern Cape York Peninsula. This low was most likely the remnants of former tropical cyclone Katrina. The low was weak and poorly structured as it moved to the north of the Kimberley during 8 February, however its organization improved considerably significantly overnight on 9 February as it moved westsouthwest away from the north Kimberley coast. An LNG tanker reported 40 knot winds near the centre during the late afternoon of 10 February and Victor was named at 1000 UTC. It continued to move westward and remained weak during 11 February but intensified during 12 February. An eye became visible during the morning of 13 February as Victor reached peak intensity. Victor continued westsouthwest but then progressively weakened. By the morning of 15 February it was fully sheared with a fully exposed low level centre apparent on satellite imagery. Victor was a small cyclone for its whole lifetime and was surrounded by very high environmental pressures of around 1012 hPa. The ship report of 40 knot winds near the cyclone centre was at a time when the ship's barometric pressure reading was 1005 hPa and the central pressure was estimated to be 1002 hPa.
Victor was briefly renamed by Mauritius as tropical cyclone Cindy as it passed west of 90°E. The time period from the initial formation of the low in the Coral Sea, through until it could no longer be identified as a low in the central Indian Ocean was at least 50 days.
North Kimberley low 16-19 April 1998
Coincident with the final active phase of the monsoon over northern Australia, a low developed northeast of Gove and moved steadily westward into the Timor Sea. It then took a south southwest track and intensified overnight on 18 April. The low crossed the north Kimberley coast as a sheared system before it had a chance to develop into a tropical cyclone. Gale force winds were experienced on the north Kimberley coast for short periods in a rain band on the southern side of the centre, and strong winds were reported for most of the day on 19 April.