Coastal Storm Activity along the Eastern North Island of New Zealand - East Cape to Wellington

Abstract

Coastal storm activity for the eastern North Island, between East Cape and Wellington, has been quantified from a meteorological perspective through the use of cyclone tracks and extreme winds and from an oceanographic perspective by using hindcast wave information. It has culminated in the production of a high quality, digital coastal storm database for the eastern North Island. Together, this information provides a new understanding of coastal storm behaviour for the eastern North Island. A regional database of historical coastal storms along the eastern North Island between 1930 and 2005 (75 years) is now available in digital format. Coastal storms were identified as bouts of strong winds (greater than or equal to 10.5 m/s) from long-term local wind records from 1962 to 2005, and prior to this period, coastal storms were qualitatively recognised as any event leading to coastal shipping disruptions/delays, large wave conditions along the coast, episodes of coastal erosion and strong onshore wind periods. This digital database consists of five informative components that include storm meteorology, storm oceanography, impacts and damages, storm photo’s and images, and data sources. It has identified a set of five storm types for the eastern North Island consisting of Trough/Ridges, East Coast Lows, Subtropical Lows, Tasman Sea Lows, and Cyclone-Anticyclone pair. The two dominant types are Trough/Ridge and East Coast Low, with the Trough pattern involving weather systems primarily from the southern ocean, whilst East Coast Lows involve large cyclones off the coast that can be distantly generated (from the Tasman Sea or subtropics) or locally generated around NZ from southern ocean troughs. The most intense coastal storms off the eastern North Island are East Coast Lows involving cyclones from the subtropics. These storm events reveal blocking-type anticyclones east of the Chatham Islands play a vital role in coastal storm activity by steering cyclones southward towards NZ and then blocking any eastward movements so that cyclones become slow-moving off the east coast. These factors increase the intensity of pressure gradients directly over eastern NZ. The Gisborne region, for the 1962-1991 period (30 years), had an annual average of three coastal storms and displays peak activity in September. These storms are overwhelming from the south and southeast. A longer dataset of local winds at Wellington, spanning 1962-2005 (44 years), produced an annual average frequency of 9 coastal storms per year. The monthly distribution revealed peak storm activity in June and heightened activity between May and August. Both short-lived, high intensity storms (winds greater than or equal to 14.5 m/s for at least 12 hours) and long-lived, lower intensity storms (winds greater than or equal to 10.5m/s for at least 24 hours) were identified for the Wellington region. Approximately 70% of these coastal storms persisted for up to two days duration and are predominately from the south and southwest. Furthermore, the more exposed nature and steep terrain surrounding Wellington means a greater likelihood of higher intensity coastal storms compared to the Gisborne region. Strong cyclonic systems in the southwest Pacific cluster in the central Tasman Sea and east of the Chatham Islands in all seasons and are most frequent in winter. It is during winter that a clear frequency maximum is spotted over North Cape and appears to be related to the presence of slow-moving cyclones rather than high counts of discrete systems. Strong cyclones tend to form in the western Tasman Sea, in the subtropics near 22-23S, and near North Cape. This local formation off North Cape could be related to the Tasman front and North Cape eddy which create warm sea surface temperature anomalies. The complete life cycle of all strong cyclones shows formation, intensification and maturity in the western-central Tasman Sea, and therefore, a large proportion of these cyclones approaching NZ are weakening systems. However, local generation and intensification near North Cape and the Chatham Islands ensures strong cyclones continue to influence eastern NZ, and further indicates weakened Tasman Sea cyclones can drive coastal storm events through interactions with ridges and high pressure systems. Strong cyclones are most frequent around NZ in August when an average of 4-5 systems per month occurs. Extreme onshore winds off the eastern coast of NZ consist principally of winds from the southwest and south with a single high latitude frequency maximum near the dateline. These winds are generated from southern ocean cyclonic activity and their northward-extending troughs that pass over NZ, and their spread onto eastern NZ means they likely represent intense coastal storm events. Southeast, east and northeast winds rarely reach up to and beyond 20 m.s-1 over the seas to the east of NZ and generally cluster north of 40-45S indicating both subtropical and higher midlatitude source mechanisms. Extreme southeast winds are generated by the eastern flanks of large anticyclones that occupy the western Tasman Sea or large anticyclones south of the Chatham Islands. The principal frequency maximum of east and northeast winds is remote from NZ appearing near 165°W, and represent distant generation areas for large swell events (rather than coastal storm activity). These winds that occur over northern NZ are associated with a Tasman Sea or subtropical cyclone off North Cape in combination with a large anticyclone or ridge over/or east of the South Island. In contrast, the distant core for eastward of NZ are generated off the backs of large anticyclones with a trough or cyclone on its northern flank. The deep-water wave climate off the eastern North Island is dominated by waves from the south. Between 9 and 13 large wave events occur each year between East Cape and Wellington and are most likely in the months of May, June and July. In contrast, large storm waves from the southeast, east and northeast have annual average frequencies of 1-3 events. The Gisborne coast was found to be the most exposed with large deep-water waves (greater than or equal to 3m) coming from the northeast through to the southwest. However, waves from the south and southwest are the largest and most persistent. The meteorology creating these waves are southern ocean troughs whilst the less frequent waves from the easterly quarter involve low pressure systems east or northeast of NZ. The different proxies for studying coastal storms all have shortfalls and arrive at different levels of coastal storm activity. It is suggested here that an optimal mix of these proxies can be used to identify damaging coastal storms along the eastern North Island.