How often is New Zealand hit by tropical cyclones?
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12 NIWA Water & Atmosphere 10(1) 2002 12 METEOROLOGICAL RESEARCH How often is New Zealand hit by tropical cyclones? Mark Sinclair Recent NIWA research shows that northern New Zealand is hit by an average of a little over one storm of tropical origin each year. The severity of these storms depends on their location and on the phase of the El Niño/La Niña cycle. Every year between December and April, storms from the tropics move south toward New Zealand. As these tropical cyclones advance poleward, they eventually lose characteristic hurricane features such as the eye and the surrounding symmetric cloud and precipitation region and progressively acquire the asymmetric cloud and thermal features more typical of a frontal mid-latitude storm. Scientists use the term extratropical transition (ET) to refer to this poleward movement and accompanying structure changes. As these storms move into New Zealand waters, they often maintain sufficient vigour to produce damaging winds, high seas and heavy rain. Occasionally, tropical cyclone remnants re- intensify in the extratropics to become potent mid-latitude storms capable of inflicting loss of life and severe property damage. Several memorable ET events have occurred in the New Zealand region. In April 1968, Tropical Cyclone Gisele re-intensified as it moved south over New Zealand, producing winds gusting to 75 m/s (270 kph) in Wellington, and sinking the interisland ferry Wahine with the loss of 51 lives. Cyclone Bola dumped over 900 mm of rain and produced hurricane-force winds in regions of northern New Zealand in March 1988. More recently, Cyclones Fergus and Drena brought torrential rain and storm-force winds to the North Island in December 1996, triggering an exodus of summer tourists from coastal resorts. Cyclones near New Zealand Tropical cyclone “best track” and intensity data are compiled and archived by the MetService and kindly provided to NIWA. This database contains time, location and estimated intensity, for all south-west Pacific tropical cyclones detected since 1939. Before the advent of satellite imagery in 1969, many storms went undetected by the sparse observational network. Therefore only data from 1970 onward were used for this study. The top illustration to the right shows the average distribution for all tropical cyclones during 1970–1997. Of the nine or so that form each year, on average, slightly more than one will pass within 5° latitude (555 km) of Northland and Auckland. There are progressively fewer close encounters farther south. In the Tasman Sea, west of New Zealand, average storm motion – shown in the middle illustration (right) – is only about 5 m/s due south. On the other hand, east of New Zealand, tropical cyclones move away to the south-east at speeds exceeding 8 m/s. Average tropical cyclone intensity – see the bottom illustration – is greatest near latitude 20–25°S, indicating Mark Sinclair was formerly based at NIWA in Wellington and is now with Embry-Riddle Aeronautical University, Prescott, Arizona, USA. Teachers: this article can be used for Planet Earth and Beyond L4 A.O. 1. See other curriculum connections at www.niwa.co.nz/ pubs/wa/resources Recommendations from International Experts’ meeting The work described in this article is being conducted following recommendations from a recent international Extratropical Transition (ET) workshop. Risk Prediction Initiative, a science/ business partnership, funded the workshop. At the meeting, leading authorities on ET proposed an agenda to guide future ET research. The full research agenda is found at www.bbsr.edu/rpi/ (then search for “extratropical transition”). Recent NIWA-sponsored research has addressed several Risk Prediction Initiative recommendations for the south-west Pacific Ocean basin. One of these recommendations is to compile basic statistics for ET for each of the ocean basins that experiences hurricanes (called tropical cyclones in our region). This research extends and synthesises a number of case studies of individual ET events in the NZ region. Classification of south-west Pacific tropical cyclones Intensity Description Speed Central Class range (kph) pressure (hPa) 1 Tropical depression <34 >995 2 Gale 34–47 995–986 3 Storm 48–63 985–975 4 Hurricane >63 <975
Transcript of How often is New Zealand hit by tropical cyclones?
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NIWA Water & Atmosphere 10(1) 2002
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METEOROLOGICAL RESEARCH
How often is New Zealandhit by tropical cyclones?Mark Sinclair
Recent NIWAresearch showsthat northern NewZealand is hit byan average of alittle over onestorm of tropicalorigin each year.The severity ofthese stormsdepends on theirlocation and onthe phase of theEl Niño/La Niñacycle.
Every year between December and April,storms from the tropics move south towardNew Zealand. As these tropical cyclonesadvance poleward, they eventually losecharacteristic hurricane features such as the eyeand the surrounding symmetric cloud andprecipitation region and progressively acquirethe asymmetric cloud and thermal featuresmore typical of a frontal mid-latitude storm.Scientists use the term extratropical transition (ET)to refer to this poleward movement andaccompanying structure changes.
As these storms move into New Zealandwaters, they often maintain sufficient vigour toproduce damaging winds, high seas and heavyrain. Occasionally, tropical cyclone remnants re-intensify in the extratropics to become potentmid-latitude storms capable of inflicting lossof life and severe property damage.
Several memorable ET events have occurredin the New Zealand region. In April 1968,Tropical Cyclone Gisele re-intensified as itmoved south over New Zealand, producingwinds gusting to 75 m/s (270 kph) inWellington, and sinking the interisland ferryWahine with the loss of 51 lives. Cyclone Boladumped over 900 mm of rain and producedhurricane-force winds in regions of northernNew Zealand in March 1988. More recently,Cyclones Fergus and Drena brought torrentialrain and storm-force winds to the North Islandin December 1996, triggering an exodus ofsummer tourists from coastal resorts.
Cyclones near New ZealandTropical cyclone “best track” and intensity dataare compiled and archived by the MetServiceand kindly provided to NIWA. This database
contains time, location and estimated intensity,for all south-west Pacific tropical cyclonesdetected since 1939. Before the advent ofsatellite imagery in 1969, many storms wentundetected by the sparse observationalnetwork. Therefore only data from 1970 onwardwere used for this study.
The top illustration to the right shows theaverage distribution for all tropical cyclonesduring 1970–1997. Of the nine or so that formeach year, on average, slightly more than onewill pass within 5° latitude (555 km) ofNorthland and Auckland. There areprogressively fewer close encounters farthersouth.
In the Tasman Sea, west of New Zealand,average storm motion – shown in the middleillustration (right) – is only about 5 m/s duesouth. On the other hand, east of New Zealand,tropical cyclones move away to the south-eastat speeds exceeding 8 m/s. Average tropicalcyclone intensity – see the bottom illustration– is greatest near latitude 20–25°S, indicating
Mark Sinclair wasformerly based atNIWA in Wellingtonand is now withEmbry-RiddleAeronauticalUniversity, Prescott,Arizona, USA.
Teachers: this articlecan be used forPlanet Earth andBeyond L4 A.O. 1.See other curriculumconnections atwww.niwa.co.nz/pubs/wa/resources
Recommendations fromInternational Experts’ meetingThe work described in this article is beingconducted following recommendations from arecent international Extratropical Transition (ET)workshop. Risk Prediction Initiative, a science/business partnership, funded the workshop. Atthe meeting, leading authorities on ETproposed an agenda to guide future ETresearch. The full research agenda is found atwww.bbsr.edu/rpi/ (then search for“extratropical transition”).
Recent NIWA-sponsored research hasaddressed several Risk Prediction Initiativerecommendations for the south-west PacificOcean basin. One of these recommendations isto compile basic statistics for ET for each of theocean basins that experiences hurricanes(called tropical cyclones in our region). Thisresearch extends and synthesises a number ofcase studies of individual ET events in the NZregion.
Classification of south-west Pacific tropicalcyclonesIntensity Description Speed CentralClass range (kph) pressure (hPa)
1 Tropical depression <34 >9952 Gale 34–47 995–9863 Storm 48–63 985–9754 Hurricane >63 <975
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that most storms moving into New Zealandwaters are weakening. Storms that undergo ETin the Tasman Sea are more vigorous than thoseeast of New Zealand.
As the graph above shows, largest numbers oftropical cyclones make it into middle latitudes(south of 35°S) in February and March. If wecompare these events with overall cyclonenumbers (which also peak in February), we seethat March is the month during which thegreatest fraction of storms will take a track intomiddle latitudes. Put another way, this meansthat, given a tropical cyclone, it is most likelyto move south of 35°S in March. Thiscorresponds to the time of warmest seatemperature, a factor that scientists have shownto be essential for tropical storm survival.
An ENSO connectionThe location and intensity of ET is also affectedby the phase of the El Niño–SouthernOscillation (ENSO) cycle. During El Niño years,tropical cyclones enter middle latitudesanywhere between the Australian coast andabout 150°W and typically weaken quickly asthey move away to the south-east.
During La Niña years, ET is largely confined tolongitudes west of the dateline (longitude 180°).These storms move more slowly toward thesouth and tend to retain their vigour for longer.Interestingly, the strongest storms appear toaffect New Zealand when the ENSO cycle isbetween El Niño and La Niña. ■
Average distribution, motion and intensity for alltropical cyclones during 1970–1997.
top: Distribution. Contours represent the number oftropical cyclones passing within 555 km of each pointper year, drawn at intervals of 0.5; values greater than2 and 3 are shaded.
middle: Average storm motion vectors, with speedcontours added every 2 m/s; speeds greater than 8and 10 m/s are shaded.
bottom: Average cyclone intensity as defined in thetable (left); values greater than 2.4 and 2.6 intensityunits are shaded.
Frequency by month of tropical cyclones progressingsouth of 35οS from 1970–1997 data. The total numberof storms was 81.
