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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.