Tropical cyclones - some basics

a. Tropical cyclones (TC) require six criteria to be met (1) -

1. low-level convergence onto a surface low-pressure region

2. warm moist tropical air (thereby possessing potential instability)

3. a local sea-surface temperature above 26° C

4. only weak vertical shear within 4 degrees latitude of the centre of convection

5. a Coriolis force at least that of places at 4 degrees latitude

6. a large-scale high-pressure region (ie divergence) in the upper troposphere

b. 95% of TC’s affecting Australia arise between 9 - 19° S. Almost all form within a ‘steering current’ in the Trade winds, just south of the ‘shear line’ between Trade winds and monsoon westerlies. In other words, TC’s are steered by subtropical highs, though the latter may be disturbed by upper-level lows. A high deflects the TC to higher latitudes, where the water is colder, so that the TC decays, releasing its energy. So TC’s transfer energy polewards (1).

c. The main characteristic of TC’s (compared with other kinds of low-pressure region) are the warm core combined with the lowest winds being the strongest. The definition of a TC adopted by the Australian Bureau of Meteorology in 1978 is as follows -

"a non-frontal synoptic-scale cyclonic rotational low-pressure system of tropical origin, in which 10-minute mean winds of at least 17.5 m/s occur, the belt of maximum winds being in the vicinity of the system’s centre."

d. The reference for Fig 13.12 in the textbook is Neuman’s (1).

e. The eastwards extension of the warm pool in the western equatorial Pacific ocean during an El Nino event means that TC’s arise further upwind of Australia, and hence are more likely to curve to the south and then west before reaching that continent. The median longitude of the origins of a group of TC’s forming during El Nino’s was 175 oE, whereas at other times the median was 156 oE, which is about 2,000 km nearer Australia (2, 3). Thus fewer TC’s damage the continent during El Nino periods.

f. The size of the eye depends on the intensity of the TC. The more intense (ie the lower the central pressure) the smaller (sic) the eye, since greater centrifugal force (ie faster rotation) is needed to balance it. For instance, a paricular TC in 1988 had a central pressure of 960 hPa, maximum winds of 200 km/h and an eye 55 km across, but 36 hours later the pressure had fallen to 888 hPa, the speed risen to 296 km/h and the eye shrunk to only 15 km (1).

g. The coastal damage caused by ‘storm surge’ when a TC strikes land is greatest if the crossing occurs at high tide, and worst on the side of the TC with onshore winds, ie south of the eye in the southern hemisphere. The seriousness of the impact depends also on the shape of the coastline. Most of the fatalities caused by TC’s are due to storm surge. A TC striking the coast creates vertical wind shear which may be enough to generate tornadoes near the shore (1).

h. Atlantic tropical cyclones are called ‘hurricanes’. Those occurring in the north Atlantic during 1951-90 (averaging 9.7 each year) were more numerous than the estimated 8.5 annually during the 40 years a century earlier (4).

(1) Neumann, C.J. 1993. Global Guide to Tropical Cyclone Forecasting WMO Tech. Doc. 560, Report no. TCP-31 (World Meteor. Organ.) 43pp.

(2) Sturman A. and N. Tapper 1996. The Weather and Climate of Australia and New Zealand (Oxford) 476pp.

(3) Hastings, P.A. 1990. Southern Oscillation influences on tropical cyclone activity in the Australian/south-west Pacific region. Internat. J. Climatology 10, 291-8.

(4) Fernandez-Partagas, J. and H.F. Diaz 1996. Atlantic hurricanes of the second half of the nineteenth century. Bull. Amer. Meteor. Soc., 77, 2899-906.