Many terms are used for the description of tropical cyclone activity and conditions. The following list provides a number of these terms and brief definitions. A list of suggested websites for further information is provided at the end of this page and on the 'Web Links' menu. In addition to the definitions below, two brief paragraphs follow, which discuss the conditions necessary for the development of a tropical cyclone and the impact of ENSO on tropical cyclone activity.

TROPICAL CYCLONE Generic term used by the World Meteorological Organisation to define weather systems developing over tropical or subtropical waters in which winds exceed 63 kph; tropical cyclones also have a definite organised surface circulation winds rotate anti-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere in the SW Indian Ocean also used as naming for tropical cyclones with maximum average windspeeds above 74 mph (119 kph)

TROPICAL DISURBANCE Weather system giving rise to a specific area of cloudiness with embedded showers and thunderstorms

TROPICAL DEPRESSION Tropical cyclone system with definite anti-clockwise wind circulation with maximum sustained winds of below 39 mph (61 kph); the system is given a number

TROPICAL STORM Tropical cyclones with maximum average windspeeds above than 39 mph (61 kph) and assigned a name

HURRICANE Tropical cyclone with maximum average windspeeds above 74 mph (119 kph) in the North Atlantic, NE Pacific (E of the dateline), S Pacific (E of 160E) organised weather system with a well defined eye and with winds rotating in an anti-clockwise spiral around a region of low pressure

TYPHOON Tropical cyclone with maximum average windspeeds above 74 mph (119 kph) in the NW Pacific (including the Philippines) (W of the dateline)

SUPER-TYPHOON Typhoon that reaches maximum sustained 1-minute surface winds of at least 150 mph (234 kph). This is the equivalent of a strong Saffir-Simpson category 4 or 5 hurricane (see chapter 19) in the Atlantic or North East Pacific. This term is used by the U.S. Joint Typhoon Warning Centre in Guam

INTENSE TYPHOON Having intense hurricane (of Saffir-Simpson category 3 and above) wind status

SEVERE TROPICAL CYCLONE Tropical cyclone with maximum average windspeeds above 74 mph (119 kph) in the SW Pacific (W of 160E), SE Indian Ocean (E of 90E)

SEVERE CYCLONIC STORM Tropical cyclone with maximum average windspeeds above 74 mph (119 kph) in the N Indian Ocean

SUB-TROPICAL CYCLONE Low-pressure system (not named) existing in the tropical or subtropical latitudes that has characteristics of both tropical cyclones and mid-latitude cyclones, often transform into true tropical cyclones eg tropical storm Leslie (2000)

SAFFIR-SIMPSON HURRICANE INTENSITY SCALE 5 category system giving an estimate of the potential flooding and damage to property based on the estimated intensity of a hurricane; originally developed for use in the Atlantic and North East Pacific, now also modified for use in the North West Pacific

HURRICANE SEASON In the Atlantic runs from 1st June to 30th November In the North East Pacific runs from 15th May to 30th November In the Central Pacific runs from 1st June to 30th November In the Australian runs from 1st December to 30th April

TYPHOON SEASON In the North West Pacific runs from 1st January to 31st December, with the majority of the activity between 1st June and the end of the year

EYE The relatively calm centre of the tropical cyclone that is more than one half surrounded by wall cloud

STORM SURGE Abnormal rise in sea level accompanying a hurricane or other intense storm

MAXIMUM SUSTAINED WINDS The highest surface wind speed averaged over a 1-minute period of time. Some weather bureaus (eg. PAGASA, Japan Meteorological Agency, etc.) average it over a 10-minute period of time. (Peak gusts over water average 20 to 25 percent higher than sustained winds)

FUJIWARA EFFECT The rotation of two storms around each other

ENSO El Niño-Southern Oscillation, with El Niño being the reversals in oceanic warming and the Southern Oscillation being the changes in atmospheric pressure. The ocean warming and pressure reversals are, for the most part, simultaneous. For example, when atmospheric pressure is low in the South Pacific high pressure cell and high over Indonesia and Australia, the Pacific trade winds weaken, upwelling of cool water on the Pacific equator and along the Peruvian coast weakens or stops, and SSTs increase in these areas where the upwelling weakens.

SOUTHERN OSCILLATION The see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific; when the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific, and vice-versa

EL NIÑO An abnormal warming of surface ocean waters in the eastern tropical Pacific, also referred to as a warm event. The term was originally used to describe the appearance of warm (surface) water from time to time in the eastern equatorial Pacific region along the coasts of Peru and Ecuador. El Niño has a return period of four to five years; when an event occurs, it often lasts from 12 to 18 months

LA NIÑA The appearance of colder-than-average sea surface temperatures (SSTs) in the central or eastern equatorial Pacific region (the opposite to conditions during El Niño). Many scientists do not like the use of the term and prefer to call it a cold event

WARM EVENT Anomalous warming of SSTs in the central and eastern equatorial Pacific, also referred to as El Niño. A warming in the regions mentioned is accompanied by a relative cooling in the western equatorial Pacific

COLD EVENT SSTs become anomalously colder compared to the long-term average for the central and eastern equatorial Pacific region, also referred to as La Niña. The opposite of a warm (El Niño) event in that region

CONDITIONS NECESSARY FOR THE DEVELOPMENT OF A TROPICAL CYCLONE A number of preliminary conditions must be met for a tropical cyclone to develop, including a warm sea temperature > 26 °C, high relative humidity (the degree to which air is saturated by vapour), atmospheric instability, and a location of at least 4 - 5 latitude degrees from the Equator. The four phases of tropical cyclone development, linked to the associated wind intensities, are tropical disturbance, tropical depression, tropical storm, hurricane. Observations of the impacts of Atlantic tropical storms and hurricanes along the coast of the US mainland indicate that the amount of damage does not increase exponentially with wind speed. Hurricane category 4, 148 mph (238 kph) windspeeds are estimated to produce on average up to 250 times the damage of a Saffir-Simpson category 1 hurricane.

IMPACT OF EL NIÑO / LA NIÑA ON TROPICAL CYCLONE ACTIVITY El Niño is widely believed to lead to reduced hurricane activity in the north Atlantic (Gray, 1984, Gray 1993), to increased typhoon occurrence in the eastern part of the northwest Pacific (Gray 1993) and to decreased typhoon occurrence in the western part of the northwest Pacific (Gray 1993). The impact of El Niño on landfalling hurricanes has been examined for US landfalling events. It has been found that the probability of two or more hurricanes making landfall on the US mainland during an El Niño year is 28% compared to 66% during a La Niña year (Bove et al 1998). Differences between El Niño and La Niña impacts on typhoon frequency have been found to be much smaller than those found for the Atlantic (Saunders and Roberts 1999). Over much of the northwest Pacific, and also for landfalling events in Japan, South Korea, Taiwan and China, El Niño has a dominant influence on increased frequency. In the northern Philippines and land areas bordering the South China Sea, typhoons have been found to be more common during La Niña months (Saunders and Roberts 1999). The eastern portion of the Northeast Pacific, the Southwest Indian, the Southeast Indian/Australian, and the North Indian basins have either shown little or a conflicting ENSO relationship and/or have not yet been examined in sufficient detail.

IMPACT OF EL NIÑO / LA NIÑA ON TROPICAL CYCLONE ACTIVITY Landsea Frequently Asked Questions www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html Typhoon 2000 www.typhoon2000.ph/ Philippines Atmospheric, Geophysicial and Astronomical Services Administration Ð Weather Branch www.pagasa.dost.gov.ph/wb/wb.htm Hurricane Warning www.hurricanewarning.net/ Canadian Hurricane Centre www.ns.ec.gc.ca/weather/hurricane/ NOAA El Niño pages www.elnino.noaa.gov/ Australian Bureau of Meteorology www.bom.gov.au/info/cyclone/

Impact of El Niño / La Niña on tropical cyclone activity References Bove, M.C. et al., 1998, Effect of El Niño on U.S.landfalling hurricanes,revisited. Bulletin of the Amer.Met. So. Gray, W.M.,1984, Atlantic seasonal hurricane frequency: Part I: El Niño and 30mb quasi-biennial oscillation influences. Mon. Wea. Rev., 112 112 112 112,1649-1668 Gray, W.M.,1993, Seasonal Forecasting. Chapter 5 in Global Guide to Tropical Cyclone Forecasting, WMO/TD-No.560 Saunders, M.A.and Roberts, F.P., 1999, El Niño impact on landfalling intense tropical yclones, Proc.23rd AMS Conference on Hurricanes and Tropical Meteorology, Dallas, January 10-15, 1999