About the cyclone trend maps
Cyclone indices
Low pressure systems (often called cyclones) include midlatitude cyclones, monsoon lows, and tropical cyclones, and are responsible for much of the rainfall and strong winds we experience. Their distribution and strength reveal rainfall locations and regions of potentially dangerous weather.
Low pressure systems are associated with a cyclonic rotation of air about a central low point in atmospheric surface pressure, which is clockwise in the southern hemisphere. The depth of the central atmospheric pressure and the gradient in pressure determine the strength of the cyclone. In terms of climate change monitoring, long-term changes in the nature or number of low pressure systems in a given region may indicate changes in storm tracks and hence impact upon rainfall and temperature changes.
Here, we provide two measures of low pressure systems:
(a) Cyclone density represents a count of the number of cyclones for a given area for a given season. This is expressed per degree of latitude (a grid area of 111 km by 111 km) squared as an average number.
(b) Cyclone intensity represents the average central pressure of the low pressure systems (in hPa) for a given region over a given season.
Analyses available
Trend maps of cyclone density and cyclone central intensity are available from 1950 to present for two regions. These are Australia (from 0° to 50°S and from 94°E to 174°E) and the whole of the southern hemisphere. Prior to (around) 1980 the data become less reliable over the Southern Ocean. Prior to the advent of satellite data in (around) 1980 the data become less reliable over the southern hemisphere, particularly over the Southern Ocean.
Interpreting the analyses
The trend maps are a useful way to compare how cyclone density and cyclone central pressure has changed in different regions over time. However, they need to be interpreted with caution. Trend values have been determined from a linear (straight line) fit to the data, but the change indicated may not have been gradual. For example, a calculated trend could be due to a relatively rapid "step" change, with the remainder of the series being fairly flat (see some of the timeseries graphs). In addition, the trends are calculated over a relatively short period of time. Users are advised to keep in mind the period over which trend values have been calculated and interpret them alongside the timeseries of spatially averaged values.
The trend values calculated here using past reanalysis/observations should not be used to imply future change. Due to the complex interactions between the natural and anthropogenic drivers, the climate system at any location is always changing. Future changes will depend on how these drivers interact, which will not necessarily be the same as in the past.
Data used
The data used to analyse cyclone density and cyclone central pressure are the NCEP/NCAR, 6-hourly, surface pressure reanalysis. These data are available from the National Center for Environmental Prediction (NCEP) website http://www.ncep.noaa.gov/. The analysis method makes use of the cyclone tracking algorithm developed at the Bureau of Meteorology and the University of Melbourne described in Jones and Simmonds (1993). The intensity of cyclones in terms of gradients of pressure is required to be at least 0.15 hPa per degree of latitude squared, as averaged within a 5 degree (~550 km) radius of the centre of the cyclone.
Please note that any use of the data analysed on these web pages should be acknowledged to the Bureau of Meteorology. Apart from the purposes of study, research, criticism and review, no part of these data may be reproduced, or redistributed for any commercial purposes, or distributed to a third party for such purpose, without written permission from the Director of Meteorology.
Further information
Murray RJ and Simmonds I 1991. A numerical scheme for tracking cyclone centres from digital data. Part I: Development and operation of the scheme. Australian Meteorology Magazine, 39, 155166.
Jones DA and Simmonds I 1993. A climatology of southern hemisphere extratropical cyclones. Climate Dynamics, 9, 131-145.
Simmonds I and Keay K 2000. Mean southern hemisphere extratropical cyclone behavior in the 40-year NCEP-NCAR reanalysis. Journal of Climate, 13, 873-885.
Simmonds I and Keay K 2000. Variability of southern hemisphere extratropical cyclone behavior, 1958-97. Journal of Climate, 13, 550-561.
Pezza AB, Simmonds I and Renwick JA 2007. Southern hemisphere cyclones and anticyclones: recent trends and links with decadal variability in the Pacific Ocean. International Journal of Climatology, 27, 1403-1419.
