About the anticyclone trend maps
Anticyclone indices
High pressure systems (often called anticyclones) are so called since they represent regions of increased atmospheric pressure. Regions of increased atmospheric pressure are associated with clear skies and no rainfall. In Australia, high pressure systems are also associated with warm daytime temperatures and cool overnight temperatures.
High pressure systems are associated with an anticyclonic rotation of air about a central high point in atmospheric surface pressure, which is anticlockwise in the southern hemisphere. The depth of the central atmospheric pressure and the gradient in pressure determine the strength of the anticyclone. In terms of climate change monitoring, long-term changes in the nature or number of high pressure systems in a given region may indicate either an increase or decrease in stable atmospheric conditions. For example, an increase in the occurrence of high pressure systems or an increase in central atmospheric pressure over time generally indicates clearer conditions and reduced rainfall.
Here, we provide two measures of high pressure systems:
(a) Anticyclone density represents a count of the number of anticyclones 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) Anticyclone intensity represents the average central pressure of the high pressure systems for a given region over a given season.
Analyses available
Trend maps of anticyclone density and anticyclone 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 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 anticyclone density and anticyclone 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 anticyclone density and anticyclone 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 (1994). The intensity of anticyclones in terms of gradients of pressure is required to be at least 0.075 hPa per degree of latitude squared, as averaged within a 5 degree (~550 km) radius of the centre of the anticyclone.
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 1994. A climatology of southern hemisphere extratropical anticyclones. Climate Dynamics, 10, 333-348.
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.
