Time-scales of the variability of the atmosphere

In this study the time-scales of variability of several weather elements are explored by season and location across the globe, emphasizing the Northern Hemisphere and especially the USA. The resulting description is useful because regions that exhibit low frequency variability (i.e. longer periods than the 2-5 days synoptic-scale) are assumed to be related more directly to changes in boundary conditions (e.g. anomalies of ENSO-related sea-surface temperature [SST], snow cover, etc.). Therefore, this low frequency variability may be predictable at greater ranges than those for which numerical weather prediction is helpful. New as well as established measures of persistence and frequency dependence are used and intercompared. In particular, the standard deviation of the differences between adjacent period means, when compared over a range of period lengths, reflects both autocorrelation and (if applicable) cycle time. Frequency dependence is thereby summarized with minimal computation. The geographical distribution of the amplitude (amount of variability depends largely on latitude and the upstream geographical environment (i.e. higher latitude and continentality of upstream environment tend to increase variability). At most locations, variability is greatest (lowest) during the cold (warm) seasons of the year. The geographical distribution of the dominant frequencies of variability are examined by season for Northern Hemisphere sea-level pressure and 700 hPa geopotential height, and USA surface temperature and precipitation. It is demonstrated that the dominant frequencies tend to vary in parallel across all four fields. In general, weather variables are found to vary at relatively low frequency (long periods) at high latitudes and, to a lesser extent, at subtropical latitudes. At mid-latitude, low frequency variability prevails most over the blocking regions in the eastern and central North Pacific and North Atlantic oceans. High frequency variability occurs in the synoptically active jet exit regions over the western oceans and the eastern and central parts of the Northern Hemisphere continents. Data from the Atmospheric Model Intercomparison Project integration of the National Centers for Environmental Prediction (formerly National Meteorological Center) medium-range-forecast general circulation model, which reproduce the Northern Hemisphere frequency dependence well at 700 hPa, indicate roughly analogous behaviour in the Southern Hemisphere. However, the longitudinal variation of mid-latitude frequency dependence is substantially less in the Southern Hemisphere, possibly because of the comparative absence of large, topographically significant land masses with favourable separation distance.