Climatic Variability and Coherence in Time and Space

Abstract

In a medium-sized country (area 10⁴–10⁵ km²) horizontal climatic variations are smaller and less important than time variability, a fact often neglected in standard texts and tables. Extreme months, seasons and years are often responsible for scarcity of food (and thus high prices), cause high demands of water or energy supply and many economic disruptions. The frequency of such events varies with time and distance, expressed in statistical terms like variance, standard deviation or interannual variability. For selected European stations and areas running 30-year averages of long records (temperature, precipitation, runoff and water budget terms) are given, together with similar values of interannual variability. In general terms, high interannual variability occurred during the early 19th century, i.e. in the final phase of the “Little Ice Age”; in some areas (not all) low variability was observed during the early 20th century. 30-year seasonal averages of rainfall vary by 20–25 percent, in the Mediterranean by 30–50 percent. The interannual variability varies substantially with time.

Anomalies of temperature, precipitation, pressure (resp. geopotential) and wind are not only mutually correlated, they are also rather distinctly correlated in the space domain. Bjerknes and Namias investigated the possible use of sea-surface temperatures to understand the physical mechanism of extreme anomalies; in this case the autocorrelation often remains significant for periods up to 6–9 months, and large-scale teleconnections such as the “Southern Oscillation” or the Walker circulation have demonstrated unexpected and promising results. Examples of such teleconnections within the tropics and between different climatic belts are presented.