Annual and Seasonal Century Scale Trends of the Precipitation
163
When using the moving averages of the scores, the number of stations with
significant trends much increases as for the regions. But relations between regional
and local trends are the same as previously.
6 Comparison Between Centuries Old and Longer Trends
When station series longer than the twentieth century are available, they have been
compared with the previous ones. Different possibilities occur:
No change, significant and non significant trends remain the same (Miqwe,
Algiers, Gibraltar, etc.),
• trends remain non significant but their signs change (Safi),
• several trends which were not significant during the last century become
significant when the series are longer (Essaouira, Tunis, etc.).
7 Conclusion
During the twentieth century, most of the Mediterranean area had a non significant
evolution of its precipitation and only Greece had a significant linear rainfall
decrease at the century scale (even if each of the Greek stations had only a non
significant decreasing evolution). A few isolated stations belonging to the other
regions with non significant century scale trends present significant ones.
So, we can conclude that when rainfall trends are calculated from scores of a
factor obtained by RPCA, they perform as averages of the area stations.
When scores are transformed by moving averages, their linear trends can be
significant when score trends are not.
Some century scale trends of isolated stations become significant only ifthey are
calculated with series longer than a century.
References
IPCC (2001) Contribution of Working Group I to the Third Assessment report of the
Intergovernmental Panel on Climate Change. In: Houghton JT, Ding Y, Griggs DJ, Noguer M,
Van der Linden PJ, Xiasu D (eds.). Third Assessment Report: Climate Change 200 I (The
Scientific Basis). Cambridge University Press, UK, 944 P
Richman MB (1985) Rotation of principal components. Journal of Climatology 6, pp 293-335
163
When using the moving averages of the scores, the number of stations with
significant trends much increases as for the regions. But relations between regional
and local trends are the same as previously.
6 Comparison Between Centuries Old and Longer Trends
When station series longer than the twentieth century are available, they have been
compared with the previous ones. Different possibilities occur:
No change, significant and non significant trends remain the same (Miqwe,
Algiers, Gibraltar, etc.),
• trends remain non significant but their signs change (Safi),
• several trends which were not significant during the last century become
significant when the series are longer (Essaouira, Tunis, etc.).
7 Conclusion
During the twentieth century, most of the Mediterranean area had a non significant
evolution of its precipitation and only Greece had a significant linear rainfall
decrease at the century scale (even if each of the Greek stations had only a non
significant decreasing evolution). A few isolated stations belonging to the other
regions with non significant century scale trends present significant ones.
So, we can conclude that when rainfall trends are calculated from scores of a
factor obtained by RPCA, they perform as averages of the area stations.
When scores are transformed by moving averages, their linear trends can be
significant when score trends are not.
Some century scale trends of isolated stations become significant only ifthey are
calculated with series longer than a century.
References
IPCC (2001) Contribution of Working Group I to the Third Assessment report of the
Intergovernmental Panel on Climate Change. In: Houghton JT, Ding Y, Griggs DJ, Noguer M,
Van der Linden PJ, Xiasu D (eds.). Third Assessment Report: Climate Change 200 I (The
Scientific Basis). Cambridge University Press, UK, 944 P
Richman MB (1985) Rotation of principal components. Journal of Climatology 6, pp 293-335