Climate Change and the European Water Dimension - Agri ...

Chapter IV.A. Proxy indicators of Climate Forcing Across
Climate Forcing Across Euro
IV.A. Introduction
Consequences of climate variability for hydrology, ecology, and economy during the
last decades were often studied using proxy indicators for complex meteorological,
atmospheric, and/or oceanographic phenomena. The use of simple time-averaged
proxies reduces by definition complex space and time variability into simple
measures and allows researchers of different disciplines to link their work to climate
forcing in a straightforward manner. Proxies with relevance for European climate
include the indices for large-scale climatic patterns such as the North Atlantic
Oscillation (NAO) and Arctic Oscillation (AO), the El Nino Southern Oscillation
(ENSO), the Gulf Stream Position, and several more regional indices.
As there are several ways to define the spatial structure of specific climate patterns,
usually several indices, i.e. proxies do exist for the description of the temporal
dynamics of a climate pattern. Most often these indices are either derived from
simple pressure differences between relevant locations or from principal component
time series of the leading eigenvector of sea level pressures. However, different
indices for one climatic pattern are usually highly correlated, and hence will not be
presented and discussed in the following overview.
The best-known circulation pattern with respect to European climate is the NAO
representing a large-scale fluctuation in the air pressure difference between the
Azores High and the Iceland Low. It dominates much of the atmospheric behaviour
over the North Atlantic, and is known to influence air temperature and precipitation
over large areas of the Northern Hemisphere (Hurrell 1995, IPCC 2001). The NAO
fluctuates between positive index values, and a negative index values. The positive
(negative) phase is characterised with higher (lower) than normal surface pressures
near the Azores combined with anomalously low (high) pressure throughout the
Arctic and sub-Arctic. As a consequence, the prevailing westerly winds across the
North Atlantic are strengthened and moved northwards during a positive phase of the
NAO, whereas the westerly winds are weakened during a negative phase of the
NAO. The NAO is closely related to the Arctic Oscillation (AO) and may be
(Thompson et al. 2003)– but this is still somewhat controversial (Hurrell et al. 2003)
a smaller-scale manifestation of the latter. Figures IV.A.1a,b).
Despite its presence throughout the year, the influence of the NOA on Europe is
strongest during winter (Figure IV.A.2). The regional pattern of NAO influences in
Europe is as follows: a positive NAO index is associated with enhanced storminess
from Southern Greenland across Iceland into northern Europe, and a modest
decrease in activity towards southern Europe. Wetter winters in high positive NAO
years have been recorded in Northern Europe, whereas drier conditions in high NAO
years occurred over much of central and southern Europe, the Mediterranean and
parts of the Middle East. As a consequence, in the Mediterranean and the Middle
East, river run-off (e.g., the Rhone, Ebro, Euphrates Rivers (Cullen and deMenocal
2000; Lloret et al. 2001)) and water levels (e.g., the Caspian Sea (Rodionov 1994))
were lower during high positive NAO phases, whereas river runoff in Irish and Welsh
rivers (Kiely 1999; Bradley and Ormerod 2001) and into the Baltic Sea was higher
(Hänninen et al. 2000). However, Spring flood discharges in the Warta and Bug
rivers in Poland were negatively related to the NAO Index, probably as a result of
decreased winter snow cover in high NAO years (Kaczmarek 2003).
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