Environmental Engineering Reference
In-Depth Information
1
Introduction
1.1 Noise-induced phenomena
Most environmental dynamics are affected by a number of random drivers. This
randomness typically results from the uncertainty inherent to the temporal or spa-
tial variability of the driving processes. For example, if we consider the temperature
record measured at a certain meteorological station, we can easily notice some obvi-
ous deterministic components of climate variability associated with the daily rotation
of the Earth or with the annual seasonal cycle. At longer time scales we might rec-
ognize some patterns of interannual climate variability (e.g., the El Ni no Southern
Oscillation or the North Atlantic Oscillation) associated with temporally and spatially
coherent anomalies in the atmospheric and oceanic circulations. These anomalies
exhibit a certain degree of regularity in addition to unpredictable random fluctua-
tions. However, besides these daily, annual, and interannual oscillations (and other
deterministic signals), the temperature record will also exhibit some disorganized
fluctuations that are typically ascribed to
environmental randomness
. In stochastic
models of environmental dynamics this randomness is commonly expressed as
noise
.
Random environmental drivers are ubiquitous in nature. The occurrence of rain-
fall, sea storms, droughts, fires, or insect outbreaks are typical examples of random
environmental processes. The noise underlying these processes is an important cause
of environmental variability. What is the effect of this noise on the dynamics of
environmental systems? Systems forced by random drivers are commonly expected
to exhibit random fluctuations in their state variables. Thus the effect of noise is
typically associated with the emergence of disorganized random fluctuations in the
state of the system about its stable state(s). However, this trivial effect of noise is
not the only possible way in which random drivers can affect a dynamical system.
In the physics literature it was reported that noise can have a more fundamental role
(e.g.,
Horsthemke and Lefever
,
1984
;
Cross and Hohenberg
,
1993
;
Garcia-Ojalvo and
Sancho
,
1999
;
Sagues et al.
,
2007
). In fact it can induce new ordered states and new
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