E. Nogueira et al., MODELING THERMOHALINE PROPERTIES IN AN ESTUARINE UPWELLING ECOSYSTEM (RIA-DE-VIGO, NW SPAIN) USING BOX-JENKINS TRANSFER-FUNCTION MODELS, Estuarine, coastal and shelf science, 44(6), 1997, pp. 685-702
Since 1987, twice weekly, hydrological variables have been monitored a
t a fixed station in the Ria de Vigo (NW Spain), aiming to examine the
time scales of variability and the relationships to meteorological co
nditions. The present paper analyses: (1) the advantage of Box-Jenkins
transfer function (TF) models (single output-multiple input), a type
of linear stochastic model, to describe the dynamic behaviour of the s
ystem; and (2) the coupling between the Ria and meteorological events
at the time scale of autonomy of this coastal inlet affected by the Ib
erian coastal upwelling, approximately a fortnightly period. In order
to achieve these objectives, thermohaline properties have been used to
characterize the estuarine ecosystem (output variables), while wind r
egime, runoff in the drainage basin and incoming solar radiation have
been considered as the main forcing variables (input variables). The u
se of the amplitude rime series, derived from principal component anal
ysis (PCA) applied to the deseasonalized meteorological variables, is
also explored as a different set of input variables. When compared wit
h standard regression models, all TF models built to describe thermoha
line behaviour had reduced residual variance. Similar TF models, as we
ll as percentage of explained variance, were also obtained when meteor
ological variables or the amplitude time series were used as input var
iables. The fitted TF models provided an insight into the 'inertial' b
ehaviour of the system and the time scales of coupling of the system w
ith the forcing variables. The plausible physical mechanisms which lin
k the response of the system with the observed meteorological variabil
ity are also discussed. As could be expected, bottom thermohaline prop
erties show a stronger inertial behaviour than the surface ones, which
is particularly marked for bottom temperature. Besides, the shelf dom
ain, by means of upwelling-downwelling events, strongly influences sur
face and bottom temperature, as well as bottom salinity; by contrast,
surface salinity is mainly influenced by the effect of wind along the
main axis of the Ria and runoff. In relation to the rime scales of cou
pling between the system and the forcing variables, thermohaline prope
rties show a dependance with the meteorological conditions in, at leas
t, the immediately preceding fortnight period. It was concluded that:
(1) TF models that incorporate meteorological information described th
e dynamic behaviour of the system adequately; and (2) this type of mod
el can be useful as a first approximation to develop more sophisticate
d (deterministic) models, since, with the purpose of modelling any sta
te variable of the system, both the coupling between different domains
and the time scales of the interactions must be taken into account. (
C) 1997 Academic Press Limited.