MODELING THERMOHALINE PROPERTIES IN AN ESTUARINE UPWELLING ECOSYSTEM (RIA-DE-VIGO, NW SPAIN) USING BOX-JENKINS TRANSFER-FUNCTION MODELS

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.