Physical forcing of the water temperature variability along the Espinheiro Channel (Ria de Aveiro, Portugal)

Abstract

Water temperature data resulting from 1-year monitoring al ong 10 km of an estuarine tidal channel located inside Ria de Aveiro lagoon (NW coast of Portugal) were analyzed to investigate its spatial and temporal variability, as well as its response to the effects of two major forcing: tides and meteorological conditions. Water temperature is being monitored since 2004, using a new technology composed by an optical sensing cable fixed along the channel bed. It extends from the lagoon inlet to the river mouth, integrating fiber Bragg grating sensors to measure the water temperature each 500 m. The data under an alysis consist in 18 annual time-series measured in 2004-05 by each sensor. Mathematical techniques as sp ectral analysis and Empirical Orthogonal Functions (EOFs) were used in order to establish the semidiurnal and seasonal variation of the water temperature inside the channel. The results showed that the water temperature variability is induced by the major forcing factors (tide and meteorological conditions), clearly indicating that th e high-frequency oscillations of the water temperature are strongly related to the solar heating daily variation and to the semidiurnal and diurnal tidal effect. On the other hand, the low-frequency oscillations are modulated by the weather and/or long-term meteorological events. EOFs analysis indicated that the variability in the wa ter temperature time series can be accounted by the first component, which is closely related to the inter-annual variation of the air temperature. Besides the day/night and tidal variations, the water temperature variability is also determined by the seasonal changes on the meteorological conditions, especially due to the shallow channel’s depth.