The annual cycle of temperature in a temperate estuary and associated heat fluxes to the coastal zone

Longitudinal and depth profiles of salinity and temperature were measured in the Tamar Estuary, southwest England, during 1985. During late autumn, winter and early spring, the observed temperature-salinity scatter plots illustrated either near-conservative behaviour or heat loss from the estuary to the atmosphere, whereas the rest of the year they illustrated heat gain from the atmosphere. Observed maximum and minimum freshwater temperatures occurred during late July (19.6°C) and mid January (3.1°C). Maximum and minimum observed coastal-water temperatures occurred during early October (15.1°C) and late February (6.2°C). Similar coastal (15.1°C) and offshore (15.4°C) surface temperatures were observed in summer as a consequence of stratified conditions offshore and the existence of a near-surface, mixed layer there. Air temperature, freshwater (zero salinity) temperature and coastal water temperature showed a strong correlation with coastal soil temperatures measured at 0.05, 0.3 and 1 m beneath ground, respectively. Simulated temperatures were derived from a model that solved the heat budget equations for 1985 on an hourly time-scale, using hourly measurements of local meteorological variables. The yearly average and standard deviation of temperatures in mid estuary were 11.6°C and 4.3°C, respectively. Seasonal temperature amplitude exceeded tidal and diurnal amplitudes by almost an order of magnitude. Temperatures exhibited strongly tidal, predominantly semidiurnal signals, as well as non-tidal, diurnal signals. The daily averaged rates of horizontal heat transport to the coastal waters from the estuary and its rivers during 1985 were typically between 1–2 GW during the spring and summer months. Heat transport was generally reversed during late autumn, winter and spring. The estuary then generally received heat from the coastal zone, with maximum rates of heat transport that were typically 3 GW. Because estuary residence times varied from less than a few days in winter to less than two weeks in summer, there was a close coupling (on seasonal time-scales) between estuarine heat fluxes and coastal seawater and terrestrial freshwater temperature differences.