Yanko Andrade

Upwelling on the Scotian Shelf Inferred from Satellite and Coastal Measurements

Thesis Approved August 1991

Satellite images of SST and related oceanographic data (i.e., coastal water temperature, wind, atmospheric pressure and sea level) are analysed as part of a study of temperature variability along the coast of Nova Scotia. Simple dynamical balances and multiple spectral analysis are used to understand and model the observed temperature changes in the summers of 1985 and 1986.

It is shown that approximately 75% of the night- pass AVHRR satellite images may be affected by undetectable fog contamination. Near the coast IR- derived SST is also shown to be biased high by about 2 C. However, the high correlation between IR- derived SST and in situ measurements gives credibility to the spatial patterns mapped by the NOAA satellites. For example, the cold water seen in the image may indicate upwelling at specific sites along the coast rather than alongshore advection of temperature signals.

Temperature-inferred upwelling velocity is highly coherent with the alongshore wind stress in the 0.04-0.15cpd frequency range: a simple wind- driven inviscid model can explain the transfer function between wind and upwelling. The inclusion of interfacial friction is shown to have a minor effect of this frequency range. Surprisingly the coherence between upwelling and nearshore temperature is low. This is due to rapid temporal changes in the vertical temperature gradient which were used to estimate upwelling rather than a climatological mean gradient. It is also shown that local sea level does not respond to wind stress according to the simple frictional balance suggested by Sandstrom (1980); large sea level changes are often unrelated to changes in alongshore wind stress. However, from spectral analysis it is shown that the alongshore sea level difference is an important forcing term for nearshore temperatures. For frequencies between 0.04-0.15cpd about 80% of the temperature variance is accounted for by changes in the alongshore head. Although simple dynamical balances cannot easily account for this empirical relation, satellite imagery suggests that it might be the result of steric effects. Regardless of the dynamical basis of this newly discovered relationship between alongshore head and temperature, it could form the basis of an effective prediction scheme for nearshore temperature based on sea level observations which are now available in real time.