Regional coherence and geographical variability in the surface water temperatures of Scottish Highland lochs
Livingstone, David M.; Kernan, Martin
In 25 lochs in the Grampians and Northwest Highlands of Scotland, miniature thermistors with integrated data loggers were employed in 2000 and 2001 to measure lake surface water temperatures (LSWTs). Results indicate that LSWTs fluctuate coherently in response to climatic forcing, and that additionally, they differ little in an absolute sense, implying that daily mean LSWTs can be upscaled easily. Regional coherence in LSWT is substantially higher in summer and autumn than in winter and spring. LSWT decreases approximately linearly with increasing altitude above sea level during winter and spring, but not otherwise; this contrasts strongly with the results of studies in other mountain regions, which show the most pronounced linear decrease in LSWT with altitude to occur in summer. LSWT showed no dependence on latitude at any time of the year, but showed a significant dependence on longitude – which can be interpreted as distance from the maritime influence of the Atlantic – especially during autumn and spring. In summer, no consistent dependence of LSWT on either altitude, latitude or longitude was found, implying that any differences in LSWT that may exist among the lochs in summer are predominantly the result of lake-specific local effects. The duration of the period during which LSWT ≤ 4°C depends on altitude and longitude, but not on latitude. The timing of the onset of this period in autumn occurs essentially simultaneously throughout northern Scotland, whereas the timing of the end of the period in spring becomes later with increasing altitude and increasing distance towards the east. This suggests that the duration of inverse stratification or circulation at temperatures less than the temperature of maximum density will tend to be less in low-lying lochs and/or lochs close to the Atlantic coastline than in high-altitude lochs and/or lochs located further from the Atlantic coast, and that this difference is entirely due to differences in climatic forcing that occur in spring.