Effects of climatic variability on the thermal properties of Lake Washington

We conducted a statistical analysis of a long-term (1964-1998) record of intra- and interannual temperature fluctuations in Lake Washington, Washington. Lake Washington has experienced a warming trend, with overall increases of 1.5 (0.045degreesC yr(-1)) and 0.9degreesC (0.026degreesC yr(-1)), respectively, for temperature data weighted over the surface (0-10 m) and entire lake volume. This warming trend was greatest for the period from April to September and was smallest and nonsignificant for November-February. A principal-components analysis of the long-term mean monthly temperature time series identified two independent modes of interannual variability. The first mode represented the months of the year when the lake warms and is warmest (e.g., March-October) and explained 54% of the variability in the overall time series. The second mode represented the months when the lake cools and is coldest (November-February) and explained 24% of the variability. The March-October mode was positively correlated with interannual variability in air temperatures and the Pacific Decadal Oscillation (PDO), multivariate r(2) = 0.65. The November-February mode was positively correlated with air temperature, PDO, relative humidity, solar radiation, and wind speed (r(2) = 0.83). A heat budget model indicated that long-term trends had a secondary role and interannual variability dominated, with exceptions being net long-wave atmospheric radiation and surface-emitted radiation, for which the long-term trend explained similar to25% and 53% of the total variance, respectively. An increase in incoming long-wave radiation fluxes was mainly associated with the increase in minimum daily temperatures (0.06degreesC yr(-1), r(2) = 0.47), especially during the March-October mode, when the linear trend accounted for 85% of the variability.