Temperature trends and interannual variability in the Strait of Georgia, British Columbia

A continuous 36 year long record of semi-monthly temperature profiles from the central Strait of Georgia, British Columbia is used to examine low frequency variability and trends through the water column. Decomposition of temperature anomalies into empirical orthogonal functions shows that the dominant mode accounts for 78% of the variance, while the principal component associated with this mode (PC1) is dominated by fluctuations on interannual time scales. To relate the variability within the Strait to that occurring over the northeast Pacific, PC1 is compared with anomalies in local air temperature, sea surface temperatures off the west coast of Vancouver Island, and upper ocean temperatures along Line-P. These comparisons suggest that much of the interannual variability observed in the Strait of Georgia occurs in response to large-scale atmospheric forcing over the northeast Pacific. However, following tropical El Niño events there are significant anomalies associated with processes occurring along the coastal oceanic wave guide. The strongest event in the entire record, the remarkable negative temperature anomaly of winter 1978/1979, appears to be associated with a deep water intrusion that was forced locally.A warming trend is observed over the period 1970–2005 through the entire water column of the Strait of Georgia, with a depth-averaged value of 0.024∘Cyr-1. The vertical variation of the linear trend in temperature is contrasted with trends observed through the upper water column in the adjoining northeast Pacific over the same time period. Comparable trends are observed within the upper 100 m of the water column. However, at greater depths trends in the Strait of Georgia exceed those observed offshore by a factor of two. This is likely a consequence of the entrainment of near-surface waters into the deep Strait by the estuarine circulation. The results illustrate how global climatic changes may be amplified at depth in a coastal sea.