Strait of Georgia (SoG) semi enclosed, maximum depth 400 meters Wind controls ocean current Spr bloom Fraser low
The role of wind in determining the timing of the spring bloom in the Strait of Georgia.(Report)
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The Strait of Georgia (SoG) is a semi-enclosed, deep sea (maximum depth 400 m) located off the coast of mainland British Columbia, Canada (Fig. 1). In the southern SoG, the near-surface physical oceanography is dominated by the Fraser River plume and the estuarine flow it produces (Pawlowicz et al. 2007). The SoG is too large to be considered a classic estuary, but its dynamics are similar to those of smaller fjord estuaries. It is productive (yearly average productivity 1.6 g C x [m.sup.-2] x [day.sup.-1]) and, being a temperate sea, has a classic diatom-dominated spring bloom and a weaker fall bloom (R. Pawlowicz, A.R. Sastri, S.E. Allen, D. Cassis, O. Riche, M. Halverson, R. El-Sabaawi, and J.F. Dower, unpublished data). The timing of the spring bloom has been observed to vary interannually by as much as 6 weeks, with blooms as early as February and as late as mid-April (R. Pawlowicz, A.R. Sastri, S.E. Allen, D. Cassis, O. Riche, M. Halverson, R. El-Sabaawi, and J.F. Dower, unpublished data). …
In this paper, we will investigate the role of the principal physical forcings on the SoG, which vary interannually (wind, freshwater flux, and cloud fraction). We wish to determine which of these physical forcings, within the observed variation of the forcing, most strongly influence the timing of the spring bloom.
To directly observe mixing-layer depth, one can use various instantaneous turbulence measurements, but it is much harder to maintain measurements over an extended period, and unlike mixed depth, which is due to the integrative effects of mixing, mixing-layer depth varies rapidly. Here, the depth of the mixing layer will be calculated using a turbulence closure model designed for the surface ocean (Large et al. 1994). Recent one-dimensional (1D) models have considered the timing of the spring bloom in Prince William Sound (Eslinger et al. 2001) and the Bering Sea (Jin et al. 2006). In both cases, the phytoplankton is light-limited and the initiation of the bloom is due to reduced mixing and increased stratification. In the Bering Sea, ice is usually a controlling factor, but in ice-free years, Jin et al. (2006) showed that wind mixing and thermal stratification controlled the mixing-layer depth and thus the timing of the bloom. They were limited in their investigation of interannual variations as they had only one year of ice-free data. Using three years of data in Prince William Sound, Eslinger et al. (2001) showed that, again, wind mixing controlled the mixing-layer depth and the timing of the spring bloom. In both these regions, thermal stratification dominates over salinity stratification; whereas in the SoG, salinity stratification dominates due to the large freshwater fluxes. For the SoG, it has therefore been postulated that the beginning of the large freshwater flux due to snowmelt (the freshet) is necessary for the mixing layer to shallow and the bloom to begin (Yin et al. 1997b).