Modeling the formation and circulation processes of water masses and sea ice in the Gulf of St. Lawrence, Canada

Abstract

[1] The seasonal cycle of water masses and sea ice in the Gulf of St. Lawrence is examined using a three-dimensional coastal ice-ocean model with realistic tidal, atmospheric, hydrologic, and oceanic forcing. The model includes a level 2.5 turbulent kinetic energy equation. A model simulation over 1997-1998 is verified against available data on sea ice, temperature, and salinity. The results demonstrate a consistent seasonal cycle in atmosphere-ocean exchanges and the formation and circulation of water masses and sea ice. The accuracy of radiative, momentum, and sensible heat exchanges at the sea surface, and the production of turbulent kinetic energy from winds and tides, are critical to the accuracy of the modeled circulation. The analysis of the mean error on near-surface temperature and salinity in the late summer and fall using standard bulk exchange coefficients and radiation (about 1°C too cold and 1 salinity unit too fresh) shows the tradeoff between tidal mixing at the head of the Laurentian Channel, and winddriven circulation and mixing in the surface waters. The results suggest year-long stratification in the estuary and northwestern Gulf, with little mixing except near the head region, where relatively deep warmer waters are mixed to the surface during winter, and cold intermediate waters are efficiently withdrawn during summer. The results suggest that the summer cold waters found at intermediate depths in the estuary and northwestern Gulf are not formed in situ. A significant fraction of these waters enters through the Strait of Belle Isle in wintertime, eventually reaching the estuary within about 6 months.

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…The warm and relatively fresh surface layer (0 to 30 m) overlies the cold intermediate layer or CIL (30-150 m deep; S P = 32.0 to 32.6) that is formed by advection of the GSL's wintertime surface mixed layer (Galbraith, 2006). Below the CIL, a warmer (2 to 6 • C) and saltier (S P = 33 to 35) bottom layer (> 150 m deep), originating from the mixing of western-central Atlantic and Labrador shelf waters that intrude at depth primarily through Cabot Strait, flows sluggishly landward (∼ 0.5 cm s −1 ; Bugden, 1988) toward the head region of the Laurentian Channel (Saucier et al, 2003;Gilbert et al, 2005).…”

“…The termination (head) of the Laurentian Channel at an abrupt and shallow sill near Tadoussac marks the region of transition between the upper and lower estuaries and is an area of complex tidal phenomena (Gratton et al, 1988). Due to rapid shoaling, tidal movements (e.g., internal tides and strong flows over the steep sill) locally generate significant mixing of surface freshwater with cold, nutrient-rich waters from the intermediate and deep layers of the GSL, resulting in a fertile surface layer that flows continuously seaward (Coote and Yeats, 1979;Saucier and Chassé, 2000) and sustains important feeding habitats for several large marine mammals (Dufour and Ouellet, 2007). The lower estuary's seaward outflow, together with the Gaspé Current, a rapidly moving coastal jet, are a major input of nutrients and zooplankton to the near-surface waters of the GSL (Coote and Yeats, 1979;Plourde and Runge, 1993).…”

“…The lowest surface-water pCO 2 values were found downstream of the MTZ in the lower reaches of the SLE near Pointe-des-Monts, where the channel widens into the GSL. Due to favorable environmental conditions (nutrients, light, and stratification), phytoplankton blooms typically occur in late spring or early summer in the LSLE (Zakardjian et al, 2000), with maximal biological production occurring in its downstream portion due to the mixing of cold nutrient-rich waters, which upwell at the head of the Laurentian Channel, with warmer freshwaters flowing in from the north-shore rivers (Savenkoff et al, 1994). Seaward of the estuary-gulf boundary, the pCO 2 gradually increased from 207 to 478 µatm, coinciding with rising surface-water temperatures (T = 3.9 to 13.7 • C).…”

Evaluation of an operational ice–ocean analysis and forecasting system for the Gulf of St Lawrence

“…A complete seasonal cycle of the GSL was simulated in Saucier et al (2003), reproducing main circulation features driven by high-frequency boundary forcing, including hourly atmospheric conditions, daily river runoffs, and tidal predictions at the scale of minutes. Modelling developments presented later in Saucier et al (2004Saucier et al ( , 2009) allowed, with the same type of forcing and without data assimilation, to produce multi-year simulations of the GSL capturing interannual variations.…”

“…Realistic simulations of oceanic conditions in the GSL with numerical models were made possible following studies from Saucier et al (2003Saucier et al ( , 2004Saucier et al ( , 2009) and other references therein. A complete seasonal cycle of the GSL was simulated in Saucier et al (2003), reproducing main circulation features driven by high-frequency boundary forcing, including hourly atmospheric conditions, daily river runoffs, and tidal predictions at the scale of minutes.…”