Deep Water Renewal in the Strait of Georgia

Abstract

The Strait of Georgia is a semi-enclosed basin on the Canadian west coast in which exchange with the shelf is restricted by narrow constrictions and shallow sills. The local dynamics are mostly dominated by the mixed tides and by the estuarine circulation that is forced mainly by discharge of fresh water from the Fraser River. The intermediate and deep water of the Strait are renewed through discrete deep water renewal events during which dense water flows over the sills and into the interior basin. Several data sets are closely examined to better understand the nature and variability of the deep water renewal process over a wide range of time scales. Measured deep water renewal (DWR) events are clearly identified, as well as their effects on the water properties in the deep basin. It is found that the events can be classified into two categories: late winter intrusions bring cold, oxygen rich water, and late summer events bring warm, saline, low oxygen water. These two DWR seasons determine the annual cycle of the deep water properties in the Strait of Georgia.For both seasons, the DWR events always occur following a neap tide, at which time the density of bottom water in the sill area peaks. However, not all neap tides are followed by a DWR event. During the DWR seasons, discrete events are found to occur every second neap tide. It is believed that this monthly periodicity is required to allow enough time for the salinity (density) of the deep water near the sill to increase sufficiently following the flushing of dense local water by the previous DWR event. Also, the timing of the two DWR seasons is explained in terms of the yearly cycle of the surface and bottom water density in the estuary: DWR events are more likely to happen at the beginning (spring) and end (fall) of the coastal upwelling season, when the Fraser River discharge is not too large. Finally, it is shown that, during El Niño years, unfavorable conditions develop that can shut down the late winter DWR season, leading to much warmer deep water during the following winter.