19-23 January 2012:

 An intense windstorm left thousands of Vancouver Islanders out of power and forced BC Ferries to suspend service to the mainland on the 22nd. Wind gusts exceeding 110km/h were recorded in places, as a train of intense low pressure systems struck Vancouver Island's west coast one after another. Wave buoy data at the La Perouse Bank (located approximately 50km northwest of Folger Node, ISDM ID online data: C46206) showed extreme waves reaching as high as 18m on 22-23 January.

Watching from Below

Our Folger Passage node is connected to two different instrument platforms, Folger Pinnacle (25m) and Folger Deep (100m). Scientists use data from these platforms to study a variety of topics, including ocean biogeochemistry, land-ocean interactions, phyto- and zooplankton, fish and marine mammals. A recent pilot study by Ocean Networks Canada data specialist Dilumie Abeysirigunawardena found that these platforms might additionally be well-suited to study storm waves and storm impacts on ocean biology.

Deep Mixing

Data from Folger Deep revealed some interesting trends. At storm onset, oxygen concentrations increased significantly, probably due to mixing from wave action, and then levelled out. Water temperature also increased during the early stages of the storm. Salinity, on the other hand, dropped throughout the event.

When Folger Pinnacle data were examined, patterns were different than those observed at Folger Deep. Folger Pinnacle water temperatures dropped sharply at the onset of the storm. Apparently, storm waves were mixing warmer surface water downward and cool water upward from deeper layers into shallow layers. Surface cooling may have also been affected by rain water temperatures, although rain data remain to be examined.

Wind & Wave Data

An Acoustic Doppler Current Profiler (ADCP) on Folger Pinnacle platform has special wave measurement capabilities for assessing directional wind and swell waves. Although the pressure sensor of this ADCP was partially damaged, it is still able to collect data which can be corrected using other fully functional pressure sensors at this site. Having multiple sensors at one location is very advantageous, as it allows us to validate the accuracy of collected data. Secondary sensors can also be used as backup in the event a primary sensor fails at a location.

At the storm's peak on 23 January, the ADCP data from Folger Pinnacle indicated significant wave heights, reaching over 10m, with larger directional distributions of wave energy. The drop in the wind speeds from 110 to 35 km/h that day caused the wave energy to diminish substantially toward the end of the day.

The following set of plots from the Folger Pinnacle ADCP gives a detailed illustration of wave energy evolution over the day. Plot A shows the evolution of wave parameters on 23 January 2012. Early in the day, the waves were quite large and gradually diminished as the wind subsided. The thick black line marks peak wave heights from the day. Plot B shows wave energy distribution at the time marked by the thick black line in plot A. Plot C shows wave energy directions. Waves were propagating from almost all directions, but strongest energy was concentrated in the north to west quadrants. Plot D shows wave energy distribution over time. High frequencies indicate large sea waves; low frequencies indicate swell waves.

Zooplankton Distributions

Another interesting feature of the ADCP is its beam intensity data, which shows how objects like bubbles, zooplankton and fish move through the water column. Zooplankton, tiny organisms that feed on phytoplankton, typically migrate up to surface waters to feed at night when there is less of a chance of being eaten by visual predators; at dawn they descend into the depths for protection. These movements can be tracked on both our ADCP and our Biosonics echosounder at Folger. Surprisingly, these regular zooplankton migrations were apparently disrupted by the storm.

On 19 January the normal zooplankton migration occurred, but as winds strengthened on 20 January, their pattern was disrupted significantly and remained so for the rest of the storm. Even following the storm it took nearly 10 days for the zooplankton's regular migration schedule to resume. We speculate that perhaps zooplankton are not strong enough swimmers to ascend against such intense waves. Another explanation is that their food (phytoplankton) was more widely dispersed than normal. A third possible explanation is that bubbles injected into the water by the storm action masked the zooplankton signals. During some storm events, waves have been observed to drive air bubbles downward 30-50m into the water column.

Still Much More to Learn

This pilot study by our data specialist illustrates just a couple examples of some new ways scientists can use our seafloor instrumentation to study ocean dynamics and biology. We invite interested scientists to delve deeper, building on these initial observations. Contact us if you would like to know more about our instrumentation and find out how to access our data archives.