Seasonal Variability and Generation Mechanisms of Nonlinear Internal Waves in the Strait of Georgia

Abstract

Seasonal variability and generation mechanisms of large nonlinear internal waves are reported, based on an analysis of 9 years of continuous observational data collected at nodes of the Ocean Networks Canada coastal observatory in the Strait of Georgia, Canada. About one thousand large nonlinear internal wave packets were identified. The timing of these packets suggests a very strong correlation with daily tides and fortnightly cycles. More waves are seen during the stronger tides of the fortnightly cycle but overall waves are seen in less than 40% of all days. Other wave characteristics show seasonal variations. In winter, when the pycnocline is weaker, both the spatial and temporal scales of internal waves are several times larger than summertime waves and wave amplitudes can approach 30 m. These waves are large enough vertically but small enough horizontally that standard acoustic Doppler current profiler (ADCP) processing algorithms present a strongly distorted view of wave shape and features. A specialized reprocessing of the beam velocities is used to correctly measure the wave velocity field and to determine wave propagation speed and direction. In addition to the previously known northward‐propagating waves in the Strait, we also found waves propagating in other directions and a noticeable correlation with winds, which may be a proxy for spatial variations in density and/or velocity shear. By considering wave propagation direction, relationship with tidal phase, and relationship with wind, three generation mechanisms were found. Two particularly interesting generation mechanisms involve waves radiating from the Sand Heads area.

Diurnal and Seasonal Variability of Near‐Surface Oxygen in the Strait of Georgia

“…Thus, our crude estimates of gross productivity are not necessarily inconsistent with the earlier measurements, but further validation would be required before they can be trusted. In particular, the large diurnal migration signal from zooplankton visible in this area (Li et al, 2018;Sato et al, 2013) may also modulate the diurnal signal of O 2 .…”

Studies of Internal Waves in the Strait of Georgia Based on Remote Sensing Images

“…Since 2008, two 150 kHz RDI Teledyne ADCPs and a 300 kHz RDI Teledyne ADCP were deployed separately at depths of 170 m, 300 m, and 100 m, respectively. More information about these observations is given by Li et al [7].…”

“…More recent studies include Wang and Pawlowicz in 2011, 2012, 2017 [4][5][6] who studied the large amplitude solitary-like internal waves in the Strait by combining airborne photo images and on-site water column measurements. The most recent studies in the Strait include the studies of nonlinear internal waves by Li et al in 2018 [7], who analyzed nine years of continuous observational data collected at three nodes of the Ocean Networks Canada coastal observatory in the Strait. Most previous field observations only looked at a small number of waves during a rather short time period.…”

“…Internal waves present extensively in the Strait of Georgia [8] due to the influence of the intense tidal current near the narrow waterways in the southern strait. Latest researches [4,7,9] showed that there are multiple internal wave generation mechanisms in the Strait of Georgia. Except for interactions associated with tides and topography, plume intrusion is another key potential formation mechanism.…”

“…Usually, in the Strait of Georgia, internal waves propagate in pycnoclines shallower than 10 m, with a typical wave length of 50~100 m. The wave fronts are as long as 10 km. The wave amplitudes (the amplitude of an internal wave is defined as the maximum displacement from the undisturbed interface) are less than 10 m in the summer, while they can reach 30 m in the winter [7]. The near-surface internal waves in the strait can cause rather strong surface currents, which facilitates the internal wave detection in remote sensing images [10,11].…”