Wind-driven upwelling and surface nutrient delivery in a semi-enclosed coastal sea

Abstract

Abstract. Wind-driven upwelling is an important control on surface nutrients and water properties in stratified lakes and seas. In this study, a high-resolution biophysical coupled model is used to investigate upwelling in the Strait of Georgia on the Canadian Pacific coast. The model is forced with surface winds from a high-resolution atmospheric forecast and has been tuned in previous studies to reproduce extensive observations of water level, temperature, salinity, nutrients and chlorophyll with competitive skill relative to similar models of the study region. A total of 5 years of hourly surface nitrate and temperature fields are analyzed in order to characterize the dominant upwelling patterns of the basin. A prevailing along-axis wind pattern steered by mountainous topography produces episodic upwelling along the western shore during the spring and fall southeasterlies and along the eastern shore during the summer northwesterlies, as indicated by positive nitrate anomalies. Principal component analysis reveals that these cross-axis upwelling patterns account for nearly one-third of the surface nitrate variance during the summer productive season. By contrast, nearly half of the surface temperature variance over the same period is dominated by a single, combined mixing and diurnal heating–cooling pattern. The principal components associated with these patterns correlate with along-axis wind stress in a manner consistent with these physical interpretations. The cross-axis upwelling response to wind is similar to other dynamically wide basins where the baroclinic Rossby deformation radius is smaller than the basin width. However, the nitrate anomaly during upwelling along the eastern shore is stronger in the northern basin, which may be indicative of an along-axis pycnocline tilt or an effect of the background along-axis stratification gradient due to the Fraser River. Our findings highlight an important spatiotemporal consideration for future ecosystem monitoring.

A clustering approach to determine biophysical provinces and physical drivers of productivity dynamics in a complex coastal sea

“…The wind forcing used (HRDPS, see model description) has 2.5 km spatial resolution and hourly temporal resolution, and it is used operationally by Environment Canada in the Canadian Pacific region. The skill of the HRDPS wind product in this region when compared to local meteorological stations has been evaluated by a previous study and accurately reproduces the climatology of observed wind magnitudes and directions (Moore-Maley and Allen, 2022). We first interpolate this product onto the model grid and then extract hourly wind speed.…”

“…In contrast, the CSoG (cluster 3/sky blue) shows the lowest variability between summer and winter energy magnitudes. Summer wind energies are actually higher in the CSoG than in the NSoG, likely due to the long wind fetch length in the CSoG, as summer winds in the Salish Sea are predominantly northerly (Thomson, 1981;Moore-Maley and Allen, 2022). Average vertical eddy diffusivity is lowest in the CSoG (Fig.…”

“…Atmospheric forcing, including winds and solar radiation, is derived from the High Resolution Deterministic Prediction System (HRDPS), a nested 2.5 km resolution operational atmospheric model (Milbrandt et al, 2016). The HRDPS model output is too coarse to accurately resolve atmospheric conditions in the northern inlets, but its wind fields have shown good agreement with observations throughout the Strait of Georgia (Moore-Maley and Allen, 2022). Coupled to the physical model is a NPZD-type biological model (SMELT -Salish Sea Lower Trophic Ecosystem Model;Olson et al, 2020), which is described in summary below.…”

“…In the model, these conditions favor high abundance of the gleaner flagellate group. In the NSoG, nutrient drawdown also occurs, but episodic wind events lead to stronger upwelling and mixing due to the comparatively weaker stratification and inject sharp pulses of nutrients into the near surface, leading to sharp, short-lived diatom blooms (Moore-Maley and Allen, 2022). In contrast, in the CSoG, despite stronger summer winds, strong stratification continues to favor gleanertype organisms.…”

Air exposure moderates ocean acidification effects during embryonic development of intertidally spawning fish

“…Year-class strength is determined by survival of the vulnerable larval stage, which in turn is strongly dependent on environmental factors affecting successful development 8 . One such environmental factor is ocean acidification (OA), to which the NE Pacific may be particularly vulnerable due to naturally high concentrations of aquatic CO 2 from both natural and anthropogenic drivers including upwelling, low buffering capacity, riverine input and eutrophication 9 – 14 . Detrimental effects of future projections of ocean acidification levels have been documented in experiments with embryonic and larval Pacific and Atlantic herring ( Clupea harengus ), with lowered condition factor, developmental delays and malformations reported at CO 2 levels as low as 1000 µatm 15 – 18 , levels which already occur episodically in coastal regions including the Salish Sea 19 – 22 .…”