Physical controls on extremes of oceanic carbon and oxygen in coastal waters

Date

2019-10-02

Authors

Engida, Zelalem M.

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Abstract

The west coast of Vancouver Island is located at the northern end of the California Current System, one of the world’s Eastern Boundary Current Systems. The region is characterized by wind driven coastal upwelling and high productivity, which supports fisheries and related industries. Climate change poses a challenge to these industries by increasing seawater acidity and decreasing dissolved oxygen levels, which are two of the multi-stressors of marine organisms. This thesis explores the relative importance of different physical and biological mechanisms that affect oxygen and carbon extremes in the region. The relatively weak local wind in the region is not well-correlated with local currents and temperature. Results of coherence analyses between multi-depth current and temperature measured at a single mooring site (48.5 ◦ N, 126 ◦ W) in the west coast of southern Vancouver Island and coincident time series of North America Regional Reanalysis (NARR) 10 m wind stress in the geographic domain 36 – 54 ◦ N, 120 – 132 ◦ W are presented. The two-decade long (1989 – 2008) current records from the three shallowest depths (35, 100 and 175 m) show a remote response to winds from as far south as 36 ◦ N. In contrast, temperature only at the deepest depth (400 m) show strong coherences with remote winds. The frequency window of maximum coherence and the estimated average time-lags are consistent with the frequencies and pole-ward propagating phase speeds of coastal trapped waves. Lack of coherence between remote winds and the 400 m currents suggests that the temperature variations at that depth are driven by vertical motion resulting from poleward travelling coastal trapped waves (CTWs). In order to study the relative roles of physical and biological processes on controlling oxygen and carbon tendencies, oxygen cycle has been successfully added to an existing biogeochemical model of the west coast of Vancouver Island. This idealized model then was forced with a long synthetic record of present-day conditions, specifically 1017 years of stochastically generated daily resolved forcing including local and remote winds. The seasonal cycles of the modelled DIC and O2 compare well with depth averaged observational data. They are also found to be strongly coupled in the lower layers, where biological processes are more important. In the upper layer, physical processes such as the differing gas exchange rates partially decouple DIC and O2 . Robust statistics on DIC and oxygen extreme events were calculated by using the long realizations of the model baseline experiment. In the upper mixed layer, O2 extreme events occur 2–3 times more frequently than DIC extreme events. Both extreme events show a much larger interannual variability in the lower layer. In this layer, oxygen extreme events events occur late in the summer, following intense upwelling events early in the upwelling season. Counter-intuitively, within the summer upwelling season, when sporadic upwelling events are expected to cause extreme conditions, the fraction of days with joint DIC–O2 extreme events is negligible. Sensitivity analysis shows that increased primary production, via increased phytoplankton growth rate, decreases the small fraction of days with joint DIC-O2 extreme events in the upper layers during the summer upwelling season but increases it in the winter downwelling season. Lowering upwelling intensities lowers the fraction of days with joint DIC–O2 extreme events. Increasing the upwelling intensities had the opposite effect on this fraction. Changes in up/downwelling intensity did not change this fraction within the summer upwelling season. A non-monotonic response by oxygen extreme events in the lower layer is observed when phytoplankton growth rate was increased. Generally, a moderate decrease in growth rate increases the chances of model lower layer O2 extreme events, while near-zero growth rate does not. In some cases, the same parameter perturbation results in different responses by the mean and the extreme events of DIC and O2 , suggesting that results of studies focusing on physical and biological forcing of the mean state may not directly translate result to extremes. This thesis has identified relative locations within the study domain of priority for effective monitoring of dissolved oxygen and carbon extremes in the study region. Finally, joint DIC- O2 extreme events are found to be common at the end of the summer. This information can be used to inform adaptation and mitigation plans aimed at protecting the economic and bequest value of the coast from potential hazards associated with oxygen and carbon extremes.

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Keywords

Carbon, Oxygen, extremes, upwelling, local forcing, remote forcing, return periods, biology, physics, statitstics

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