Reverse estuarine circulation due to local and remote wind forcing, enhanced by the presence of along-coast estuaries

TitleReverse estuarine circulation due to local and remote wind forcing, enhanced by the presence of along-coast estuaries
Publication TypeJournal Article
Year of Publication2017
AuthorsGiddings S.N, MacCready P.
JournalJournal of Geophysical Research-Oceans
Date Published2017/12
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000422732100051
Keywordsgreenland glacial fjords; juan-de-fuca; mixed estuaries; nitrogen-sources; oceanography; pacific-northwest; phytoplankton blooms; plain estuary; Residual circulation; river plume; willapa bay

Estuarine exchange flow governs the interaction between oceans and estuaries and thus plays a large role in their biogeochemical processes. This study investigates the variability in estuarine exchange flow due to offshore oceanic conditions including upwelling/downwelling, and the presence of a river plume offshore (from a neighboring estuary). We address these processes via numerical simulations at the mouth of the Salish Sea, a large estuarine system in the Northeast Pacific. An analysis of the Total Exchange Flow indicates that during the upwelling season, the exchange flow is fairly consistent in magnitude and oriented in a positive (into the estuary at depth and out at the surface) direction. However, during periods of downwelling favorable winds, the exchange flow shows significantly more variability including multiple reversals, consistent with observations, and surface intrusions of the Columbia River plume which originates 250 km to the south. Numerical along-strait momentum budgets show that the exchange flow is forced dominantly by the pressure gradients, particularly the baroclinic. The pressure gradient is modified by Coriolis and sometimes advection, highlighting the importance of geostrophy and local adjustments. In experiments conducted without the offshore river plume, reversals still occur but are weaker, and the baroclinic pressure gradient plays a reduced role. These results suggest that estuaries along strong upwelling coastlines should experience significant modulation in the exchange flow during upwelling versus downwelling conditions. Additionally, they highlight the importance of nearby estuaries impacting one-another, not only in terms of connectivity, but also altering the exchange flow. Plain Language Summary Estuarine systems provide extensive biological and ecological functions as well as contribute to human uses and economies. However, estuaries are susceptible to change and most estuaries have been significantly impacted, threatening their important functionality. Understanding estuarine dynamics is critical to understanding estuarine ecosystems. Hydrodynamic connectivity between estuaries and the coastal ocean is a key dynamical driver impacting critical biological and biogeochemical processes such as ocean/estuarine nutrient and phytoplankton exchange and regulation of estuarine residence time, dissolved oxygen, and acidification levels. Typically estuarine-ocean exchange brings oceanic water into the estuary at depth, mixes it upwards within the estuary, and returns an outflowing mixture of oceanic and riverine water at the surface to the ocean. This manuscript documents seasonal reversals to this typical circulation pattern and the hydrodynamic drivers of the reversals. It highlights the importance of offshore winds and connectivity with neighboring estuaries. Improved understanding of these mechanisms can help us predict how estuaries will respond to a changing climate.

Short TitleJ Geophys Res-Oceans
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