Anomalous Near-Surface Low-Salinity Pulses off the Central Oregon Coast

Annually, after the spring transition1, winds off the Oregon and Washington coast are predominantly southward (upwelling-favorable) and the Columbia River Plume (CRP) is advected to the south by the coastal upwelling jet, and offshore by the cumulative Ekman transport, detaching the plume from the coast. The shelf currents and wind forcing overwhelm the natural tendency of the plume to turn to the right due to the Coriolis effect, and as a result, relatively fresh water is observed off the Oregon coast2. Departures from this average state, however, can be quite dramatic, with a bi-directional CRP also often influencing the Washington shelf during summer3,4. The location of plume water as well as its freshwater content are important to the Oregon-Washington shelf ecosystem as it affects sediment deposition, nutrient concentrations as well as circulation and stratification3. Recent work has shown the importance of the CRP to shelf ecosystems and biogeochemistry including increased primary productivity, enhanced macrozooplankton concentration, delivery of biologically important micronutrients, and offshelf chlorophyll export5,6,7,8. Because rivers and estuaries differ in their carbon chemistry dynamics from the open shelf, there is increased interest in how these regions interact with respect to ocean acidification. River-influenced water on the open continental shelf can enter coastal estuaries and influence both the physics, including stratification and circulation, and biogeochemistry found there. Since the Columbia River is the largest freshwater source on the U.S. west coast, it is important to estimate the reach of its alongshore influence on both continental shelf ecosystems and coastal estuaries. While a number of observations9,10,11,12,13 reveal frequent intrusions of the CRP into estuaries north of the Columbia River, along the Washington coast, to our knowledge observations of CRP waters entering estuaries south of the Columbia River have not been previously reported, despite the fact that the CRP may extend over 500 km south of the Columbia River mouth2. From mid-May through August 2011, extreme Columbia River discharge reached more than two standard deviations above the mean for most of this period, having a peak discharge of nearly 17,000 m3/s in early June (Fig. 1c). This anomalous discharge as well as the atmospheric circulation and climate anomalies observed over the Pacific Northwest region during spring of 2011 can be explained by an extreme La Nina event14,15. The months of February and March reached the highest monthly Southern Oscillation Index (SOI) since 1866, while the month of April had the highest value since 1903 and the 2nd-highest on record. The impacts of this La Nina in the Pacific Northwest, with approximately a 3-month lag, were characterized by a longer storm season, increased spring snowfall in the Cascade Range, and negative temperature anomalies14. Just as extreme weather events influence terrestrial ecosystems and have potential economic impacts16, it is important to understand the impact of such events on marine ecosystems. Figure 1 (a) Average precipitation across the Pacific Northwest for May 2011. Map obtained from the National Oceanic and Atmospheric Administration (http://www.nwrfc.noaa.gov/water_supply/sea_wy_summary.cgi?wy=2011). (b) Mountain snowpack across the Columbia River ... Effects of the anomalous discharge from the Columbia River on the coastal ocean thermohaline structure were observed offshore of Newport, Oregon, approximately 180 km south of the Columbia River mouth. Using a suite of sensors and platforms, including three moorings, land-based HF-radar, satellite remote sensing and underwater gliders, we describe the onshore advection of the CRP off Newport, OR, from the mid-shelf, nearly 20km offshore, to the coast and eventually into the Yaquina Bay (Newport) during a sustained wind reversal event. In the next section we present our results, which are structured as follows: first the broader scale weather patterns are characterized as well as their effect on the CR streamflow during the spring/summer of 2011; then the spring transition and its effects on thermohaline structure of the coastal ocean are presented; lastly, a case study is presented, in which both satellite remote sensing and in situ moored observations are used to demonstrate the onshore propagation of the CRP onto the inner-shelf and into the Yaquina bay estuary and the consequent impact in the estuarine biogeochemistry. The results section is followed by the discussion section in which we place our findings in context and summarize and discuss our conclusions. Finally in the methods section we describe the data and methods used in this study.