Inverse Estuarine Circulation in Marginal Bays in Greater Puget Sound

Transcription

1 Inverse Estuarine Circulation in Marginal Bays in Greater Puget Sound Skip Albertson, Jan Newton Environmental Assessment Program, Washington State Dept of Ecology, Olympia, WA Applied Physics Lab, University of Washington, Seattle, WA Abstract - Most estuaries have fresher water at the head and saltier water at the mouth, which drives a residual estuarine circulation. However, large expulsions of buoyant freshwater down the main longitudinal axis of a basin (e.g., Main, Whidbey, or Strait of Georgia) can effectively pinch off marginal embayments, inside which salinity and density remain higher than at the mouth. This inversion in the density gradient causes the net tidally-averaged residual flow pattern to become inverse and increases the residence time of the bay. Factors leading to the creation of an inverse salinity gradient are the magnitude and timing of riverine input from unfrozen precipitation and warming that causes snowmelt. If the timing of a freshet coincides with the kinetic energy of a storm, instead of lagging after it, the flushing potential of the storm could be nullified by inversion. Climate change (e.g., global warming) could partially explain why this might be happening more frequently in recent years since mid-winter precipitation is not stored in the mountains as snowpack but instead released immediately as runoff. Examples are given from Hood Canal, Sequim Bay, Discovery Bay and elsewhere within the inland waters of Washington State. This effect could play a role in stimulating hypoxia or other water quality issues related to flushing. Methods - For each marginal bay (i.e., on the side, or off the main channel axis) where data exist at an internal station (black dots on maps) we take the nearest available external station (white dots on maps, joined with a white tie-line) and subtract the internal salinity at each depth from the external salinity. Data are either taken monthly with a seaplane by Ecology (Fig. a) as part of the Puget Sound Assessment and Monitoring Program (PSAMP) or several times a day by the University of Washington Oceanic Remote Chemical/optical Analyzer buoy (UW-ORCA; Fig. b) as part of the Hood Canal Dissolved Oxygen Program (HCDOP). The ORCA buoy is an autonomous moored profiler, which has been placed in different locations in Puget Sound (Dunne et al. ; Ruef et al. 3). At the surface a float supports a platform carrying a meteorological package and a cell phone to transmit data in real time. a) b)

2 Fig. Locations of interior and exterior stations used for salinity difference calculations used for a) monthly Ecology stations, b) hourly UW-ORCA stations. Results - Negative values of salinity difference are plotted in red, which indicate the potential for an inverse, or at least reduced residual flow in the estuary; positive salinity differences correspond to a normal estuary that contains fresher water relative to external locations and are indicated in blue. The UW-ORCA data are plotted with respect to depth but the Ecology data are not mainly because they are more spread out in time. These salinity differences show aspects of seasonality and interannual variability. Some locations, such as East Sound at Orcas Island, appear persistently saltier than outlying waters. Bellingham Bay GRG (outside) - BLL9 (inside) Port Angeles harbor ADM (outside) - PAH3 (inside)

3 Budd Inlet DNA (outside) - BUD (inside) Sequim Bay ADM (outside) - JDF (inside) Discovery Bay ADM (outside) - DIS (inside) Quartermater Harbor EAP (outside) - QMH (inside) Penn Cove SAR3 (outside) - PNN (inside) Holmes Harbor SAR3 (outside) - HLM (inside) Hood Canal ADM (outside) - HCB (inside) East Sound GRG (outside) - EAS (inside)

4 Carr Inlet GOR (outside) - CRR (inside) Lynch Cove HCB3 (outside) - HCB (inside) Figure. Monthly Ecology data showing relation between exterior and interior salinity in marginal bays. A negative relation is shown in red. Figure 3. Time-series data from Hood Canal ORCA mooring showing relation between exterior and interior salinity. Episodic salinity reversals are shown in red Discussion - The degree to which the inverse condition exists may affect a given bay s sensitivity to hypoxia and other water quality conditions. Further analysis is planned to compare the frequency of inverse condition with reductions in water quality, such as low dissolved oxygen. We note that the frequency of the inverse condition is highly dynamic, changing with freshwater input driven largely by climate forcing and therefore influenced by climate change. Offshore winds that act at a distance to move water masses in and out

5 of Greater Puget Sound can also have an effect (Cannon, ). Substantial interannual variability is shown for northern Hood Canal. The residual circulation in Hood Canal is a function of the density of water external to Hood Canal, as represented at the North Buoy These waters are a complex combination of end-members (sources) such as interior basins (e.g., Whidbey Basin), Georgia Strait, and Pacific Ocean end-members. For more discussion on sill dynamics and deep fjord water renewal via Admiralty Straits please see the abstract by Dmitri Leonov from the Water Quality and Quantity GB-PS 7 poster session. Acknowledgements We thank Al Devol and Wendi Ruef of UW School of Oceanography for Oceanic Remote Chemical-optical Analyzer (ORCA) data, and numerous Ecology staff who have collected the PSAMP data over the years represented here. References Cannon, G.A., and D.E.Bretschneider (). Interchanges between coastal and fjord circulation, in Contaminant Fluxes through the Coastal Zone, G. Kullenberg (ed.), Rapp. P.-v. Reun. Cons. int. Explor. Mer, 88, Dunne, J. P., A. H. Devol, and S. Emerson.. The Oceanic Remote Chemical/Optical Analyzer (ORCA): An autonomous moored profiler. J. Atmos. Oc. Tech 9, Ruef, W. A. Devol, S. Emerson, J. Dunne, J. Newton, R. Reynolds, and J. Lynton. 3. In situ and remote monitoring of water quality in south Puget Sound: The ORCA time series. Proc. of the 3 Georgia Basin/Puget Sound Research Conference, 3 March-3 April 3, Vancouver, BC, Canada.