1. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Outline: 1)Background and Motivation 2)Role of Physical Forcing 3)Simplified Modeling Approach 4)Sensitivity Studies 5)Physical Mechanisms and Oxygen Budget 6)Historical Observations of Hypoxia 7)Conclusions Center for Coastal Physical Oceanography Old Dominion University CCPO Seminar: Monday, March 28, 2011 Center for Coastal Physical Oceanography

2. From Chesapeake Bay Program newsletter: http://ian.umces.edu/pdfs/do_letter.pdf Map of Mean Dissolved Oxygen -- Summer 2005 Low DO has significant impact on a wide array of biological and ecological processes. Large regions of Chesapeake Bay are impacted by hypoxia/anoxia. Over $ 3.5 billion was spent on nutrient controls in Chesapeake Bay between 1985-1996 (Butt & Brown, 2000) Assessing success/failure of reductions in nutrient loading requires understanding of the physical processes that contribute to the inter-annual variability.

3. From Chesapeake Bay Program newsletter: http://ian.umces.edu/pdfs/do_letter.pdf Conceptual Model for Hypoxia in Chesapeake Bay Physical forcing is thought to play an important role in extent and severity of hypoxia: 1) River Discharge; 2) Temperature; 3) Wind forcing

4. Seasonal and Inter-Annual Variability in Hypoxic Volume (from CBP data 1984-2009) Maximum observed Minimum observed Data compiled from Murphy et al. (submitted)

5. Regional Ocean Modeling System (ROMS) ChesROMS Model Grid Model forcing Realistic tidal and sub-tidal elevation at ocean boundary Realistic surface fluxes from NCEP (heating and winds) Observed river discharge for all tributaries. Temperature and salinity at ocean boundary from World Ocean Atlas. Very simple oxygen model

6. Oxygen Model Oxygen is introduced as an additional model tracer. Oxygen consumption (respiration) is constant in time, with depth- dependent vertical distribution. No oxygen consumption outside of estuarine portion of model No oxygen production. Open boundaries = saturation Surface flux using wind speed dependent piston velocity following Marino and Howarth, 1993. No negative oxygen concentration and no super-saturation. Depth-dependent Respiration Formulation Surface Oxygen Flux using Piston Velocity: From Marino and Howarth, Estuaries, 1993 Model assumes biology is constant so that the role of physical processes can be isolated!

7. Comparison with Bottom DO at Chesapeake Bay Program Stations

8. July 19-21, 2004August 9-11, 2004 Comparison with Chesapeake Bay Program Data Bottom Dissolved Oxygen Concentration (mg/L)

9. Simple model captures seasonal cycle of hypoxia as well as a more complicated bio-geo-chemical model.

10. In addition to seasonal cycle, model captures some of the inter-annual variability 485 km 3 days 476 km 3 days 707 km 3 days Model predicts roughly 50% more hypoxia in 2004 than in 2005, solely due to physical variability.

11. Variability of Physical Forcing What is relative importance of different physical forcings in controlling seasonal cycle of hypoxia?

12. Sensitivity to River Discharge

13. Sensitivity to Temperature

14. Sensitivity to Wind

15. Differences between 2004 and 2005 are almost entirely due to wind forcing

16. Sensitivity to Summer Wind Magnitude Average Monthly Wind Speed from Model at PNAS Wind speed during May-August was increased/decreased by 15%

17. Changes in average summer wind speed of %15 result in roughly 2-fold change in hypoxic volume.

18. Base Summer Winds Positive 90° Negative 90° 180° Sensitivity to Summer Wind Direction Modeled summer wind direction

19. Sensitivity to Summer Wind Direction Along axis winds result in less total hypoxic volume

20. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Outline: 1)Background and Motivation 2)Role of Physical Forcing 3)Simplified Modeling Approach 4)Sensitivity Studies 5)Physical Mechanisms and Oxygen Budget 6)Historical Observations of Hypoxia 7)Conclusions Center for Coastal Physical Oceanography Old Dominion University Center for Coastal Physical Oceanography CCPO Seminar: Monday, March 28, 2011

21. Lateral Advection Mechanisms for Oxygen “Ventilation” Direct Vertical Mixing Along-Channel Advection detrainment

22. July 2004 average bottom Oxygen Fixed Volume for Budget calculations Oxygen Budget Calculations Rate of change Advection: Turbulent mixing Respiration horizontal lateral Integrate all terms over entire volume

23. Monthly Averaged Sub-Pycnocline Oxygen Budget

24. Ekman wind stress North Wind x Ekman wind stress South Wind Ekman wind stress West Wind x Ekman wind stress East Wind Response of Chesapeake Bay to Wind Forcing is Strongly Impacted by Rotation

27. For most of the deep areas of the Bay, the gradient Richardson almost never drops below 0.25 in pycnocline (year round!). Richardson Number for CPB Station 4.3

28. Sensitivity to Summer Wind Direction Along axis winds result in less total hypoxic volume

29. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Outline: 1)Background and Motivation 2)Role of Physical Forcing 3)Simplified Modeling Approach 4)Sensitivity Studies 5)Physical Mechanisms and Oxygen Budget 6)Historical Observations of Hypoxia 7)Conclusions Center for Coastal Physical Oceanography Old Dominion University Center for Coastal Physical Oceanography CCPO Seminar: Monday, March 28, 2011

30. Historical Observations of Hypoxia in Chesapeake Bay Data from Jim Hagy 1)Multiple regression based on estimated nitrogen loading explains relatively small amount of observed variance. 2)Residuals to fit suggest hypoxic volume is increasing, despite recent reductions in estimated nitrogen loading. 3)One interpretation is that Bay is less able to assimilate nutrient inputs because of ecosystem degradation. Observed hypoxic volume (< 1 mg/L) Regression Model Residual

31. Duration of Summer Wind River Discharge Nitrogen Loading NNEESESSWWNW < 2 mg/L0.000.080.18-0.49-0.370.040.690.320.160.36 < 1 mg/L-0.020.040.15-0.48-0.340.030.710.360.240.44 < 0.2 mg/L-0.10-0.080.05-0.42-0.17-0.100.550.300.330.62 * values in red denote significance at 95% confidence interval Correlation of Historic Data (Hagy et al.) with Wind Direction Wind data from Patuxent Naval Air Station (1950--2007)

32. Multiple Regression based on Nitrogen Loading and Duration of Westerly Winds When you account for changes in wind direction, residual slope is no longer significant. Observed hypoxic volume (< 1 mg/L) Regression Model Residual

33. Have the Winds over Chesapeake Bay Changed in recent Decades?

34. Conclusions 1)A relatively simple model with no biological variability can reasonably account for the seasonal cycle of hypoxia in Chesapeake Bay. 2)Wind speed and direction are the two most important physical variables controlling hypoxia in the Bay. 3)Model results are largely insensitive to variations in river discharge. 4)The model suggests that the dominant balance controlling hypoxia is between respiration and advective processes not vertical mixing. 5)During winter months ventilation is dominated by longitudinal advection. 6)During the summer months ventilation is greater by lateral advection. 7)Because of the width of Chesapeake Bay, the rotational response to wind forcing is greater for along-channel winds than for across-channel winds. 8)Winds from the north enhance the residual circulation, increasing the longitudinal flux of oxygen into the hypoxic zone. 9)However, winds from then north are not common during the summer months and subtle shifts between south and west winds may play a significant role in the observed inter-annual variability.