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Stratification and Hypoxia: Inter-Annual Patterns and Spring-Neap Cycle Relationships

This study investigates the dynamics of water stratification and hypoxia over monthly to inter-annual timescales with a focus on how hypoxic event timing might relate to spring-neap tidal cycles. The findings reveal that while large-scale density structures remained uniform, inter-annual variations are primarily driven by runoff. Hypoxic events are weakly linked to tidal cycles, suggesting that river flow plays a more crucial role. The analysis spans 2001-2009, providing insights into seasonal stratification patterns and potential climate impacts.

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Stratification and Hypoxia: Inter-Annual Patterns and Spring-Neap Cycle Relationships

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  1. Stratification and hypoxia on monthly to inter-annual timescales… plusIs hypoxic event timing related to spring-neap cycles?Codiga(GSO)Mar 21, 2013

  2. Monthly to inter-annual timescalesJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C12004, doi:10.1029/2012JC008473, 2011 Key points: • Large-scale density structure unexpectedly uniform despite complex geometry • Inter-annual variations, linked to hypoxia: runoff-driven, insensitive to wind • Long-term trends controlled by climate-driven runoff increases not warming

  3. Horizontal Density Structure • ~uniform gradient near surface • Minor east/west differences • Deep water: weak gradient!

  4. Bullocks Reach – Representative • T stratification gone by fall • S stratification more persistent; more variable inter-annually • Density stratification peaks in late spring (S is >80% responsible)

  5. Response to river flow • Large scatter • Not widely at odds w/ theory: • Stratification ~2/3 power law • Horiz. gradient ~1/3 power law

  6. Driving factors considered • River flow (USGS) • Surface heat flux (N.A.R.R.) • Wind: speed, direction, constancy (N.A.R.R.) • 2001-2009 period  9-yr means and stddevs of monthly-means

  7. Inter-annual stratification and driving factors • Strong link to river flow • Relationship to heat flux and winds not evident

  8. Stratification and hypoxia Inter-annual • There is a relationship with late spring stratification (but not the 5-month mean stratification) • It is not very strong! Kendall’s Tau 0.611, p=0.025, n=9

  9. Climate trends and stratification • Eqn of state: 1 kg m-3 density change requires • 5 C in temperature OR 1 PSS in salinity • Stratification increase due to: • observed 1–2 oC warming: • upper bound 0.1–0.2 kg m-3 • assumes shallow warms fully, deep not at all • observed +13% river flow: • estimated ~0.5 kg m-3 • based on power law relationship • At least twice the estimated warming influence • Inter-annual variability insensitive to winds

  10. Hypoxic event timing and Spring-Neap cycles • From MLR and other analyses we know that spring-neap cycles are more weakly related to hypoxic event timing than river flow • Nonetheless, explore potential linkage: • Nine years of hypoxic events 2001-2009 • Events defined using MWT with 2.9 mg/l, 1 day minimum length, and 9 hr trigger duration • Tidal range definitions • Neap = < 0.9 m, Spring = > 1.2 m

  11. Bullocks Reach Number of STARTS Each bar is one hypoxic event Left end: tidal range at event start Right end: “ “ end Spring Number of ENDS Between Neap Neap Spring Between

  12. Mount View (West Passage)

  13. Greenwich Bay

  14. Conclusions • Some stations have more event starts during neap and more event ends during spring • BR & MV (northern and western areas) • Not Greenwich Bay • Pattern is moderately strong • Results sensitive to spring-neap tidal range definition • Expect limited ability for forecasting

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