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

Monthly to inter-annual timescales JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C12004, doi: /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

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

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)

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

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

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

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

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 o C 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

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 – Events defined using MWT with 2.9 mg/l, 1 day minimum length, and 9 hr trigger duration – Tidal range definitions Neap = 1.2 m

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

Mount View (West Passage)

Greenwich Bay

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