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 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

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.)• 2001-2009 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 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

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

Bullocks ReachEach bar is one hypoxic event

Left end: tidal range at event startRight 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