influence of richardson number on dissolved oxygen concentration ...

Influence <strong>of</strong> Richardson Number
on dissolved oxygen concentration
on the eastern Texas-Louisiana Shelf
Steven F. DiMarco and Piers Chapman
Department <strong>of</strong> Oceanography
Texas A&M University
ERF 2007
Providence, RI
8 November 2007

Outline
• Objectives/methods/data
• Case studies
– Mid-summer: stable water column, persistent hypoxia
– Late summer: erosion <strong>of</strong> shoreward edge <strong>of</strong> hypoxia
– Spring: variable stability, onset <strong>of</strong> depleted oxygen layer
• Conclusions

Objectives
• How do dissolved oxygen concentrations
vary on short (order <strong>of</strong> hours) time scales
• How does this variability relate to changes
in water column stability and current shear
Management implications: essential to sample at proper spatial and
temporal scales to provide reasonable parameterizations for
numerical modeling

Richardson Number (Ri)
• Relates water column stability (stratification) to current
shear
– Stability metric: Brunt-Vaisala frequency (N 2 )
– Current Shear: (∂u/∂z) 2
– Ri = (N 2 ) / (∂u/∂z) 2
– When shear is greater than stability -> mixing
• Mixing Questions
– Where
– When
– Strength
– Relation to DO concentration

2007 Field Year
• 3 MCH Cruises
– March 22-29 (M8)
– July 16-19 (M9)
– September 6-9 (M10)
• Mixing processes
– 6 anchor stations (12-37 hours)
– Velocity (RDCP)/CTD (Seabird 911) pr<strong>of</strong>iles

July 2007: M9 Cruise
QuickTime and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Mid-summer, stable water column
Source: N. Rabalais LUMCON

Time Series Station D
July 2007
salinity
• Well mixed surface
layer
• Strong pynocline at
~14 m depth
• Thin (2 m) near
bottom layer
Salinity

Time Series Station D
July 2007
Dissolved oxygen
• Well mixed surface
layer
• Strong oxycline at
~14 m depth
• Thin (2 m) near
bottom hypoxic layer
• Low variability
Dissolved DO Oxygen

Time Series Station D
July 2007
Current speed
• Low energy surface
currents
• Moderate current
variability
– ~15 m depth (3-5
mab)
– Above hypoxic
layer
speed

Time Series Station D
July 2007
Dissolved oxygen vs Ri
• Mixing throughout water
column above pycnocline
– Mixing in surface layer
• Low current shear
• BUT high instability
• Shear is enough to
overcome water
column stratification
– mid-water mixing above
oxycline
• Not affecting stable
hypoxic layer
• Weak mixing beneath
oxycline
• Stable hypoxic layer

September 2007: M10 Cruise
Late summer: erosion <strong>of</strong> shoreward edge

Time Series Station C
6 September 2007
Salinity
• Weakly stratified
– Early record
– Low variability
• Fresh water plume
– Near surface
– North wind
• High frequency
variability after
plume event
– Internal wave
• Rise <strong>of</strong> deep
isohalines
Salinity

Time Series Station C
6 September 2007
Fluorometer
• High surface fluorescence
associated with plume event
– Low nutrients
• Patchy water column values
– Occurs at same isopycnal
layer
• High near bottom
fluorescence
– High nutrients
– Remineralization
Fluorometer

Time Series Station C
6 September 2007
Dissolved Oxygen Concentration
• Benthic source <strong>of</strong>
low DO
• Considerable
temporal variability
in lower layers
• Suggests upward
mixing <strong>of</strong> low DO
Weakly stratified
Stronger stratification

Time Series Station C
6 September 2007
Velocity
• Baroclinic structure
– Two-layer
– Semidiurnal
• Surface and midwater
velocity maxima
– Up to 50 cm/s
• High temporal variability
• Shear throughout record

Time Series Station C
6 September 2007
DO and Ri
• Surface mixing above
pycnocline
• Downward injection
events
– 12 hours apart
– Across pycnocline
• Mixing at upper edge <strong>of</strong>
low DO
– Erosion <strong>of</strong> upper edge <strong>of</strong>
oxycline

March 2007: M8 Cruise
C
Frontal passage case

Time Series Station C
March 2007
Salinity
Fresh
Salty
• 24 hours
• Weakly Stratified
– Beginning <strong>of</strong>
record
• Current/wind shift
• Rising halocline
• Strongly stratified
– mid to end <strong>of</strong>
record

Time Series Station C
March 2007
Dissolved Oxygen Concentration (ml/l)
• Oxygenated surface
waters
• Oxygen depleted
near-bottom water
• Complex vertical
structure
• DO falls under
stronger pycnocline

Time Series Station C
March 2007
Dissolved Oxygen vs Ri
• Surface mixing
– Throughout record
• Subpycnocline mixing
– Increases in second half
<strong>of</strong> record
• Mid-water column
– Pre-front
• Mixing along upper
edge <strong>of</strong> pycnocline
– Hour 9
• upward mixing <strong>of</strong> low
DO near-bottom water
– Hour 16
• Strong vertical mixing
• Pycnocline at 6 m
• Downward mixing <strong>of</strong>
larger DO upper
waters

Conclusions
• Eastern TX-LA shelf has a variety <strong>of</strong> physical processes
that can affect stratification (freshwater input, wind
forcing, internal waves) and complex vertical current
structure
• Temporal variability <strong>of</strong> stratification, current structure, and
dissolved oxygen concentration can be on the order <strong>of</strong>
hours
• Vertical current structure combined with water column
stability controls vertical mixing
• Mixing contributes to control <strong>of</strong> temporal variability <strong>of</strong> DO

Acknowledgements
• This work was funded by NOAA CSCOR under
NGOMEX-2006.
• Data processing: Dr M. K. Howard (TAMU).
• Thanks to the crew <strong>of</strong> the R/V Pelican.
• Special thanks to scientists/students who participated on
the cruises.