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Temporal Variability in the Physical Dynamics at Seamounts and its Consequence for Bio-Physical Interactions
2006 ROMS/TOMS European WorkshopUniversidad Alcalá, Alcalá de Henares, Spain
November 6-8, 2006
Christian Mohn & Martin WhiteDept. Earth and Ocean Sciences
National University of Ireland, Galway
Current and recent research, mapping and management initiatives
Oceanic Seamounts:An Integrated Study
www.earthref.org
and:Theme session at AGU fall meeting 2006:
Seamounts: Intersection of the Biosphere, Hydrosphere, and Lithosphere
Seamount dynamics: Parameter space
(after Beckmann, 1999)
Important ingredients to describe the dominant physicalprocesses and their interactions
Stratification conditionsCoriolis parameter
(geographical location)
Seamount geometry
Steady forcing
Periodic forcing
?
1
23
4
Euphotic Zone
NutrientRich Water
Nutrient Depleted Surface layer
Uplifting of deep water
Available nutrients
Mixing
1 - Retention of organic material and larvae by 3-D circulation 2 - Downwelling of organic material to benthic communities3 - Upstream advection and entrainment into seamount area4 - Downstream advective loss and patchiness development
Vertical Nutrient Fluxes
• Surface layer nutrient depletion in summer• Additional nutrient supply to surface layer through upwelling/mixing of nutrient-rich deep water
Biophysical interactions
Advective Processes
(after White et al., 2005)
? ?
N
Deployment period: late July to early December, 2003
• 1 mooring at summit level (depth range: 780-900m)
• 3 moorings at mid flank (depth range 1450-1550m)
• 1 mooring at deep flank (2250m depth)
OASIS project case study: Sedlo Seamount
Location Mooring array
Weekly mean surface flow from AVISO satellite altimetry for a location immediately SW of Sedlo Seamount
Sedlo Seamount: Forcing and response
Direction of flow around seamount:
< 0
> 0
Seamount response (relative vorticity from a summit mooring triangle)
Biological implications: Sedlo Seamount
SeaWifs Chlorophyll-a
• Climatology - Enhanced levels of Chlorophyll over seamount• But: Patchiness of same scale around seamount• High inter-annual variability
7 years, August monthly mean August, 7 year mean
Summit depth: 750 m, subtropical North Atlantic
Biological implications: Great Meteor Seamount
Summit depth: 280 m, subtropical North Atlantic
SeaWifs Chlorophyll-a
• As for Sedlo• But: more consistent pattern over the summit
7 years, August monthly mean August, 7 year mean
Idealized seamount model: Description
• Rutgers/UCLA Regional Ocean Model System (ROMS version 2.2)
• Model Domain: E-W-periodic channel (L=1024 km, M =512km), Gaussian seamount centered at x=L/4 and y=M/2, summit depth = 200m
• Resolution: 256 x 128 horizontal grid points (4km), 20 vertical levels with high resolution at surface and bottom layers (Θs = 5, Θb =1)
• Initialisation: analytical approximation of NE-Atlantic summer subtropical stratification conditions taken from CTD measurements at Great Meteor Seamount, linear equation of state
Main aim: To estimate the influence of low-frequency variations of the far-field forcing on the distribution of passive tracers at a seamount
Key question: Can long-term variations of the far-field forcing contribute to passive tracer patchiness development?
Idealized seamount model: Description (contd.)
• Forcing: Analytical formulation for a periodically varying free surface elevation at the northern and southern edge according to:
T = 0 T = 15 T = 30 (days)
1. Steady flow (U = 10 cm/s)
2. Amplitude modulated flow(U = 5 – 15 cm/s)
3. Bidirectional flow (W-N,U = 10 cm/s)
4. Bidirectional flow (W-E, U = 10 cm/s)
Calculation begins from rest with constant barotropic forcing of U0 = 10 cm/s and analytical stratification
Idealized seamount model: Experimental strategy
Onset of forcing modulation and initialization of passive tracers after 40 days
Transient response
U0
15 L/U ~ 40 days
U0
L = 25 km
Passive tracer distributions
Amplitude modulation, uni-directional far field forcing(1: steady inflow, 2: amplitude modulated inflow)
Solution: 60 days after tracer release, 10 day averages, at 100 m depth
0.0 0.05 0.1 0.15 0.2
Main result: Advective loss and different levels of downstream patchiness development
1 2
400 km
280
km
Passive tracer distributions (contd.)
Variation of inflow direction(3: West-North, 4: West-East)
Solution: 60 days after tracer release, 10 day averages, at 100 m depth
0.0 0.05 0.1 0.15 0.2
Main result: re-entrainment and enhanced tracer retention within a circular area of up to 2 seamount radii away from the central summit
3 4
400 km
280
km
Passive tracer distributions: SPEM
Solution: 60 days after tracer release, 10 day averages, at 100 m depth
0.0 0.05 0.1 0.15 0.2
1 2
3 4
400 km
280
km
ROMS/SPEM differences:
• Sharp, frontal structures which are retained over the ROMS integration period ( weak horizontal mixing / exchange)• 2 Δx wave like patterns in lee of the seamount (not apparent in density fields)• Negative tracer concentrations and ‘over-shooters’
But:Qualitative agreement of tracer distribution patterns as a response to different types of forcing
Ongoing work:
• Sensitivity studies (test runs using different tracer advection schemes)• Validation of model results (analysis of remote sensing data at different locations as part of a 4th year student project)
Differences and possible causes / strategies:
Conclusions
• Model results show that variations of the far field forcing can significantly contribute to variability and patchiness of passive biological material in the vicinity of seamounts.
• But:How realistic are these results? Comprehensive model validation is needed.
• Better understanding of ROMS and its sensitivity to changes of computational options, choice of mixing schemes and boundary conditions
Other ROMS related projects at NUIG:
Regional model of Irish oceanic and shelf waters to simulate egg distribution and larval growth and transport of commercial fish species in strategic regions (in collaboration with the Irish Marine Institute)
Acknowledgements
ROMS user forum
Captain and crew of RV Arquipelago (mooring deployment) and RF Meteor (mooring recovery) and the rest of the OASIS
team
NDP Marine RTDI Fund 2000 - 2006