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