Temporal Variability in the Physical Dynamics at Seamounts and its Consequence for Bio-Physical Interactions 2006 ROMS/TOMS European Workshop Universidad Alcalá, Alcalá de Henares, Spain November 6-8, 2006 Christian Mohn & Martin White Dept. Earth and Ocean Sciences National University of Ireland, Galway

Current and recent research, mapping and management initiatives Oceanic Seamounts: An Integrated Study 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 physical processes and their interactions Stratification conditions Coriolis parameter (geographical location) Seamount geometry Steady forcing Periodic forcing

? Euphotic Zone Nutrient Rich 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 communities 3 - Upstream advection and entrainment into seamount area 4 - 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, mooring at summit level (depth range: m) 3 moorings at mid flank (depth range m) 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 (Θ s = 5, Θ b =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 U 0 = 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 U0U0 15 L/U ~ 40 days U0U0 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 Main result: Advective loss and different levels of downstream patchiness development 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 Main result: re-entrainment and enhanced tracer retention within a circular area of up to 2 seamount radii away from the central summit km 280 km

Passive tracer distributions: SPEM Solution: 60 days after tracer release, 10 day averages, at 100 m depth 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 4 th 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