Ocean circulation, transport and mixing at seamounts and biological consequences

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Ocean circulation, transport and mixing at
seamounts and biological consequences
Christian Mohn and J. William Lavelle
SEAMOUNTS09 workshop Exploration, Biogeosciences
and Fisheries Scripps Institution of
OceanographyMarch 19-21, 2009
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The long journey towards a detailed picture
etopo-5 (NGDC, NOAA, 1988)
Smith and Sandwell (1997)
Foundation Seamount Chain, south Pacific
(satellite gravimetry) http//www.ngdc.noaa.gov/mg
g/image/seafloor.html
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Global distribution of seamounts and seamounts
sampled
censeam.niwa.co.nz
Predicted locations of 14300 seamounts
(Kitchingman and Lai, 2004) Biologically sampled,
but no data available Some level of data
available at about 300 seamounts Data available
(taxonomy, abundance, species compostion,
sampling strategy) at about 50 seamounts
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Physical parameter space
Coriolis parameter, geographical location
Vertical heat and salt flux
f
Stratification
? (s,t,p)
QT,QS
Umean
H
currents, forcing
Uvar
hm
L
Seamount morphology
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Flow-topography interaction Transient response
and Taylor cap formation
Transient state
Steady state
1
2
2
s
strong flow
STRONG
weak flow
strong flow
weak inflow Butterfly pattern strong inflow
Taylor cap circulation, cold water (dense)
dome dominant in regions with strong mean flows
and weak tidal variability
Taylor cap disappears for very strong flows
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Flow-topography interaction Tidal rectification
Transient state
Steady state
1
2
2
s
STRONG
RECTIFIED
transient response Rotating trapped wave
pattern and tidal amplification time-mean
rectified anti-cyclonic recirculation, cold water
dome dominant in regions where mean far field
flows are weak (Fieberling Guyot)
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Idealized flow pattern at seamounts The steady
state
Wind
Wind mixing, wind-driven currents
Umean
Umean
Local turbulence
Internal waves
Tidal currents

• Bottom-intensified recirculation cell, isopycnal
  doming
• Downwelling in the seamount center
  BUT
• Upwelling along the outer seamount flanks

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Seamount processes Flow amplification at
seamount summits
Lavelle (unpublished)
Flow at seamount summits can be a factor of 20
(or more) higher than the surrounding far field
flow!
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Local processes Internal wave generation and
propagation
Internal wave energy propagation
Slope criticality
Lavelle (in prep.)
Generated by flow over topography and sea surface
winds C1 bathymetry ? is considered critical,
potential internal wave formation sites,
associated with intense fluid dynamics
Dissipation of internal waves affects
large-scale circulation and plankton distribution
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Local response to long-term variability of
impinging flows
? dv/dx du/dy
-

Sedlo Seamount, 780 m summit depth
Sub-tidal far field flow, SW of the seamount
(AVISO currents)
Temporary destabilization of Taylor Cap flow at
the summit correlated with disturbance in the
far field (White et al., 2007Mohn et al., 2009)
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Summary I
Knowledge base still very limited, mainly based
on individual snapshots and modelling
studies Each seamount is a unique environment,
defined by its very own physical parameter
space Idealized steady state Taylor cap and/or
tidally rectified toroid Steady state
permanently exposed to concurrent/intermittent
oceanic processes (sub-tidal flow variability,
wind events, tidal currents, internal waves,
local flow amplification)
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Bio-physical interactions A conceptual model
Passive particle retention, enhanced primary
productivity, vertical particle scattering
Nutrient-depleted surface layer
Uplifting of nutrient-rich deep water
Attraction and aggregation of higher trophic
levels
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Enhanced levels of primary productivity?
SeaWifs Chlorophyll-a (White et al., 2007)
June-August, 7 year mean
7 years, August monthly mean
summit
Great Meteor Seamount (summit depth 280 m,
subtropical North Atlantic), Climatology -
enhanced levels of Chlorophyll over
seamount But Patchiness of same scale around
seamount High inter-annual variability
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Restricted particle dispersal and retention?
Top View 150 m depth
Great Meteor Seamount (Beckmann Mohn, 2002)
Fieberling Guyot (Mullineaux Mills, 1997)
Retention above seamounts mainly influenced by
physical processes?
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Mechanisms for particle displacement Vertical
scattering
Porcupine Bank
Rockall Bank
vertical
d (m)
d (m)
days after tracer release
days after tracer release


horizontal


280 km
Strong tidal influence on tracer displacement up
to several 100 meters (vertically) and several
kilometers (horizontally) within one tidal
cycle (Mohn and White, 2007)
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Mechanisms for particle displacement Internal
waves
Large vertical movement of the 10ºC isotherm at
northern Porcupine Bank slope
Elevated levels of suspended material at the
seabed near the location of the carbonate mounds
(Mienis et al., 2007)
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Passive tracers Response to sub-tidal flow
variations?
Initial tracer release area
Inflow Modulation of amplitude and
direction Modulation period 30 days No tides
Simulated Eulerian tracer concentrations (the
last 30 days of a 90 days simulation were
averaged) (Mohn and White, 2009)
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Passive tracers Response to sub-tidal flow
variations - Tracer variability patterns -
Normalized tracer variance (90 day simulation
period) (Mohn and White, 2009)
High variability in regions of particle loss and
accumulation (not limited to the summit)
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Patterns of seasonal variability
Repeated surveys at Great Meteor Seamount, summit
depth 280 m, subtropical North Atlantic
(Mourino et al., 2001) Seasonal changes in
mixed layer depth and stratification translates
into 2-3 fold changes in depth-integrated Chl-a
and primary production rates.
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Summary II
Changes in hydrographic and flow conditions
(high- and low-frequency) are an important factor
for shaping seamount communities (and controlling
bio-communication?) Sphere of influence is not
restricted to the seamount (implications for
sampling strategies), but penetrates deep into
the oceanic far field. Complex co-existence of
physical controls and biological distribution
patterns requires a more comprehensive ecosystem
modelling approach (full spectrum forcing).
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Outlook Full spectrum forcing
Lavelle (in prep.)
Response of passive tracer patterns to mean and
combined mean tidal forcing (representing only
part of the spectrum of oceanic motions) More
retention when tidal flow is added
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Outlook Full spectrum forcing
Current meter time series from the East Pacific
Rise EPR (Lavelle, 2009)
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Outlook Full spectrum forcing
Tracer response to a more realistic forcing
spectrum as determined from the EPR time series.
Lavelle (in prep.)
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Conclusions
Seamounts are very good habitats to study and
understand bio-physical interactions well enough
to predict abundance and distribution of marine
species. Is our knowledge good enough?
Adaptation of sampling and modelling strategies
(sphere of influence, full-spectrum forcing and
biological response, robustness of observed
patterns against long-term changes) A broad,
multi-disciplinary ecosystem approach is required
for both research and sustainable
protection/management. Intensification of
exisiting networks.