ROLE OF IRREGULAR COASTLINES IN LARVAL

DISPERSAL

Tim Chaffey, Satoshi Mitarai, Dave Siegel

Results and research plan

WHY IS AN IRREGULAR COASTLINE IMPORTANT TO LARVAE?

• Topographic eddies may be important in determining habitat connectivity.– Larvae can be entrained at the edges of eddies for time

scales equivalent to their PLD.• High local recruitment is observed in island wake eddies

produced by unidirectional flows (Swearer et al, 1999)– Similar clear recruitment patterns will not be observed in

coastal eddies where currents are less persistent in direction (Largier, 2001).

• Filament formation off topographic features may be important for rapid offshore transport of larvae (Haidvogel, 1991).

• Offshore filaments present an obstacle to nearshore settlement in PLD.

GOALS• Estimate the role of irregular coastlines on larval dispersal using idealized

ROMS simulations.– Do headlands create a consistent or stochastic connectivity? – Over what time scale does connectivity become consistent?– Is there a critical headland amplitude/width to create a consistent connectivity?– How important is headland spacing?– If there is a consistent connectivity, is it temporally bounded by different wind

regimes?– Understand the fluid dynamics of headland induced filament formation and eddy

recirculation.

POSSIBLE PHYSICAL EXPLANATIONS

• Flow is along the headland and no flow separation is seen

• Filaments transport larvae offshore.• NearshoreEddies retain larvae altering travel distances and location• Offshore Eddies return larvae to nearshore region.

Critical Size HeadlandHeadland Size < Critical Size

Model Setup

• Bathymetry slope is similar to California Coast– Inshore isobaths follow coastline becoming north-south offshore

• Wind field estimated from July buoy measurements– The principal axis of the winds stress is rotated to along the

isobaths• The pressure gradient term is quantified using dynamic

height data from the CalCOFI July surveys.– The principal axis of the alongshore pressure gradient is rotated

to along the isobaths. • Model spun up using a fully developed flow field.• Each realization only over a 180-day period.• Life History - All larvae are contained in the top 10

meters.

Coastlines Examined

• Straight Coastline • Idealized California Coastline - Modeled after

real coastline data.• Both share domain of 288 km in the cross-shore

and 256 km in the alongshore dimension.• Periodic boundary condition at alongshore

boundaries.

Idealized California Coastline

Larvae Dispersal for Two Coastlines

Straight Idealized CA

Connectivity Matrixes for a Straight Coastline

Connectivity Matrixes for Idealized CA CoastlineBase Case Base Case 2 Base Case 3

SUMMARY

• All conclusions are for a single 180-day realization for a chosen life history.

• Larvae are entrained along edges of eddies.• Connectivity is stochastic for the straight

coastline case.• Connectivity is only weakly consistent for

idealized CA coastline.• No persistent physical features seen amongst

different coastlines or amongst realizations.

Discussion Ideas• Over what time scales does consistent connectivity start to

exist? • How do cross-shelf wind stress variations around a

headland affect upwelling? Are there upwelling shadows created?

• Does the buoyancy front around an upwelling shadow present a barrier for larval transport?

• What is “criticial” to produce consistent connectivity over defined time scales?– Wind, headland size (Rossby radius?), headland

spacing, bathymetry, model tuning • Are the physical interactions between critical and non-

critical parameters important?

IDEALIZED HEADLAND DESIGN

• Gaussian-shape headland/headlands in idealized simulations– Three parameters

2. Width (w)(twice the std of Gaussian function)

1. Amplitude (a)

3. Domain size (d) = distance between headlands

a = 10 kmw = 20 kmd = 256 km

FUTURE PLANS

Amplitude (km)

Wid

th (

km)

Retention regime

Turbulent regime

• Examine winter (weak upwelling) case• Examine different life histories (PLD, vertical behaviors)• Change headland parameters

– Clarify role of headland size (see diagram below)– Reduce domain size to see headland interactions

• Do more realizations & obtain statistics• Change wind parameter to reflect wind around headlands

DISCUSSION (1)

• Critical size would be Rossby radius– When the headland size is comparable to Rossby

radius, eddies are greatly affected by headland, resulting in enhanced turbulence

• In reality, these two types co-exist, perhaps affecting each other– Interactions may be more important for small

headland case (e.g., make it more turbulent?)

• Which one is more significant? – Consistency around headland or stochasticity driven

by coastal circulations (straight coastline case)?