ROLE OF HEADLAND IN LARVAL DISPERSAL

Satoshi Mitarai

Preliminary results and research plan (Maybe for the next F3 meeting)

BACKGROUND

• Topographic eddies may be important in determining habitat connectivity– Eddies can retain eddies for time scales comparable

with their PLD

• High local recruitment is observed in island wake eddies (e.g., Swearer et al, 1999)– But, such clear pattern will not be observed in coastal

eddies where currents are less persistent in direction (Graham & Largier, 1997)

• Few notable studies (Largier, 2003)– Geostrophic size & flow time scale to be considered

GOAL OF THIS STUDY

• Estimate the role of coastal headland eddies on larval dispersal using idealized ROMS simulations– By comparing the obtained results with straight

coastline case– Are there any critical sizes?

HEADLAND DESIGN

• Gaussian-shape headland in idealized simulations– Three parameters

2. Width (w)(twice the STD)

1. Amplitude (a)

3. Domain size or distance between headlands (d)

a = 10, 20 & 40 kmw = 20 kmd = 256 km

SST EVOLUTION

LARVAL DISPERSALSmall headland (a = 10 km) Large headland (a = 40 km)

• Eddies affected by headland

Red dots = settlement (PLD = 20 to 40 days)

ONLY SETTLERSSmall headland (a = 10 km) Large headland (a = 40 km)

• More turbulent & offshore transport

Red dots = settlement (PLD = 20 to 40 days)

SAMPLE CONNECTIVITY(Connectivity is normalized so that summation becomes unity)

• Significant retention• Headland is not strong sink

• Longer travel distance (than straight coastline)• Headland is strong sink

ANOTHER REALIZATION

• Looks similar to the previous one…

AND ANOTHER

• Connectivity seems to be consistent interannually• Some variations for intermediate size, though

(POSSIBLE) PHYSICAL EXPLANATION

H

Small headland (a = 10 km) Large headland (a = 40 km)

Wind stress

Water accumulation HHeadland eddies

Pressure

• Additional pressure reduces larval displacement & create retention zone

• Headland eddies bring larvae offshore, leading to longer travel & headland settlement

SUMMARY

• The role of head land in larval dispersal highly depends on the size (i.e., nearly opposite effect)

• When small (~ 10 km) – Create retention zone around them, leading to

significant number of self recruitments– Not a strong source or sink

• When large (~ 40 km)– Makes flow field more turbulent, resulting in longer

travel distance (than straight coast line)– Strong sink (i.e., accumulates settlers from upstream)

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)?

DISCUSSION (2)

• Can we support the simulation results with biological data (e.g., otoliths)?– Pick two types of habitats in central coast; i) around

small headland and ii) large headland – Estimate dispersal scale using otoliths data (e.g.,

Siegel et al, 2003)– And see if there is significant difference between

estimated dispersal scale

• Same thing happens in “realistic” simulations?– Collaboration with UCLA, SIO, OSU?

FUTURE PLANS (TIM’S WORK)

Tim will work on these

Amplitude (km)

Wid

th (

km)

Retention regime

Turbulent regime

• Examine winter (weak upwelling) case• Change headland parameters

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

• Do more realizations & obtain statistics