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Density Temperature ( oC) Salinity Fluorescein (g/l) Fluorescein vs density North-south velocity

Density Temperature ( oC) Salinity Fluorescein (g/l) Fluorescein vs density North-south velocity

Red to northBlue to south

Shipboard ADCP

A series of dye releases from a small boat during the summers of 2001/2002 investigated cross-shelf circulation in a region dominated by intermittent wind-driven upwelling. The aim was to make direct, Lagrangian observations of the pathways of cross-shelf flow during active upwelling. Lagrangian techniques provide insight that is not available from Eulerian measurements because of the dominance of alongshelf flow. Here we focus on a release from August 2002, made as winds transitioned from weakly downwelling-favorable to weakly upwelling-favorable. Dye was seeded in an alongshore streak at 20m depth in 55m of water, a little beneath the pycnocline. Of note was the rapid onshore spreading of the dye patch, having a cross-shelf extent of 5km after 48 hours, and extending into shallow water (less than 20m bottom depth). The dye remained largely isopycnal, following the upper edge of the bottom boundary layer onshore. The offshore edge of the patch became associated with a strong temperature inversion – a layer which appears as a warm anomaly as cooler, fresher water from the north overrides it at the surface.

Andrew Dale1 (acd@coas.oregonstate.edu), Jay Austin2, Jack Barth1 and Murray Levine1

1 College of Atmospheric and Oceanic Sciences, Oregon State University, Corvallis, OR.2 Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA.

Abstract

Do temperature inversions and interleaving features tell us anythingabout secondary (cross-shelf or cross-frontal) circulation?

Rapid onshore spreading of the dye patch

Often they don’t because alongshelf variability is important too.

This material is based upon work supported by the National Science Foundation under Grant No. 0136900. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Work was carried out from the 54ft OSU research vessel Elakha, skippered by Perry York. Technical assistance from Dennis Root and Anatoli Erofeev. ADCP advice etc. from Mike Kosro and Steve Pierce. Assistance at sea from Yvan Alleau, Marc Barnes, Renato Castelao, Scott Couch, Cidney Howard, Jason Killian, Anthony Kirincich, Jamie Lescinski, Larry O’Neill, Michael Ott, Daniel Palacios.

Acknowledgements

Flo

ure

scein

Tem

pera

tur

e

Densit

y

Salin

ity

Cascade Head

(Photo John Bane)

(Photos Jay Austin)

Dye tank and Minibat launch

Minibat configured for dye release

Dye in hose

Drogued ARGOS/GPSdrifter

Pre

-re

lease

1 d

ay p

ost-

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2 d

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54ft R/V Elakha.

Undulating Minibat vehicle with fluorometer and CTD.

Shipboard ADCP.

The dye release was made on 6 August 2002 during an extended period of predominantly upwelling-favorable (northerly) winds, but immediately following two days of weakly downwelling-favorable (southerly) winds. Survey periods are highlighted above.

September experiment not described here

Wind reversal coincides with dye release

Equipment

Dye

ReleaseTracking/Survey

45 kg fluorescein fluorescent dye.

Density adjusted to release location.

Dye patch relocated on successive days, using a drogued GPS/ARGOS drifter as guidance.

Survey consisted of Minibat tows on cross-shelf lines extending as close to shore as practical (to 10-20 m bottom depth).

Topography is relatively uniform alongshore

Dye released by pumping via a hose to the Minibat while towing at a constant density.

Initial dye patch was an alongshelf streak.

Dye re

lease – a

lon

gsh

elf s

treak

(~ 4

km

)

The strong temperature inversion encountered one day after dye release. Temperature increases with depth by as much as 2 oC over 5 m. The temperature maximum is associated with high turbidity and chlorophyll, as well as a a strong fluorescein signal.

Tem

pera

tur

eD

ensity

Fluore

scein

Light transmissio

n

In addition to the August 2002 dye release described here, dye releases were successfully tracked during August 2001 (investigating interaction of the equatorward upwelling jet with Stonewall Bank), and in September 2002 (in a similar situation to that described here).

Planned fieldwork for 2003 will comprise three further dye releases, from a larger vessel, permitting high resolution surveys and 24-hour tracking.

Mid-depth temperature maxima are regularly observed off the Oregon coast, often associated with high turbidity and chlorophyll. It has been speculated that these features trace a cross-shelf pathway of the subduction of near-shore surface water.Dye released on August 6 2002 was

not associated with a temperature maximum.

One day after release, the dye was contained in a temperature maximum, associated with the arrival of relatively cool, fresh near-surface water (see T-S ).

After two days the temperature maximum was no longer visible.

The dye core did not move cross-shelf during this period.

It seems clear that the August 7 temperature inversion arose because of the intermittent presence of near-surface water of different T-S properties. It is suggested that this was a result of sheared alongshore advection of alongshelf variability, rather than cross-shelf transport. Upwelling-favorable winds would have pushed surface water offshore during this period.

See vertical profile Pre-release T-S

1 day post-release

T-S

Gre

en is T

-S d

urin

g re

lease

Gre

en is T

-S o

f dye

obse

rved

Dye was injected as an alongshelf streak of approximately 4 km in length at close to 20 m depth, with minor corrections made to the injection depth to maintain constant density. Such a release approximates a point source in a 2D sense (relative to a cross-shelf section). A double circle shows the injection relative to a pre-release section at 45oN (below). As upwelling-favorable winds blew over subsequent days, alongshelf velocities became increasingly southward. In this transition, the inner-shelf led velocities further offshore. The dye patch initially moved slowly to the north, stalled, then began to move south. Dye spread rapidly onshore, remaining largely isopycnal, lying along the upper boundary of a developing bottom Ekman layer. The core of the dye patch (greatest dye concentrations) remained close to the isobath of the release (50-60 m), despite some of the dye reaching bottom depths of 15 m or less.

1 d

ay p

ost-

rele

ase

2 d

ays p

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Dye rapidly spread onshore along an isopycnal surface, although the dye core remained in water of 50-60 m bottom depth.

Density Fluorescein

1 day post-release (55 m)

2 days post-release (51 m)

On release, the dye streak presumably had a width equal to the diffuser, approximately 1 m.

Soon after release (2-4 hours) the cross-shelf scale of the dye patch was already 0.5-1 km.

One day after release, the onshore edge of the dye patch had reached water of depth 20 m for a cross-shelf scale of nearly 5 km.

Two days after release, the cross-shelf scale was still around 5 km, although dye concentrations in shallow water were increasing (assuming the difference between the sections surveyed is representative of the temporal change).

Flo

ure

scein

The relative contributions of advection, diffusion and shear dispersion to this cross-shelf spreading are not yet clear.

A vertical profile through the near-bottom dye tongue two days after dye injection reveals that, although the tongue appears to parallel the bottom, dye is not concentrated in the bottom boundary layer. In fact, it follows an isopycnal surface approximately 5 m above the bottom. The well-mixed bottom boundary layer is only a few meters thick, and does also contain dye. The onshore edge of the dye patch is located where this isopycnal intersects the bottom.

Values in parenthesis give the bottom depth at the location of maximum dye load.

Bottom boundary layer.

Dye maximum above bottom boundary layer.

Dye release