SeaSoar Observations During Coastal Mixing and Optics

Jack Barth and Mike Kosro

College of Oceanic and Atmospheric Sciences

Oregon State University


This project is part of the Office Of Naval Research "Coastal Mixing and Optics (CMO) Accelerated Research Initiative". The objective of the Coastal Mixing and Optics ARI is to quantify and understand the role of vertical mixing processes (in a laterally-varying environment) in determining the mid-shelf vertical structure of hydrographic and optical properties and particulate matter.

As part of the Coastal Mixing and Optics ARI, we are investigating spatial distributions of mixing and optical properties on the continental shelf and relating them to larger scale features in the shelf density and velocity fields (e.g., wind-driven or geostrophic currents and fronts). By defining spatial variations in not only the continental shelf and slope circulation, but also in the mixing and optical properties in this region we can determine the relationship between lateral processes and observed variations in local vertical mixing processes. To accomplish these objectives,we made contemporaneous measurements of density, microstructure and light absorption/attenuation using sensors mounted on SeaSoar, a towed undulating measurement platform. The SeaSoar sensor suite includes a dual-sensor Seabird CTD, a new microstructure instrument (MicroSoar) and a nine-wavelength spectral absorption and attenuation meter (WETlabs ac9). Horizontal velocity is measured directly, at the same time and space scales as the hydrographic, mixing and optical fields are measured, using a shipborne ADCP.

Seasoar data reports: Hydrographic, ADCP, AC-9 (optical), and Microstructure

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Figure 1

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Schematic of SeaSoar equipped with a Seabird 911+ conductivity temperature depth (CTD) instrument, a WETlabs multispectral optical absorption and attenuation meter (ac9) and MicroSoar.

Figure 2

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SeaSoar, a towed, undulating measurement package. The moveable wings are contolled by a topside computer which sends up-down signals via the conducting tow cable. The wings are moved by a hydraulic unit which is powered by an impeller on the tail of SeaSoar. Data is returned to a topside computer network via the conducting cable.

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Figure 3

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SeaSoar being deployed off the R/V Endeavor

Figure 4

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Closeup of sensors mounted on SeaSoar: dual Seabird temperature-conductivity sensors pointing forward through the nose of SeaSoar; inlet-outlet for water supply to ac9 optical instrument just above Seabird sensors; MicroSoar fast-response temperature and conductivity sensors located beneath the Seabird sensors. The MicroSoar sensors are protected by four probe guards.

We made rapid surveys during two 21-day field experiments in the Middle Atlantic Bight centered near 40.5N, 70.5W, south of Marthas Vineyard. A summer survey, when the shelf is stratified, was made from 14-Aug to 1-Sep 1996 and a spring survey, when the shelf water tends to be well-mixed and separated from offshore water by a shelf-slope front, was made from 25-Apr to 15-May 1997. During each field experiment we made repeated large-region surveys over a 80 x 70 km box, which we cover with high-resolution every 2 days. We also concentrated our measurements in a small 20 x 20 km box centered around a mid-shelf location where vertical mixing processes were intensively sampled by moored instrumentation and vertical profiling from a stationary ship.

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Figure 5

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Study region for CMO and SeaSoar survey patterns.

Figure 6

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"Cross-Shelf Section along 70 30'W sampled with SeaSoar on 25-Aug-96 0300--0500 (UTC)"

Light dots are data points: 2-db and 0.5-km averages for T, S and and 1-sec averages for the optics.

T and S: warm, salty intrusion of slope water at offshore edge of section; cold, bottom water

: strong pycnocline at 10-20 m; sloping isopycnals in offshore half of section in geostrophic balance with westward jet

Chlorophyll absorption: biological maximum at base of pycnocline; absence of biological material in shallow, salty slope water intrusion at offshore edge

Attenuation: this is an indication of the total amount of suspended material and shows the influence of biological material at the base of the pycnocline as well as a maximum in the bottom nepheloid layer which has distinct cross-shelf structure

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Figure 7

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"Maps of Temperature and Salinity at 35 m during CMO SeaSoar Surveys"

These are from the "Small Sampling Box" centered around the CMO moorings

Light dots are 0.5-km-averaged SeaSoar observations and the cross-shelf lines are separated by 5 km

Rapid evolution of the distinct mesoscale fields is evident

The vertical section shown in the "Cross-Shelf Section" figure is from the third line from the west in the upper map

Maps at other depths -- not shown -- (e.g., 35-m to show the salty slope water intrusion and 55-m to show the bottom water distributions) confirm that the mesoscale structure and its time evolution is strongly three-dimensional

Figure 8

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"Large-amplitude soliton sampled with SeaSoar during CMO"

Light dots are 1-sec-averaged SeaSoar observations using both the up and down profiles for T and S, and just the up profiles for the optics

The tops and bottoms of each SeaSoar profile are separated horizontally by about 300 m and at mid-depth in the profile the horizontal separation is
half that, or about 150 m. There are several SeaSoar profiles within this large-amplitude soliton

The soliton penetrates to the bottom and, as it propagates up the shelf, it displaces the salty slope water near the bottom

The optical properties are strongly influenced by the soliton, both in displacing the pycnocline maximum in chlorophyll absorption and in influencing the suspension of material in the bottom nepheloid layer (only hinted at in this section of chl-absorption, but more evident in sections of 650-nm attenuation not shown here)

Velocities from the 300-kHz ADCP (not shown), switched to 10-sec ensembles as we sampled the soliton, show a classic mode-one response with about 30 cm s in the upper and lower layer

Any Questions? e-mail me.