posted Jul 6, 2011 1:32 PM by John Mickett
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updated Dec 14, 2011 12:58 PM by WaveChasers APL
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Fiber-optic Salinity/Density Sensor: the ocean refractometer
Understanding the smallest motions in the ocean, which have scales smaller than a millimeter, is (ironically) important to advance our understanding of the climate, since circulation models cannot resolve turbulence and have to parameterize it. These scales are poorly understood because they are so small, and because salinity is very hard to measure. Using technology modified from AIDS research, I developed a sensor half the diameter of a human hair (figures 1 and 2) to sense these smallest-scale fluctuations in ocean refractive index. These are closely related to salinity fluctuations, which to-date have been measured by conductivity and temperature measurements. Measuring salinity from one small sensor reduces "spiking" resulting from mismatched sensor responses. Testing began summer 2001 in Puget Sound, Washington, and initial results showed that in fact the so-called Batchelor cutoff for salinity could be resolved (figure 3). In addition, however, the sensor responds to the velocity signal of the turbulence, making it an optical "shear probe," but precluding resolution of low-frequency signals. I was able to reduce, but not eliminate, this effect.
Mooring Technology
Current moored profiling technology is severely limited by battery power. This means that if we want to observe with a profiling mooring for a year, one must suffice with daily profiles - or worse. Since internal waves, which contain about half the ocean's energy, have higher frequencies than this, we wish to profile at least once an hour or so - resulting in a short (44-day) time series. To improve on this situation, we are developing a next-generation moored profiling system. With Bruce Howe and Time McGinnis (also at APL), we are modifying a McLane profiler, to be inductively charged from an external power supply. This can be a cabled observatory node, or a large moored battery pack for deployment at non-cabled sites. We are also developing the needed inductive, acoustic and radio communication technologies to allow real-time telemetry back to a nearby ship or shore.
We are also working with McLane to add new instruments and capability to the McLane whenever we can, beginning with different gearbox ratios for faster profiling speeds, rechargeable battery packs, a Nitrate sensor, and a Nortek aquadopp velocity sensor. Working with our colleagues at OSU, we have successfully integrated chi-pods for direct turbulence measurements into two of our profilers. |
posted Jul 6, 2011 1:32 PM by John Mickett
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updated Mar 27, 2012 10:16 AM by WaveChasers APL
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Embedded in the California Current System (CCS), the low-frequency oceanography of the Washington (WA) continental shelf (Figure 1)
has been studied in a great detail over the last several decades owing in part to its high productivity, and lately owing to its sensitivity to
harmful alga blooms, hypoxic and anoxic events, and ocean acidification. Overall biological productivity in the CCS, which flow along the
western boundary of the United States and southern Canadian Pacific coast, is generally attributed to seasonal upwelling of nutrient-rich
deep waters to the continental shelf (Hickey and Banas, 2003). This upwelling is usually caused by the stress of winds blowing equatorward
on the ocean's surface along the coastal boundary. It might be expected that overall productivity along the boundary coast would be correlated
with the strength of the upwelling-favorable wind stress at a given location. However, in the CCS, this relationship does not hold in a way that
the seasonal average coastal chlorophyll concentrations increase fivefold from northern California to southern Vancouver Island, counter to
the magnitude of alongshelf wind stress, which decreases by a factor of eight over this region (Hickey and Banas, 2008).  Figure
1: Map showing the location of the NEMO moorings(star) and gliderline
(blackline), as well as the NDBC Cape Elizabeth meteorological buoy
46021.
Observed barotropic tidal flow and time-mean vector of the
California Current are shown with a black ellipse and arrow,
respectively, with scaleas indicated.
The red and blue arrows are the
semidiurnal energy flux computed from observations and model,
respectively. The colormap shows conversion of barotropic
tidal energy
to baroclinic tides. The black and yellow dashed line is the boundary of
the Olympic Coast National Marine Sanctuary. Contour levels are each
100m until 500m, then each 200m thereafter.
Previous studies suggested that the northern CCS has several mechanisms that can produce upwelled nutrient concentrations comparable to those
in regions with much greater wind stress. Top candidates include a persistent nutrient supply through the dynamics of the Strait of Juan de Fuca
(Crawford and Dewey, 1988), freshwater input from the Columbia river plume (MacCready et al., 2009), the presence of the Juan de Fuca eddy, and
enhanced upwelling by the Juan de Fuca canyon (MacFadyen and Hickey, 2010). In ways that are not yet fully understood, the unique aspects of the
Washington continental shelf may conspire to boost its productivity (Hickey and Banas, 2008).
Internal waves and mixing are likely key players in transportation of nutrients and DO from the deep layer to the eutrophic zone. Limited by the observation
data on the WA continental shelf, no previous studies have reported a detailed study on the internal waves or mixing in this regions. We have recently
designed and deployed a new three-component observing system consisting of two moorings and a glider line off the Washington coast. A rich, variable
internal wave field is seen, appearing to feature some of the stronger nonlinear internal waves (NLIW) yet reported on continental shelves, when viewed
as a fraction of the water depth. I propose to study the characteristics of internal waves on the WA continental shelf, the generation mechanism and
dissipation of NLIWs and their potential influence on the nutrient supply in this region.
We're interested in the oceanography off our coast. As part of the NANOOS program, we now maintain the Cha-Ba surface buoy, its subsurface analogue,
and a glider line. We collectively refer to these systems as NEMO. |
posted Jul 6, 2011 1:31 PM by John Mickett
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updated Dec 14, 2011 1:00 PM by WaveChasers APL
]
General circulation models (e.g. Samelson, 1998) indicate that the thermohaline circulation is highly sensitive to the global magnitude and distribution of mixing. Recent observations (e.g. Polzin et al 1997) highlight the inhomogeneity of mixing. Therefore, a global map of mixing would benefit climate modeling, as well as our understanding of biological productivity and pollutant dispersal. Since global measurements of mixing are impractical, another way of estimating the global mixing distribution is to map the energy flux into internal waves, and then to measure their subsequent long-range propagation.
Figure 1: (top) Depth-integrated, annual-mean near-inertial energy-flux vectors are plotted from 60 historical moored records. The length of each arrow is logarithmic, with references indicated at upper left. Moorings with |F|< 0.1 kW/m appear as a black dot without an arrow; white dots indicate moorings which were unusable. The few instances of poleward propagation are plotted in white. Color map indicates annual-mean energy-flux from the wind to near-inertial mixed-layer motions from Alford (2001). The color scale is logarithmic and is indicated at upper left. The box in the NW Pacific is discussed in the text. (bottom) Arrows are as in (a) but for the M2-tidal band. Color map denotes internal-tide conversion using the TPXO5 model (Egbert and Ray, 2000), courtesy of G. D. Egbert. The lines in the western Pacific and near Hawaii are discussed in the text. The inset in each panel shows a histogram of the poleward component of the flux for all moorings.
The first step is to map the distribution of the two primary internal-wave sources, the wind, which generates near-inertial waves, and surface-tidal flow over topography (Egbert and Ray, 2000), leading to internal tides. The wind-flux portion is calculated (Alford, 2001) by using the NCEP/NCAR reanalysis wind fields, which incorporate observations over many years and locations into a dynamically consistent framework, to drive a simple model of the mixed-layer response. The energy flux is then given by the scalar product of the wind stress and the mixed-layer current. Ongoing work involves examining the high-latitude dependence of the fluxes (as the response gets faster at high latitude, the NCEP winds become more and more inadequate for the job), and the interannual variability of the fluxes.The global distribution of the energy flux available for internal waves is plotted below for the wind (top) and the tides (bottom, courtesy of G. Egbert). The greatest wind inputs occur at midlatitudes during wintertime, associated with travelling storms. Large tidal inputs (red regions) occur where the surface tide flows perpendicular to rough bottom features. The subsequent horizontal flux for each can be measured using historical moored records by solving for the lowest two modes and computing <u'p'>, where u' is the baroclinic velocity and p' is the baroclinic pressure anomaly. Near-inertial fluxes are large following wintertime storms, and are equatorward, since waves generated at the inertial frequency become evanescent poleward of their generation site. The tidal fluxes are usually directed away from strong topography. Since then, Zhongxiang Zhao and I have been working on further refining our estimates of the global distribution of internal-tide energy flux from both moorings and altimetry, which has resulted in a series of papers and an exciting new NSF proposal. Harper Simmons and I are working together to compare models and observations in order to determine the analogous fate of the near-inertial motions. |
posted Jul 6, 2011 1:29 PM by John Mickett
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updated Dec 14, 2011 1:02 PM by WaveChasers APL
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Ocean flows are spatially complex and rapidly evolving. Measurements resolve a combination of the two dimensions (space and time) with varying degrees of success: moorings (excellent temporal coverage, poor spatial coverage), shipboard surveys (good spatial coverage at the cost of synopticity) and single-point profiles (good vertical resolution but poor temporal and horizontal coverage). I am interested in developing better observational techniques for simultaneously resolving their spatial and temporal resolution. |
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