3D-Plus-Time Oceanography
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. The projects described next are examples of such attempts, and are also interesting in their own right as small-scale process studies.
 | The Aegean Sea Project |
This project was together with Mike Gregg and our Greek colleagues at the Hellenic Center for Marine Research. Working in the Aegean Sea (figure 1) offered a unique chance to investigate internal waves and mixing in a region with very rugged bathymetry and negligible tides. Owing to the complicated dynamics producing mixing in shallow water, the best approach was to remove one of the major forcing variables. Lacking tides, internal waves in the Aegean should be forced primarily by the wind and in some places secondarily by low frequency flows, such as the eddies revealed by the drifter tracks in earlier studies by Don Olson. In view of these opportunities, our general goals and a preliminary outline of our intended measurements were:
1. To compare internal waves and mixing at shallow (100 m) and intermediate-depths (500 m). The steep slopes around the Cycladic Plateau (figure 2) produce separations of only a few kilometers between level sites on the plateau and in nearby basins. Taking microstructure profiles alongside ADCP and moored internal-wave measurements on the plateau and in a basin provided data similar to that collected during CMO and thereby let us determine the generality of those results.
2. To relate internal wave variability to changes in local winds. We
took the internal wave measurements for 2-3 months,
centered around the intensive microstructure measurements. By
coordinating our measurements with work being proposed by Kathie Kelly
and with Greek colleagues, we had access to satellite and model
wind fields (figure 1) over the Aegean during this time.
3. To observe how the internal wave field and mixing evolve between
the basin and the plateau. By installing a small line array from the
basin to the plateau (figure 2), we observed whether changes in internal
waves on the plateau were related to (primarily wind-forced) waves
propagating onto the plateau from deeper water.
Moorings went in the water August 2004 from the Greek ship R/V Aegeo.
The intensive towed and microstructure cruise was onboard R/V
Oceanus during Oct/Nov 2004.
Preliminary analysis shows a surprisingly strong internal tide, counter to conventional wisdom for the Med, and a rich, spatially variable mixing and near-inertial internal wave field. Analysis is ongoing.
| 3-D Evolution of Thermohaline Intrusions | ^ TOP |
Lagrangian floats ( Eric D'asaro's site), microstructure profilers and towed CTD/ADCPs (SWIMS) were deployed from two ships in a high-tech attempt to map and track thermohaline intrusions in Puget Sound. These "blobs" of warm/salty or cold/fresh water are ubiquitous in the world oceans, and may be key in effecting isopycnal mixing, especially near fronts. The combination of
microstructure, floats and towed surveys enabled us to determine the blob's structure (figures 3 and 4), and to determine that its
observed evolution appeared to be due to lateral rather than diapycnal
mixing.
The results are in JPO (Download the PDF file).
In a subsequent NSF-funded project, Mike and I attacked the open-ocean problem. In a cruise summer 2007, we used microstructure and towed surveys, again centered on floats, to map the locations of intrusions relative to the subtropical front, and to measure their structure and evolution. Andrey Scherbina is analyzing the data.
| Internal Tides in Mamala Bay, Oahu (ONR) | ^ TOP |
Internal tides generated from surface-tidal flow past Oahu's Kaena Ridge and Makapuu point (figure 5) impinge upon Mamala Bay (outside the mouth of Pearl Harbor), leading to large internal tidal displacements (> 100 m) and currents (> 1 knot) at the M2 tidal frequency (12.4 hours). In September 2002 we deployed a Moored Profiler and conducted repeated towed surveys with SWIMS II, with aims of understanding its dynamics and the effects on pollutant dispersal and acoustic transmission. The findings also impact the more general problems of flows in embayments and internal-wave radiation.
The results are published in JPO (Download the PDF file). The most interesting findings so far are:
• Model and observed fluxes agree well with each other (figure 6), lending confidence to both. A striking feature is a convergence near the western portion of the Bay.
• The strongest mixing, inferred from overturns, occurs in the western portion of the Bay (figure 6), in the same region and of the correct order of magnitude to balance the flux convergence. The mixing appears due to shear instability above a submarine ridge (figure 7).
• M2 phase increases toward the west, implying wavelengths of about 60 km, which agrees well with expected mode-1 values.
• M2 phase is strongly temporally variable, and is consistent with modulated propagation due to time-variable stratification between the source at Makapuu Point, 50 km away, and Mamala Bay (figure 7).
• Using model output from Merrifield (POM), Kim Martini, a graduate student, has been examining the spatial dependence of energy (E), energy flux (F) and group velocity, cg=F/E. Preliminary analysis indicates that standing versus pure propagating waves can be identified via group velocity measured this way.
The results thus indicate a complicated superposition of eastward- and westward-travelling waves. Owing to the time-varying stratification between sources and the bay, the arrival times, and thus the interference pattern, shifts in time. We believe that the results from this highly-resolved study will shed light on the more general problem of long-distance internal-wave propagation.
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