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Murray D. Levine
and
James D. Irish

Abstract

We have developed a statistical model describing a random field of internal waves passively oscillating a random, locally horizontally uniform temperature finestructure. Here we define finestructure to be non-internal wavelike temperature fluctuations of any vertical scale caused by phenomena such as horizontal intrusions or geostrophic eddies. The model allows one to examine the effects of finestructure upon all relevant measurable statistical quantities of the temperature field as a function of the vertical scale of the finestructure. Also, the effects of the time variation of the finestructure itself are considered.

The model was fit to data obtained during the 3-week Mid-ocean Acoustic Transmission Experiment (MATE) in summer 1977 new Cobb seamount in the northeastern Pacific. Various spectra and coherences estimated from temperature time series and vertical profiles were used to make an assessment of the finestructure as well as establish the consistency of the model.

Temperature variance measured at frequencies above the local Väisäiä frequency was used to estimate the magnitude of the high vertical wavenumber finestructure. This high wavenumber finestructure (0.05–1 m−1) with a (wavenumber)−−2.5 dependence can consistently explain all the observed variance in the measured high vertical wavenumber spectra. At lower wavenumbers (0.002–0.020 m−1) a (wavenumber)−2 dependence was observed, and the spectral level found to be approximately equally divided between finestructure and internal waves advecting a constant temperature gradient. The contribution of the finestructure effects to the internal wave frequency spectrum was found to be about a factor of 10 less than that of internal waves advecting a constant temperature gradient.

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Wendell S. Brown
and
James D. Irish

Abstract

The annual evolution of the geostrophic flow structure in the Gulf of Maine was studied with a combined set of moored pressure time-series measurements and five hydrographic surveys from August 1986 through September 1987. A series of quasi-synoptic dynamic height maps depicts a gulf flow structure whose typical spatial scales decrease from order 100 km during the winter to about half that in the summer, when the evolution of surface, intermediate, and deep water masses is most rapid and complex. The unusually large amount of freshwater in the gulf during 1987 was partially responsible for the establishment of a north–south across-gulf front during the summer. Year-long time series of bottom pressure and internal pressure (derived from temperature and conductivity measurements in Georges and Jordan basins) have been differenced with coastal synthetic subsurface pressures (SSP) to yield a history of the basin-scale geostrophic flow variability. The basin-scale geostrophic transport was dominated by cyclonic flow (>0.5 × 106 m3 s−1) through the gulf during autumn 1986. During early January 1987, the flow around Jordan Basin became anticyclonic as relatively fresh Scotian shelf water flowed into the eastern gulf. Though temporarily disrupted during April and May, the Jordan Basin anticyclone persisted through July. Inflows of slope water to Georges Basin helped to establish a robust (0.5 × 106 m3 s−1) cyclonic flow around Georges Basin in June and July. Though somewhat weakened (∼0.3 × 106 m3 s−1), the cyclonic gyre migrated with the flow of slope and Maine bottom water to Jordan Basin in August. The delay in the establishment of the cyclonic gyre in Jordan Basin in 1987 appears to have been related in part to the effects of the anomalously large amount of freshwater in the gulf during 1987. A conceptual model of the annual evolution of gulf-scale flow, based on a hypothesized interplay of pressure gradient forcing produced by variable inflows (outflows), thermohaline forcing, and, to a lesser extent, wind forcing, is presented.

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Gunnar I. Roden
and
James D. Irish

Abstract

Spikes are often observed in salinity profiles computed from measurements of conductivity, temperature and pressure. Many of these spikes are not real and are the result of a mismatch in the response functions of the sensors. Some of the spikes are also due to the sequential sampling technique used by most digitizers whereby the sensors are not sampled at the same time or position. We derive expressions to linearly correct for these two causes of spikes. When the corrections are applied to measurements in the North Pacific, a significant reduction in the number and size of the spikes is observed in high gradient regions such as the thermocline.

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James D. Irish
,
Wendell S. Brown
, and
Thomas L. Howell

Abstract

Geophysical signals are often intermittent, having statistics which vary with time. Optimal sampling of these signals requires a so-called “conditional sampling” scheme, a technique which changes the sampling program to match the time scales of the processes of interest. To optimize the limited tape storage capacity of remote oceanographic instruments, a conditional sampling scheme has been implemented using the computational power of microprocessor-controlled instruments, and several deployments have been made with various configurations of the conditional sampling algorithm. This algorithm monitors short-term changes in the energy of an incoming signal within a designated high-frequency band (by digital filtering techniques) and compares the resulting intensity with the longer term statistics of the signal. If the energy exceeds an intensity defined as critical according to some criteria, then an “event” is declared and the data are recorded at a higher than normal rate for the duration of the event. When the statistics of the expected signals are not well known, and criteria cannot be predetermined with confidence, an “adaptive” technique is required whereby the instrument makes an in situ determination of the critical intensity level for each signal based on the statistics of that signal.

Several deployments of the conditional sampling instruments have been made which demonstrate the operation of the technique. In Massachusetts Bay, a burst of high-frequency internal wave energy was identified and recorded by the adaptive critical algorithm applied to a moored temperature sensor array. On the northern California shelf, salinity was calculated in situ from moored temperature and conductivity sensors, and the resulting salinity time series conditionally sampled to identify salinity events as separate from temperature or pressure events.

Conditional sampling techniques may not be optimum for exploratory work. However, where the processes and expected signals are intermittent and have a specific signature, then the use of a conditional sampling technique can make more efficient use of the limited storage capacity of remote instrumentation.

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Wendell S. Brown
,
Neal R. Pettigrew
, and
James D. Irish

Abstract

The Nantucket Shoals Flux Experiment (NSFE) was a collaborative effort to measure the alongshelf transport of mass, heat, salt and nutrients from March 1979 through April 1980 with a dense army consisting of moored current, temperature and bottom pressure instruments in an across-shelf and upper-slope transect south of Nantucket Island. The pressure component of that experiment is described here.

Bottom pressure recorders were deployed at stations N1 (46 m), N2 (66 m), N4 (105 m), and N5 (196 m) during two half-year periods. spring–summer 1979 (SUMMER) and fall–winter 1979/80 (WINTER). A synthetic subsurface pressure (SSP) record was formed from atmospheric pressure and sea level observations at Nantucket Island. The low-pass filtered (periods > 36 h) or subtidal pressures were used for the subsequent analysis. It was found that Nantucket SSP and BP are very nearly equivalent for fluctuation periods less than about 50 days if steric changes in sea level, due to density changes above the seasonal pycnocline, were removed from the SSP record. However, if coastal SSP and bottom pressures are to be contrasted on seasonal time scales, then an internal pressure (or equivalent dynamic height) correction to bottom pressure is required. An empirical orthogonal function (EOF) analysis of the pressure field shows that most of the pressure variance for periods between 2 and 40 days (WINTER, 91.9%; SUMMER, 83.5%) is contained in a mode that (a) is characterized by a decreasing amplitude and small phase-lag increase at successive seaward locations and (b) is coherent with local alongshelf (75°T) wind stress. Pressure differences between stations were used to compute across-shelf pressure gradients, whose fluctuation distribution exhibits a conspicuous intensification over the outer shelf during the WINTER in contrast to the relatively uniform SUMMER distribution.

The excellent comparison found between “geostrophic currents,” which were inferred from bottom pressure differences, and suitably averaged “observed currents” represents the first direct confirmation of the quasi-geostrophy of alongshelf flow. Subtidal fluctuations of geostrophic currents are highly coherent with observed currents and lag them by periods that are consistent with typical geostrophic adjustment times for shelf flow fields. Frequency-domain EOF results indicate that most of the geostrophic current (pressure gradient) variance is contained in the primary mode, which is characterized by an outer-shelf amplitude maximum and an approximate two-day across-shelf phase lag. A variety of statistical results shows that most of the geostrophic current (pressure gradient) variance in the 2–14-day band is highly coherent with and lass alongshelf wind stress by 0–2 days in WINTER. SUMMER geostrophic currents (pressure gradients) show less coherence and phase lag with the considerably less energetic winds but larger response per unit stress than in WINTER. Such a response is consistent with a locally wind-forced shelf flow in which friction is important.

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