Melbourne G. Briscoe

Large Diurnal Sea Surface Temperature Variability: Satellite and In Situ Measurements

Data from a surface mooring located in the Sargasso Sea at 34°N, 70°W between May 1982 and May 1984 were compared with satellite data to investigate large diurnal sea surface temperature changes. Mooring and satellite measurements are in excellent agreement for those days on which no clouds covered the site at the time of the satellite pass. During the summer half-year at this site, there is a 20% charm of diurnal warming of more than 0.5°C, with values of up to 3.5°C observed in the two-year period. Diurnal warming observed at the mooring has been simulated well by a one-dimensional model driven by local beat and momentum fluxes. Under the conditions of very light wind and strong insolation that produce the Largest surface warming, the surface mixed-layer depth reduces to the convection depth, and wind-mixing becomes unimportant. The thermal response is then limited to depths between 1 and 2 m, making it likely that such events have been underreported in routine ship observations. In all cases observed, the spatial extent of warming events as determined by satellite data are well correlated with the corresponding atmospheric pressure patterns. Conditions giving rise to the largest diurnal warming events are often associated with a westward-extending ridge of the Bermuda high. In the region studied, 57°–75°W and 29°–43°N, diurnal warming of more than 1°C was found on occasion to cover areas in excess of 300 000 km2, with warming of more than 2°C coveting areas in excess of 130 000 km2.

Preliminary results from the trimoored internal wave experiment (IWEX)

A three-dimensional array of 20 current meters, temperature sensors, and vertical temperature gradient sensors was successfully deployed for 40 days in late 1973 in the main thermocline over the Hatteras Abyssal Plain southeast of Bermuda. Sensor spacings in the main array were 1.4–1600 m in the horizontal, 2.1–1447 m in the vertical. The minimum sampling interval was 225 s. The ultimate purpose of the experiment was to estimate a vector wave number-frequency spectrum of internal waves without the usual assumptions of simple modal structure, horizontal isotropy, and linearity. The purpose of this paper is to describe some of the early results. Autospectra from the array normalize quite well in depth according to the WKBJ ‘high-mode’ solutions. Spectra of vertical displacements show a significant contribution from the internal semidiurnal tide. Samples of 1760 cross spectra calculated (based on a 40-day averaging interval) suggest horizontal isotropy, vertical homogeneity, and a possible degradation of current coherences because of fine structure in the velocity profile. Coherence of vertical displacements (i.e., temperature fluctuations) for measurements separated horizontally decays with increasing separation according to ƒ1/2X = 330 m·cph, where ƒ1/2 (cph) is the frequency at which the coherence falls to one half and X (m) is the horizontal separation. This empirical rule is based on 1600 m > X > 140 m; for smaller X, ƒ1/2 exceeds the local buoyancy frequency. Autospectra and cross spectra of vertical displacements sometimes show peaks at frequencies just less than the local buoyancy frequency; current spectra do not show such peaks. Inverse modeling of the internal wave field is in progress; expected results are a vector wave number-frequency spectrum and a description in parameter space that hopefully will permit future experiments to be less elaborate.

Preliminary results from the long-term upper-ocean study (LOTUS)

Abstract The Long-Term Upper-Ocean Study (LOTUS) is a two-year experiment at 34°N, 70°W, in deep water over the Hatteras abyssal plain, 330 km from major topography and the mean path of the Gulf Stream. The observations began in May 1983. The principal measurements are from an array of three subsurface moorings and one surface mooring carrying meteorological instrumentation as well as near-surface current meters. Some basic meteorological and engineering data are telemetered back via the ARGOS system, but most of the data are self-recorded. Other observations in the program include XBT sections, CTD profiles, and cooperative efforts with other investigators to obtain satellite imagery and other in situ measurements. The main scientific goals of the experiment are: to obtain a description of the local response of the upper ocean to a potpourri of atmospheric forcing in a variety of environmental situations, such as in the presence of Gulf Stream rings or near oceanic fronts, and even to try and describe the statistical response to the fluctuating forcing; to investigate the energy level of the internal wave field over several seasons and during the passage of various forcing and environmental events as mentioned above; and to obtain the complete profile of eddy kinetic energy. We display here selected results from the first year of LOTUS, especially those that do not require the analysis of the complete array of measurements and supporting data. Of particular interest is the eddy energy kinetic energy profile showing a nearly constant energy in the top 500 m; the abyssal eddy energy is also about half the expected level for the site. The high-frequency internal wave field at 200–500 m depths shows a seasonal variation (high in the winter, low in the early fall) that cannot be ascribed to changes in the hydrography; at greater depths the winter maximum is still visible. At each depth the energy varies from a half or a third to 2 or 3 times the mean energy at that depth. Some of the energetic events in the near-inertial field are apparently unconnected to local 3ind events, and may instead be related to the edge of strong mesoscale oceanic currents; perhaps they are being radiated during the process of geostrophic adjustment by the mesoscale features.

Riding the crest : a tale of two wave experiments

This paper gives a general overview of two ocean wave experiments. The experimental goals of the Surface Wave Processes Program (SWAPP) and of the Surface Wave Dynamics Experiment (SWADE) are quite different but complementary. In general terms, SWAPP is focused on local processes: principally wave breaking, upper mixed layer dynamics, and microwave and acoustic signatures of wave breaking. SWADE, on the other hand, is concerned primarily with the evolution of the directional wave spectrum in both time and space, improved understanding of wind forcing and wave dissipation, the effect of waves on the air-sea coupling mechanisms, and the radar response of the surface. Both programs acknowledge that wave dissipation is the weakest link in our understanding of wave evolution on the ocean. SWAPP takes a closer look at wave dissipation processes directly, while SWADE, with the use of fully non-linear (third generation) wave models and carefully measured wind forcing, provides an opportunity to study the effect o...

Vertical Coherence of the Internal Wave Field from Towed Sensors

Abstract Constant depth and isopycnal‐following tows are used to estimate the, towed vertical coherence of the internal wave field, at vertical separations of 8.5, 18, 28 and 70 m. The depths of the tows are ∼750 m at the maximum of the buoyancy frequency in the main thermocline of the Sargasso Sea, and near 350 m in the buoyancy frequency minimum between the main and seasonal thermoclines. The towed spectra and towed vertical coherence are compared with three model spectra (GM75, GM76 and IWEX): at 750 m the agreement between data and models is very good, with IWEX being slightly better. At 350 m several of the measured towed vertical coherence spectra are more complex than the spectra from the deeper tows, there are anomalously high coherences in a band from 0.7 to 2 cycles per kilometer that are not predictable by the models. We suggest this coherence bump may be evidence of Eckart resonance, i.e., modes tunneling between the two thermoclines into the region of low buoyancy frequency.

Internal waves in JASIN

The internal wavefield during the Joint Air—Sea Interaction (JASIN) experiment was monitored by moored current meters and moored and towed thermistor chains. The observations were concentrated in the upper ocean near the centre of Rockall Trough, but velocity measurements were also made near topographic features and throughout the water column. Observed spectra are compared with results from the deep ocean, as represented by the Garrett-Munk (GM) model of the spectral continuum, and are generally found to have spectral levels equal to or greater than the GM spectrum. The greatest deviation from the GM spectrum occurs at high frequencies and wavenumbers where the observed spectra often exhibit a spectral shoulder and high vertical coherence. These features, also found in other upper-ocean spectra, are explained by a model composed of three vertically standing modes. The spatial variation of internal wave variance is related to topography: variance is highest near rough topography. The ratio of variance in the semidiurnal tidal band to variance in a band in the continuum is approximately constant. The possibility of a dynamical link between the two frequency bands requires further investigation. The semidiurnal internal tide varies temporally and spatially. Rockall Bank is identified as the source of an energetic beam of tidal oscillations during a one-week period.

Observations and EOF analysis of low-frequency variability in the western part of the gulf stream recirculation

Abstract This study of low-frequency oceanic variability is based on data collected during the Long Term Upper Ocean Study (LOTUS), which was a two year program of (mainly) moored meteorological and oceanographic measurements. The mooring arrays were centered at 34°N, 70°W over the Hatteras Abyssal Plain. With a distance of about 300 km to the mean Gulf Stream axis and the continental slope, LOTUS was the most northern and western long-term mooring site in the Gulf Stream recirculation region to date. The observed low-frequency variability is dominated by zonally elongated motions of the secular time scale (periods <100 days) even great depths (4000 m). In contrast to observations in other parts of the recirculation region, the spectral shapes are strongly depth dependent. The vertical structure of the variability was examined by EOF analysis. Different kinds of EOFs were tested; the best representation of the observed variability was obtained by an EOF representing a undirectional nonrotating (with depth...

Observations on the Energy Balance of Internal Waves during JASIN [and Discussion]

Depth-integrated horizontal kinetic energy (HKE) in the frequency band 0.1-4 cycles per hour is used to estimate the time variation at one site of internal wave energy over a 40 day period during JASIN 1978. The HKE smoothed over three days varies from 400 to 1500J m-2. The canonical Garrett-Munk total energy of 3800J m-2 would provide about 1400J m-2 of hke in this high frequency band; a scaled-down estimate based on local mean buoyancy frequency and water depth suggests 450J m-2. The first part of the record (1 to about 16 August) slowly grows then decays in energy to the record minimum at rates between 0.6 and — 1 mW m-2, followed by about 11 days of sporadic growth and decay at rates between 3 and — 0.8 mW m-2 to the record maximum, and then a week of fast decay at rates between — 2 and — 1 mW m-2. A speculative balance of possible energy sources, sinks, and advection/ propagation is discussed. The tentative conclusion is that horizontal shears and wind stress fluctuations are the principal energy sources, the latter possibly via interacting surface waves, and that loss of energy from the internal wave continuum to the near-inertial band may be a major sink of hke. A surprising heuristic correspondence is shown between the amplitude of the local surface wavefield, and the internal wave energy 11 days later.

On current finestructure and moored current meter measurements of internal waves

Four closely-spaced vector-averaging current meters near 2000-m depth in the Sargasso Sea have been used to provide estimates of the intensity and vertical coherence scale of non-internal wave motions in the inertial-to-buoyancy frequency range. Based on a model that describes the measured current coherence as a degraded temperature coherence, where the degradation depends upon the signal-to-noise ratio (signal = internal waves; noise = other processes) and the coherence of the noise field, the noise spectrum is calculated (f−1 slope; 0.07 cm2s−2 variance) and a vertical half-coherence scale is estimated [0(1 m) near the buoyancy frequency, indeterminant near the inertial frequency, zero elsewhere]. It is concluded that the ‘noise’ field is caused mainly by current finestructure rather than coming from the instruments; the conclusion is supported by previous horizontal and vertical current-coherence results from the tri-moored Internal Wave Experiment (IWEX).

Distinguishing Propagating Waves and Standing Modes: An Internal Wave Model

Abstract This paper examines high-frequency (0.1-0.5 cph) internal waves, waves previously characterized by the Garrett and Munk spectral fits (GM72, GM75, GM79) as being vertically symmetric propagating waves (or equivalently “smeared” standing modes—GM72). Coherences at large vertical separations measured with deep sea moorings show significant differences from the GM79 predictions. The differences can be explained by modifying the purely propagating model to one that includes the spectral truncation and phase locking associated with a spectrum of standing modes. A model consisting of only standing modes, however, is also inadequate, whereas a more general spectral model, which effectively allows both propagating waves and standing modes, is not. These results show that much of the simplicity of the GM79 spectral fit can be attributed to lack of spectral resolution in the set of measurements to which the GM models were fit. The GM79 simplicity is not an intrinsic property of the internal wave field.